patch, 12 partsTowards a better merge resolution support

diff --git a/xdiff/xmerge.c b/xdiff/xmerge.c
index 82b3573..6ffaa4f 100644
--- a/xdiff/xmerge.c
+++ b/xdiff/xmerge.c
@@ -113,65 +113,84 @@ static int xdl_recs_copy(xdfenv_t *xe, int i, int count, int add_nl, char *dest)
 	return size;
 }
 
-static int xdl_fill_merge_buffer(xdfenv_t *xe1, const char *name1,
-		xdfenv_t *xe2, const char *name2, xdmerge_t *m, char *dest)
+static int fill_conflict_hunk(xdfenv_t *xe1, const char *name1,
+			      xdfenv_t *xe2, const char *name2,
+			      int size, int i,
+			      xdmerge_t *m, char *dest)
 {
 	const int marker_size = 7;
 	int marker1_size = (name1 ? strlen(name1) + 1 : 0);
 	int marker2_size = (name2 ? strlen(name2) + 1 : 0);
-	int conflict_marker_size = 3 * (marker_size + 1)
-		+ marker1_size + marker2_size;
-	int size, i1, j;
-
-	for (size = i1 = 0; m; m = m->next) {
-		if (m->mode == 0) {
-			size += xdl_recs_copy(xe1, i1, m->i1 - i1, 0,
-					dest ? dest + size : NULL);
-			if (dest) {
-				for (j = 0; j < marker_size; j++)
-					dest[size++] = '<';
-				if (marker1_size) {
-					dest[size] = ' ';
-					memcpy(dest + size + 1, name1,
-							marker1_size - 1);
-					size += marker1_size;
-				}
-				dest[size++] = '\n';
-			} else
-				size += conflict_marker_size;
-			size += xdl_recs_copy(xe1, m->i1, m->chg1, 1,
-					dest ? dest + size : NULL);
-			if (dest) {
-				for (j = 0; j < marker_size; j++)
-					dest[size++] = '=';
-				dest[size++] = '\n';
-			}
-			size += xdl_recs_copy(xe2, m->i2, m->chg2, 1,
-					dest ? dest + size : NULL);
-			if (dest) {
-				for (j = 0; j < marker_size; j++)
-					dest[size++] = '>';
-				if (marker2_size) {
-					dest[size] = ' ';
-					memcpy(dest + size + 1, name2,
-							marker2_size - 1);
-					size += marker2_size;
-				}
-				dest[size++] = '\n';
-			}
-		} else if (m->mode == 1)
-			size += xdl_recs_copy(xe1, i1, m->i1 + m->chg1 - i1, 0,
-					dest ? dest + size : NULL);
+	int j;
+
+	/* Before conflicting part */
+	size += xdl_recs_copy(xe1, i, m->i1 - i, 0,
+			      dest ? dest + size : NULL);
+
+	if (!dest) {
+		size += marker_size + 1 + marker1_size;
+	} else {
+		for (j = 0; j < marker_size; j++)
+			dest[size++] = '<';
+		if (marker1_size) {
+			dest[size] = ' ';
+			memcpy(dest + size + 1, name1, marker1_size - 1);
+			size += marker1_size;
+		}
+		dest[size++] = '\n';
+	}
+
+	/* Postimage from side #1 */
+	size += xdl_recs_copy(xe1, m->i1, m->chg1, 1,
+			      dest ? dest + size : NULL);
+	if (!dest) {
+		size += marker_size + 1;
+	} else {
+		for (j = 0; j < marker_size; j++)
+			dest[size++] = '=';
+		dest[size++] = '\n';
+	}
+
+	/* Postimage from side #2 */
+	size += xdl_recs_copy(xe2, m->i2, m->chg2, 1,
+			      dest ? dest + size : NULL);
+	if (!dest) {
+		size += marker_size + 1 + marker2_size;
+	} else {
+		for (j = 0; j < marker_size; j++)
+			dest[size++] = '>';
+		if (marker2_size) {
+			dest[size] = ' ';
+			memcpy(dest + size + 1, name2, marker2_size - 1);
+			size += marker2_size;
+		}
+		dest[size++] = '\n';
+	}
+	return size;
+}
+
+static int xdl_fill_merge_buffer(xdfenv_t *xe1, const char *name1,
+		xdfenv_t *xe2, const char *name2, xdmerge_t *m, char *dest)
+{
+	int size, i;
+
+	for (size = i = 0; m; m = m->next) {
+		if (m->mode == 0)
+			size = fill_conflict_hunk(xe1, name1, xe2, name2,
+						  size, i, m, dest);
+		else if (m->mode == 1)
+			size += xdl_recs_copy(xe1, i, m->i1 + m->chg1 - i, 0,
+					      dest ? dest + size : NULL);
 		else if (m->mode == 2)
-			size += xdl_recs_copy(xe2, m->i2 - m->i1 + i1,
-					m->i1 + m->chg2 - i1, 0,
-					dest ? dest + size : NULL);
+			size += xdl_recs_copy(xe2, m->i2 - m->i1 + i,
+					      m->i1 + m->chg2 - i, 0,
+					      dest ? dest + size : NULL);
 		else
 			continue;
-		i1 = m->i1 + m->chg1;
+		i = m->i1 + m->chg1;
 	}
-	size += xdl_recs_copy(xe1, i1, xe1->xdf2.nrec - i1, 0,
-			dest ? dest + size : NULL);
+	size += xdl_recs_copy(xe1, i, xe1->xdf2.nrec - i, 0,
+			      dest ? dest + size : NULL);
 	return size;
 }
 

builtin-merge-file.c, t/t6023-merge-file.sh, xdiff/xdiff.h, xdiff/xmerge.c

diff --git a/builtin-merge-file.c b/builtin-merge-file.c
index 3605960..5b4f020 100644
--- a/builtin-merge-file.c
+++ b/builtin-merge-file.c
@@ -4,7 +4,7 @@
 #include "xdiff-interface.h"
 
 static const char merge_file_usage[] =
-"git merge-file [-p | --stdout] [-q | --quiet] [-L name1 [-L orig [-L name2]]] file1 orig_file file2";
+"git merge-file [-p | --stdout] [--diff3] [-q | --quiet] [-L name1 [-L orig [-L name2]]] file1 orig_file file2";
 
 int cmd_merge_file(int argc, const char **argv, const char *prefix)
 {
@@ -13,6 +13,8 @@ int cmd_merge_file(int argc, const char **argv, const char *prefix)
 	mmbuffer_t result = {NULL, 0};
 	xpparam_t xpp = {XDF_NEED_MINIMAL};
 	int ret = 0, i = 0, to_stdout = 0;
+	int merge_level = XDL_MERGE_ZEALOUS_ALNUM;
+	int merge_style = 0;
 
 	while (argc > 4) {
 		if (!strcmp(argv[1], "-L") && i < 3) {
@@ -25,6 +27,10 @@ int cmd_merge_file(int argc, const char **argv, const char *prefix)
 		else if (!strcmp(argv[1], "-q") ||
 				!strcmp(argv[1], "--quiet"))
 			freopen("/dev/null", "w", stderr);
+		else if (!strcmp(argv[1], "--diff3")) {
+			merge_style = XDL_MERGE_DIFF3;
+			merge_level = XDL_MERGE_EAGER;
+		}
 		else
 			usage(merge_file_usage);
 		argc--;
@@ -46,7 +52,7 @@ int cmd_merge_file(int argc, const char **argv, const char *prefix)
 	}
 
 	ret = xdl_merge(mmfs + 1, mmfs + 0, names[0], mmfs + 2, names[2],
-			&xpp, XDL_MERGE_ZEALOUS_ALNUM, &result);
+			&xpp, merge_level | merge_style, &result);
 
 	for (i = 0; i < 3; i++)
 		free(mmfs[i].ptr);
diff --git a/t/t6023-merge-file.sh b/t/t6023-merge-file.sh
index 42620e0..f3484e3 100755
--- a/t/t6023-merge-file.sh
+++ b/t/t6023-merge-file.sh
@@ -161,4 +161,41 @@ test_expect_success 'ZEALOUS_ALNUM' '
 
 '
 
+cat >expect <<\EOF
+Dominus regit me,
+<<<<<<< new8.txt
+et nihil mihi deerit;
+
+
+
+
+In loco pascuae ibi me collocavit;
+super aquam refectionis educavit me.
+|||||||
+et nihil mihi deerit.
+In loco pascuae ibi me collocavit,
+super aquam refectionis educavit me;
+=======
+et nihil mihi deerit,
+
+
+
+
+In loco pascuae ibi me collocavit --
+super aquam refectionis educavit me,
+>>>>>>> new9.txt
+animam meam convertit,
+deduxit me super semitas jusitiae,
+propter nomen suum.
+Nam et si ambulavero in medio umbrae mortis,
+non timebo mala, quoniam TU mecum es:
+virga tua et baculus tuus ipsa me consolata sunt.
+EOF
+
+test_expect_success '"diff3 -m" style output' '
+	test_must_fail git merge-file -p --diff3 \
+		new8.txt new5.txt new9.txt >actual &&
+	test_cmp expect actual
+'
+
 test_done
diff --git a/xdiff/xdiff.h b/xdiff/xdiff.h
index 413082e..deebe02 100644
--- a/xdiff/xdiff.h
+++ b/xdiff/xdiff.h
@@ -50,10 +50,16 @@ extern "C" {
 #define XDL_BDOP_CPY 2
 #define XDL_BDOP_INSB 3
 
+/* merge simplification levels */
 #define XDL_MERGE_MINIMAL 0
 #define XDL_MERGE_EAGER 1
 #define XDL_MERGE_ZEALOUS 2
 #define XDL_MERGE_ZEALOUS_ALNUM 3
+#define XDL_MERGE_LEVEL_MASK 0x0f
+
+/* merge output styles */
+#define XDL_MERGE_DIFF3 0x8000
+#define XDL_MERGE_STYLE_MASK 0x8000
 
 typedef struct s_mmfile {
 	char *ptr;
diff --git a/xdiff/xmerge.c b/xdiff/xmerge.c
index 6ffaa4f..29cdbea 100644
--- a/xdiff/xmerge.c
+++ b/xdiff/xmerge.c
@@ -30,17 +30,32 @@ typedef struct s_xdmerge {
 	 * 2 = no conflict, take second.
 	 */
 	int mode;
+	/*
+	 * These point at the respective postimages.  E.g. <i1,chg1> is
+	 * how side #1 wants to change the common ancestor; if there is no
+	 * overlap, lines before i1 in the postimage of side #1 appear
+	 * in the merge result as a region touched by neither side.
+	 */
 	long i1, i2;
 	long chg1, chg2;
+	/*
+	 * These point at the preimage; of course there is just one
+	 * preimage, that is from the shared common ancestor.
+	 */
+	long i0;
+	long chg0;
 } xdmerge_t;
 
 static int xdl_append_merge(xdmerge_t **merge, int mode,
-		long i1, long chg1, long i2, long chg2)
+			    long i0, long chg0,
+			    long i1, long chg1,
+			    long i2, long chg2)
 {
 	xdmerge_t *m = *merge;
 	if (m && (i1 <= m->i1 + m->chg1 || i2 <= m->i2 + m->chg2)) {
 		if (mode != m->mode)
 			m->mode = 0;
+		m->chg0 = i0 + chg0 - m->i0;
 		m->chg1 = i1 + chg1 - m->i1;
 		m->chg2 = i2 + chg2 - m->i2;
 	} else {
@@ -49,6 +64,8 @@ static int xdl_append_merge(xdmerge_t **merge, int mode,
 			return -1;
 		m->next = NULL;
 		m->mode = mode;
+		m->i0 = i0;
+		m->chg0 = chg0;
 		m->i1 = i1;
 		m->chg1 = chg1;
 		m->i2 = i2;
@@ -91,11 +108,13 @@ static int xdl_merge_cmp_lines(xdfenv_t *xe1, int i1, xdfenv_t *xe2, int i2,
 	return 0;
 }
 
-static int xdl_recs_copy(xdfenv_t *xe, int i, int count, int add_nl, char *dest)
+static int xdl_recs_copy_0(int use_orig, xdfenv_t *xe, int i, int count, int add_nl, char *dest)
 {
-	xrecord_t **recs = xe->xdf2.recs + i;
+	xrecord_t **recs;
 	int size = 0;
 
+	recs = (use_orig ? xe->xdf1.recs : xe->xdf2.recs) + i;
+
 	if (count < 1)
 		return 0;
 
@@ -113,9 +132,19 @@ static int xdl_recs_copy(xdfenv_t *xe, int i, int count, int add_nl, char *dest)
 	return size;
 }
 
+static int xdl_recs_copy(xdfenv_t *xe, int i, int count, int add_nl, char *dest)
+{
+	return xdl_recs_copy_0(0, xe, i, count, add_nl, dest);
+}
+
+static int xdl_orig_copy(xdfenv_t *xe, int i, int count, int add_nl, char *dest)
+{
+	return xdl_recs_copy_0(1, xe, i, count, add_nl, dest);
+}
+
 static int fill_conflict_hunk(xdfenv_t *xe1, const char *name1,
 			      xdfenv_t *xe2, const char *name2,
-			      int size, int i,
+			      int size, int i, int style,
 			      xdmerge_t *m, char *dest)
 {
 	const int marker_size = 7;
@@ -143,6 +172,20 @@ static int fill_conflict_hunk(xdfenv_t *xe1, const char *name1,
 	/* Postimage from side #1 */
 	size += xdl_recs_copy(xe1, m->i1, m->chg1, 1,
 			      dest ? dest + size : NULL);
+
+	if (style == XDL_MERGE_DIFF3) {
+		/* Shared preimage */
+		if (!dest) {
+			size += marker_size + 1;
+		} else {
+			for (j = 0; j < marker_size; j++)
+				dest[size++] = '|';
+			dest[size++] = '\n';
+		}
+		size += xdl_orig_copy(xe1, m->i0, m->chg0, 1,
+				      dest ? dest + size : NULL);
+	}
+
 	if (!dest) {
 		size += marker_size + 1;
 	} else {
@@ -170,14 +213,15 @@ static int fill_conflict_hunk(xdfenv_t *xe1, const char *name1,
 }
 
 static int xdl_fill_merge_buffer(xdfenv_t *xe1, const char *name1,
-		xdfenv_t *xe2, const char *name2, xdmerge_t *m, char *dest)
+				 xdfenv_t *xe2, const char *name2,
+				 xdmerge_t *m, char *dest, int style)
 {
 	int size, i;
 
 	for (size = i = 0; m; m = m->next) {
 		if (m->mode == 0)
 			size = fill_conflict_hunk(xe1, name1, xe2, name2,
-						  size, i, m, dest);
+						  size, i, style, m, dest);
 		else if (m->mode == 1)
 			size += xdl_recs_copy(xe1, i, m->i1 + m->chg1 - i, 0,
 					      dest ? dest + size : NULL);
@@ -342,9 +386,11 @@ static int xdl_simplify_non_conflicts(xdfenv_t *xe1, xdmerge_t *m,
  */
 static int xdl_do_merge(xdfenv_t *xe1, xdchange_t *xscr1, const char *name1,
 		xdfenv_t *xe2, xdchange_t *xscr2, const char *name2,
-		int level, xpparam_t const *xpp, mmbuffer_t *result) {
+		int flags, xpparam_t const *xpp, mmbuffer_t *result) {
 	xdmerge_t *changes, *c;
-	int i1, i2, chg1, chg2;
+	int i0, i1, i2, chg0, chg1, chg2;
+	int level = flags & XDL_MERGE_LEVEL_MASK;
+	int style = flags & XDL_MERGE_STYLE_MASK;
 
 	c = changes = NULL;
 
@@ -352,11 +398,14 @@ static int xdl_do_merge(xdfenv_t *xe1, xdchange_t *xscr1, const char *name1,
 		if (!changes)
 			changes = c;
 		if (xscr1->i1 + xscr1->chg1 < xscr2->i1) {
+			i0 = xscr1->i1;
 			i1 = xscr1->i2;
 			i2 = xscr2->i2 - xscr2->i1 + xscr1->i1;
+			chg0 = xscr1->chg1;
 			chg1 = xscr1->chg2;
 			chg2 = xscr1->chg1;
-			if (xdl_append_merge(&c, 1, i1, chg1, i2, chg2)) {
+			if (xdl_append_merge(&c, 1,
+					     i0, chg0, i1, chg1, i2, chg2)) {
 				xdl_cleanup_merge(changes);
 				return -1;
 			}
@@ -364,11 +413,14 @@ static int xdl_do_merge(xdfenv_t *xe1, xdchange_t *xscr1, const char *name1,
 			continue;
 		}
 		if (xscr2->i1 + xscr2->chg1 < xscr1->i1) {
+			i0 = xscr2->i1;
 			i1 = xscr1->i2 - xscr1->i1 + xscr2->i1;
 			i2 = xscr2->i2;
+			chg0 = xscr2->chg1;
 			chg1 = xscr2->chg1;
 			chg2 = xscr2->chg2;
-			if (xdl_append_merge(&c, 2, i1, chg1, i2, chg2)) {
+			if (xdl_append_merge(&c, 2,
+					     i0, chg0, i1, chg1, i2, chg2)) {
 				xdl_cleanup_merge(changes);
 				return -1;
 			}
@@ -385,19 +437,26 @@ static int xdl_do_merge(xdfenv_t *xe1, xdchange_t *xscr1, const char *name1,
 			int off = xscr1->i1 - xscr2->i1;
 			int ffo = off + xscr1->chg1 - xscr2->chg1;
 
+			i0 = xscr1->i1;
 			i1 = xscr1->i2;
 			i2 = xscr2->i2;
-			if (off > 0)
+			if (off > 0) {
+				i0 -= off;
 				i1 -= off;
+			}
 			else
 				i2 += off;
+			chg0 = xscr1->i1 + xscr1->chg1 - i0;
 			chg1 = xscr1->i2 + xscr1->chg2 - i1;
 			chg2 = xscr2->i2 + xscr2->chg2 - i2;
 			if (ffo > 0)
 				chg2 += ffo;
-			else
+			else {
+				chg0 -= ffo;
 				chg1 -= ffo;
-			if (xdl_append_merge(&c, 0, i1, chg1, i2, chg2)) {
+			}
+			if (xdl_append_merge(&c, 0,
+					     i0, chg0, i1, chg1, i2, chg2)) {
 				xdl_cleanup_merge(changes);
 				return -1;
 			}
@@ -414,11 +473,14 @@ static int xdl_do_merge(xdfenv_t *xe1, xdchange_t *xscr1, const char *name1,
 	while (xscr1) {
 		if (!changes)
 			changes = c;
+		i0 = xscr1->i1;
 		i1 = xscr1->i2;
 		i2 = xscr1->i1 + xe2->xdf2.nrec - xe2->xdf1.nrec;
+		chg0 = xscr1->chg1;
 		chg1 = xscr1->chg2;
 		chg2 = xscr1->chg1;
-		if (xdl_append_merge(&c, 1, i1, chg1, i2, chg2)) {
+		if (xdl_append_merge(&c, 1,
+				     i0, chg0, i1, chg1, i2, chg2)) {
 			xdl_cleanup_merge(changes);
 			return -1;
 		}
@@ -427,11 +489,14 @@ static int xdl_do_merge(xdfenv_t *xe1, xdchange_t *xscr1, const char *name1,
 	while (xscr2) {
 		if (!changes)
 			changes = c;
+		i0 = xscr2->i1;
 		i1 = xscr2->i1 + xe1->xdf2.nrec - xe1->xdf1.nrec;
 		i2 = xscr2->i2;
+		chg0 = xscr2->chg1;
 		chg1 = xscr2->chg1;
 		chg2 = xscr2->chg2;
-		if (xdl_append_merge(&c, 2, i1, chg1, i2, chg2)) {
+		if (xdl_append_merge(&c, 2,
+				     i0, chg0, i1, chg1, i2, chg2)) {
 			xdl_cleanup_merge(changes);
 			return -1;
 		}
@@ -449,7 +514,7 @@ static int xdl_do_merge(xdfenv_t *xe1, xdchange_t *xscr1, const char *name1,
 	/* output */
 	if (result) {
 		int size = xdl_fill_merge_buffer(xe1, name1, xe2, name2,
-			changes, NULL);
+			changes, NULL, style);
 		result->ptr = xdl_malloc(size);
 		if (!result->ptr) {
 			xdl_cleanup_merge(changes);
@@ -457,14 +522,14 @@ static int xdl_do_merge(xdfenv_t *xe1, xdchange_t *xscr1, const char *name1,
 		}
 		result->size = size;
 		xdl_fill_merge_buffer(xe1, name1, xe2, name2, changes,
-				result->ptr);
+				      result->ptr, style);
 	}
 	return xdl_cleanup_merge(changes);
 }
 
 int xdl_merge(mmfile_t *orig, mmfile_t *mf1, const char *name1,
 		mmfile_t *mf2, const char *name2,
-		xpparam_t const *xpp, int level, mmbuffer_t *result) {
+		xpparam_t const *xpp, int flags, mmbuffer_t *result) {
 	xdchange_t *xscr1, *xscr2;
 	xdfenv_t xe1, xe2;
 	int status;
@@ -501,7 +566,7 @@ int xdl_merge(mmfile_t *orig, mmfile_t *mf1, const char *name1,
 		} else {
 			status = xdl_do_merge(&xe1, xscr1, name1,
 					      &xe2, xscr2, name2,
-					      level, xpp, result);
+					      flags, xpp, result);
 		}
 		xdl_free_script(xscr1);
 		xdl_free_script(xscr2);

diff --git a/xdiff/xmerge.c b/xdiff/xmerge.c
index 29cdbea..7dcd405 100644
--- a/xdiff/xmerge.c
+++ b/xdiff/xmerge.c
@@ -427,7 +427,7 @@ static int xdl_do_merge(xdfenv_t *xe1, xdchange_t *xscr1, const char *name1,
 			xscr2 = xscr2->next;
 			continue;
 		}
-		if (level < 1 || xscr1->i1 != xscr2->i1 ||
+		if (level == XDL_MERGE_MINIMAL || xscr1->i1 != xscr2->i1 ||
 				xscr1->chg1 != xscr2->chg1 ||
 				xscr1->chg2 != xscr2->chg2 ||
 				xdl_merge_cmp_lines(xe1, xscr1->i2,
@@ -449,12 +449,11 @@ static int xdl_do_merge(xdfenv_t *xe1, xdchange_t *xscr1, const char *name1,
 			chg0 = xscr1->i1 + xscr1->chg1 - i0;
 			chg1 = xscr1->i2 + xscr1->chg2 - i1;
 			chg2 = xscr2->i2 + xscr2->chg2 - i2;
-			if (ffo > 0)
-				chg2 += ffo;
-			else {
+			if (ffo < 0) {
 				chg0 -= ffo;
 				chg1 -= ffo;
-			}
+			} else
+				chg2 += ffo;
 			if (xdl_append_merge(&c, 0,
 					     i0, chg0, i1, chg1, i2, chg2)) {
 				xdl_cleanup_merge(changes);
@@ -505,9 +504,10 @@ static int xdl_do_merge(xdfenv_t *xe1, xdchange_t *xscr1, const char *name1,
 	if (!changes)
 		changes = c;
 	/* refine conflicts */
-	if (level > 1 &&
+	if (XDL_MERGE_ZEALOUS <= level &&
 	    (xdl_refine_conflicts(xe1, xe2, changes, xpp) < 0 ||
-	     xdl_simplify_non_conflicts(xe1, changes, level > 2) < 0)) {
+	     xdl_simplify_non_conflicts(xe1, changes,
+					XDL_MERGE_ZEALOUS < level) < 0)) {
 		xdl_cleanup_merge(changes);
 		return -1;
 	}

builtin-merge-file.c, xdiff/xmerge.c

diff --git a/builtin-merge-file.c b/builtin-merge-file.c
index 5b4f020..1e92510 100644
--- a/builtin-merge-file.c
+++ b/builtin-merge-file.c
@@ -27,10 +27,8 @@ int cmd_merge_file(int argc, const char **argv, const char *prefix)
 		else if (!strcmp(argv[1], "-q") ||
 				!strcmp(argv[1], "--quiet"))
 			freopen("/dev/null", "w", stderr);
-		else if (!strcmp(argv[1], "--diff3")) {
+		else if (!strcmp(argv[1], "--diff3"))
 			merge_style = XDL_MERGE_DIFF3;
-			merge_level = XDL_MERGE_EAGER;
-		}
 		else
 			usage(merge_file_usage);
 		argc--;
diff --git a/xdiff/xmerge.c b/xdiff/xmerge.c
index 7dcd405..d9737f0 100644
--- a/xdiff/xmerge.c
+++ b/xdiff/xmerge.c
@@ -392,6 +392,15 @@ static int xdl_do_merge(xdfenv_t *xe1, xdchange_t *xscr1, const char *name1,
 	int level = flags & XDL_MERGE_LEVEL_MASK;
 	int style = flags & XDL_MERGE_STYLE_MASK;
 
+	if (style == XDL_MERGE_DIFF3) {
+		/*
+		 * "diff3 -m" output does not make sense for anything
+		 * more aggressive than XDL_MERGE_EAGER.
+		 */
+		if (XDL_MERGE_EAGER < level)
+			level = XDL_MERGE_EAGER;
+	}
+
 	c = changes = NULL;
 
 	while (xscr1 && xscr2) {

diff --git a/rerere.c b/rerere.c
index 323e493..bf74b26 100644
--- a/rerere.c
+++ b/rerere.c
@@ -75,7 +75,10 @@ static int handle_file(const char *path,
 {
 	SHA_CTX ctx;
 	char buf[1024];
-	int hunk = 0, hunk_no = 0;
+	int hunk_no = 0;
+	enum {
+		RR_CONTEXT = 0, RR_SIDE_1, RR_SIDE_2,
+	} hunk = RR_CONTEXT;
 	struct strbuf one, two;
 	FILE *f = fopen(path, "r");
 	FILE *out = NULL;
@@ -98,20 +101,20 @@ static int handle_file(const char *path,
 	strbuf_init(&two,  0);
 	while (fgets(buf, sizeof(buf), f)) {
 		if (!prefixcmp(buf, "<<<<<<< ")) {
-			if (hunk)
+			if (hunk != RR_CONTEXT)
 				goto bad;
-			hunk = 1;
+			hunk = RR_SIDE_1;
 		} else if (!prefixcmp(buf, "=======") && isspace(buf[7])) {
-			if (hunk != 1)
+			if (hunk != RR_SIDE_1)
 				goto bad;
-			hunk = 2;
+			hunk = RR_SIDE_2;
 		} else if (!prefixcmp(buf, ">>>>>>> ")) {
-			if (hunk != 2)
+			if (hunk != RR_SIDE_2)
 				goto bad;
 			if (strbuf_cmp(&one, &two) > 0)
 				strbuf_swap(&one, &two);
 			hunk_no++;
-			hunk = 0;
+			hunk = RR_CONTEXT;
 			if (out) {
 				fputs("<<<<<<<\n", out);
 				fwrite(one.buf, one.len, 1, out);
@@ -127,9 +130,9 @@ static int handle_file(const char *path,
 			}
 			strbuf_reset(&one);
 			strbuf_reset(&two);
-		} else if (hunk == 1)
+		} else if (hunk == RR_SIDE_1)
 			strbuf_addstr(&one, buf);
-		else if (hunk == 2)
+		else if (hunk == RR_SIDE_2)
 			strbuf_addstr(&two, buf);
 		else if (out)
 			fputs(buf, out);
@@ -146,7 +149,7 @@ static int handle_file(const char *path,
 		fclose(out);
 	if (sha1)
 		SHA1_Final(sha1, &ctx);
-	if (hunk) {
+	if (hunk != RR_CONTEXT) {
 		if (output)
 			unlink(output);
 		return error("Could not parse conflict hunks in %s", path);

diff --git a/rerere.c b/rerere.c
index bf74b26..4e2c9dd 100644
--- a/rerere.c
+++ b/rerere.c
@@ -77,7 +77,7 @@ static int handle_file(const char *path,
 	char buf[1024];
 	int hunk_no = 0;
 	enum {
-		RR_CONTEXT = 0, RR_SIDE_1, RR_SIDE_2,
+		RR_CONTEXT = 0, RR_SIDE_1, RR_SIDE_2, RR_ORIGINAL,
 	} hunk = RR_CONTEXT;
 	struct strbuf one, two;
 	FILE *f = fopen(path, "r");
@@ -104,9 +104,13 @@ static int handle_file(const char *path,
 			if (hunk != RR_CONTEXT)
 				goto bad;
 			hunk = RR_SIDE_1;
-		} else if (!prefixcmp(buf, "=======") && isspace(buf[7])) {
+		} else if (!prefixcmp(buf, "|||||||") && isspace(buf[7])) {
 			if (hunk != RR_SIDE_1)
 				goto bad;
+			hunk = RR_ORIGINAL;
+		} else if (!prefixcmp(buf, "=======") && isspace(buf[7])) {
+			if (hunk != RR_SIDE_1 && hunk != RR_ORIGINAL)
+				goto bad;
 			hunk = RR_SIDE_2;
 		} else if (!prefixcmp(buf, ">>>>>>> ")) {
 			if (hunk != RR_SIDE_2)
@@ -132,6 +136,8 @@ static int handle_file(const char *path,
 			strbuf_reset(&two);
 		} else if (hunk == RR_SIDE_1)
 			strbuf_addstr(&one, buf);
+		else if (hunk == RR_ORIGINAL)
+			; /* discard */
 		else if (hunk == RR_SIDE_2)
 			strbuf_addstr(&two, buf);
 		else if (out)

Documentation/config.txt, builtin-merge-file.c, t/t6023-merge-file.sh, xdiff-interface.c, xdiff-interface.h

diff --git a/Documentation/config.txt b/Documentation/config.txt
index af57d94..cb4c4ca 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -893,6 +893,14 @@ man.<tool>.path::
 	Override the path for the given tool that may be used to
 	display help in the 'man' format. See linkgit:git-help[1].
 
+merge.conflictstyle::
+	Specify the style in which conflicted hunks are written out to
+	working tree files upon merge.  The default is "merge", which
+	shows `<<<<<<<` conflict marker, change made by one side,
+	`=======` marker, change made by the other side, and then
+	`>>>>>>>` marker.  An alternate style, "diff3", adds `|||||||`
+	marker and the original text before `=======` marker.
+
 mergetool.<tool>.path::
 	Override the path for the given tool.  This is useful in case
 	your tool is not in the PATH.
diff --git a/builtin-merge-file.c b/builtin-merge-file.c
index 1e92510..f009e73 100644
--- a/builtin-merge-file.c
+++ b/builtin-merge-file.c
@@ -15,6 +15,15 @@ int cmd_merge_file(int argc, const char **argv, const char *prefix)
 	int ret = 0, i = 0, to_stdout = 0;
 	int merge_level = XDL_MERGE_ZEALOUS_ALNUM;
 	int merge_style = 0;
+	int nongit;
+
+	prefix = setup_git_directory_gently(&nongit);
+	if (!nongit) {
+		/* Read the configuration file */
+		git_config(git_xmerge_config, NULL);
+		if (git_xmerge_style > 0)
+			merge_style = git_xmerge_style;
+	}
 
 	while (argc > 4) {
 		if (!strcmp(argv[1], "-L") && i < 3) {
diff --git a/t/t6023-merge-file.sh b/t/t6023-merge-file.sh
index f3484e3..93ec517 100755
--- a/t/t6023-merge-file.sh
+++ b/t/t6023-merge-file.sh
@@ -192,10 +192,17 @@ non timebo mala, quoniam TU mecum es:
 virga tua et baculus tuus ipsa me consolata sunt.
 EOF
 
-test_expect_success '"diff3 -m" style output' '
+test_expect_success '"diff3 -m" style output (1)' '
 	test_must_fail git merge-file -p --diff3 \
 		new8.txt new5.txt new9.txt >actual &&
 	test_cmp expect actual
 '
 
+test_expect_success '"diff3 -m" style output (2)' '
+	git config merge.conflictstyle diff3 &&
+	test_must_fail git merge-file -p \
+		new8.txt new5.txt new9.txt >actual &&
+	test_cmp expect actual
+'
+
 test_done
diff --git a/xdiff-interface.c b/xdiff-interface.c
index 944ad98..8457601 100644
--- a/xdiff-interface.c
+++ b/xdiff-interface.c
@@ -249,3 +249,23 @@ void xdiff_set_find_func(xdemitconf_t *xecfg, const char *value)
 		value = ep + 1;
 	}
 }
+
+int git_xmerge_style = -1;
+
+int git_xmerge_config(const char *var, const char *value, void *cb)
+{
+	if (!strcasecmp(var, "merge.conflictstyle")) {
+		if (!value)
+			die("'%s' is not a boolean", var);
+		if (!strcmp(value, "diff3"))
+			git_xmerge_style = XDL_MERGE_DIFF3;
+		else if (!strcmp(value, "merge"))
+			git_xmerge_style = 0;
+		else
+			die("unknown style '%s' given for '%s'",
+			    value, var);
+		return 0;
+	}
+	return git_default_config(var, value, cb);
+}
+
diff --git a/xdiff-interface.h b/xdiff-interface.h
index 558492b..b3b5c93 100644
--- a/xdiff-interface.h
+++ b/xdiff-interface.h
@@ -17,5 +17,7 @@ int read_mmfile(mmfile_t *ptr, const char *filename);
 int buffer_is_binary(const char *ptr, unsigned long size);
 
 extern void xdiff_set_find_func(xdemitconf_t *xecfg, const char *line);
+extern int git_xmerge_config(const char *var, const char *value, void *cb);
+extern int git_xmerge_style;
 
 #endif

builtin-merge-recursive.c, ll-merge.c

diff --git a/builtin-merge-recursive.c b/builtin-merge-recursive.c
index dfb363e..f3b6ede 100644
--- a/builtin-merge-recursive.c
+++ b/builtin-merge-recursive.c
@@ -1348,7 +1348,7 @@ static int merge_config(const char *var, const char *value, void *cb)
 		merge_rename_limit = git_config_int(var, value);
 		return 0;
 	}
-	return git_default_config(var, value, cb);
+	return git_xmerge_config(var, value, cb);
 }
 
 int cmd_merge_recursive(int argc, const char **argv, const char *prefix)
diff --git a/ll-merge.c b/ll-merge.c
index 9837c84..4a71614 100644
--- a/ll-merge.c
+++ b/ll-merge.c
@@ -63,6 +63,7 @@ static int ll_xdl_merge(const struct ll_merge_driver *drv_unused,
 			int virtual_ancestor)
 {
 	xpparam_t xpp;
+	int style = 0;
 
 	if (buffer_is_binary(orig->ptr, orig->size) ||
 	    buffer_is_binary(src1->ptr, src1->size) ||
@@ -77,10 +78,12 @@ static int ll_xdl_merge(const struct ll_merge_driver *drv_unused,
 	}
 
 	memset(&xpp, 0, sizeof(xpp));
+	if (git_xmerge_style >= 0)
+		style = git_xmerge_style;
 	return xdl_merge(orig,
 			 src1, name1,
 			 src2, name2,
-			 &xpp, XDL_MERGE_ZEALOUS,
+			 &xpp, XDL_MERGE_ZEALOUS | style,
 			 result);
 }
 
@@ -95,10 +98,15 @@ static int ll_union_merge(const struct ll_merge_driver *drv_unused,
 	char *src, *dst;
 	long size;
 	const int marker_size = 7;
-
-	int status = ll_xdl_merge(drv_unused, result, path_unused,
-				  orig, src1, NULL, src2, NULL,
-				  virtual_ancestor);
+	int status, saved_style;
+
+	/* We have to force the RCS "merge" style */
+	saved_style = git_xmerge_style;
+	git_xmerge_style = 0;
+	status = ll_xdl_merge(drv_unused, result, path_unused,
+			      orig, src1, NULL, src2, NULL,
+			      virtual_ancestor);
+	git_xmerge_style = saved_style;
 	if (status <= 0)
 		return status;
 	size = result->size;

builtin-checkout.c, t/t7201-co.sh

diff --git a/builtin-checkout.c b/builtin-checkout.c
index b380ad6..9b33f3a 100644
--- a/builtin-checkout.c
+++ b/builtin-checkout.c
@@ -76,6 +76,15 @@ static int read_tree_some(struct tree *tree, const char **pathspec)
 	return 0;
 }
 
+static int skip_same_name(struct cache_entry *ce, int pos)
+{
+	while (++pos < active_nr &&
+	       !strcmp(active_cache[pos]->name, ce->name))
+		; /* skip */
+	return pos;
+}
+
+
 static int checkout_paths(struct tree *source_tree, const char **pathspec)
 {
 	int pos;
@@ -107,6 +116,20 @@ static int checkout_paths(struct tree *source_tree, const char **pathspec)
 	if (report_path_error(ps_matched, pathspec, 0))
 		return 1;
 
+	/* Any unmerged paths? */
+	for (pos = 0; pos < active_nr; pos++) {
+		struct cache_entry *ce = active_cache[pos];
+		if (pathspec_match(pathspec, NULL, ce->name, 0) &&
+		    ce_stage(ce)) {
+			errs = 1;
+			error("path '%s' is unmerged", ce->name);
+			pos = skip_same_name(ce, pos) - 1;
+			continue;
+		}
+	}
+	if (errs)
+		return 1;
+
 	/* Now we are committed to check them out */
 	memset(&state, 0, sizeof(state));
 	state.force = 1;
@@ -114,6 +137,10 @@ static int checkout_paths(struct tree *source_tree, const char **pathspec)
 	for (pos = 0; pos < active_nr; pos++) {
 		struct cache_entry *ce = active_cache[pos];
 		if (pathspec_match(pathspec, NULL, ce->name, 0)) {
+			if (ce_stage(ce)) {
+				pos = skip_same_name(ce, pos) - 1;
+				continue;
+			}
 			errs |= checkout_entry(ce, &state, NULL);
 		}
 	}
diff --git a/t/t7201-co.sh b/t/t7201-co.sh
index 1dff84d..303cf62 100755
--- a/t/t7201-co.sh
+++ b/t/t7201-co.sh
@@ -369,4 +369,27 @@ test_expect_success \
     'checkout with --track, but without -b, fails with too short tracked name' '
     test_must_fail git checkout --track renamer'
 
+test_expect_success 'checkout an unmerged path should fail' '
+	rm -f .git/index &&
+	O=$(echo original | git hash-object -w --stdin) &&
+	A=$(echo ourside | git hash-object -w --stdin) &&
+	B=$(echo theirside | git hash-object -w --stdin) &&
+	(
+		echo "100644 $A 0	fild" &&
+		echo "100644 $O 1	file" &&
+		echo "100644 $A 2	file" &&
+		echo "100644 $B 3	file" &&
+		echo "100644 $A 0	filf"
+	) | git update-index --index-info &&
+	echo "none of the above" >sample &&
+	cat sample >fild &&
+	cat sample >file &&
+	cat sample >filf &&
+	test_must_fail git checkout fild file filf &&
+	test_cmp sample fild &&
+	test_cmp sample filf &&
+	test_cmp sample file
+'
+
 test_done
+

builtin-checkout.c, t/t7201-co.sh

diff --git a/builtin-checkout.c b/builtin-checkout.c
index 9b33f3a..49c43d9 100644
--- a/builtin-checkout.c
+++ b/builtin-checkout.c
@@ -20,6 +20,17 @@ static const char * const checkout_usage[] = {
 	NULL,
 };
 
+struct checkout_opts {
+	int quiet;
+	int merge;
+	int force;
+	int writeout_error;
+
+	const char *new_branch;
+	int new_branch_log;
+	enum branch_track track;
+};
+
 static int post_checkout_hook(struct commit *old, struct commit *new,
 			      int changed)
 {
@@ -85,7 +96,8 @@ static int skip_same_name(struct cache_entry *ce, int pos)
 }
 
 
-static int checkout_paths(struct tree *source_tree, const char **pathspec)
+static int checkout_paths(struct tree *source_tree, const char **pathspec,
+			  struct checkout_opts *opts)
 {
 	int pos;
 	struct checkout state;
@@ -121,8 +133,12 @@ static int checkout_paths(struct tree *source_tree, const char **pathspec)
 		struct cache_entry *ce = active_cache[pos];
 		if (pathspec_match(pathspec, NULL, ce->name, 0) &&
 		    ce_stage(ce)) {
-			errs = 1;
-			error("path '%s' is unmerged", ce->name);
+			if (!opts->force) {
+				errs = 1;
+				error("path '%s' is unmerged", ce->name);
+			} else {
+				warning("path '%s' is unmerged", ce->name);
+			}
 			pos = skip_same_name(ce, pos) - 1;
 			continue;
 		}
@@ -178,17 +194,6 @@ static void describe_detached_head(char *msg, struct commit *commit)
 	strbuf_release(&sb);
 }
 
-struct checkout_opts {
-	int quiet;
-	int merge;
-	int force;
-	int writeout_error;
-
-	const char *new_branch;
-	int new_branch_log;
-	enum branch_track track;
-};
-
 static int reset_tree(struct tree *tree, struct checkout_opts *o, int worktree)
 {
 	struct unpack_trees_options opts;
@@ -569,15 +574,15 @@ no_reference:
 			die("invalid path specification");
 
 		/* Checkout paths */
-		if (opts.new_branch || opts.force || opts.merge) {
+		if (opts.new_branch || opts.merge) {
 			if (argc == 1) {
-				die("git checkout: updating paths is incompatible with switching branches/forcing\nDid you intend to checkout '%s' which can not be resolved as commit?", argv[0]);
+				die("git checkout: updating paths is incompatible with switching branches.\nDid you intend to checkout '%s' which can not be resolved as commit?", argv[0]);
 			} else {
-				die("git checkout: updating paths is incompatible with switching branches/forcing");
+				die("git checkout: updating paths is incompatible with switching branches.");
 			}
 		}
 
-		return checkout_paths(source_tree, pathspec);
+		return checkout_paths(source_tree, pathspec, &opts);
 	}
 
 	if (new.name && !new.commit) {
diff --git a/t/t7201-co.sh b/t/t7201-co.sh
index 303cf62..b7274eb 100755
--- a/t/t7201-co.sh
+++ b/t/t7201-co.sh
@@ -391,5 +391,27 @@ test_expect_success 'checkout an unmerged path should fail' '
 	test_cmp sample file
 '
 
-test_done
+test_expect_success 'checkout with an unmerged path can be ignored' '
+	rm -f .git/index &&
+	O=$(echo original | git hash-object -w --stdin) &&
+	A=$(echo ourside | git hash-object -w --stdin) &&
+	B=$(echo theirside | git hash-object -w --stdin) &&
+	(
+		echo "100644 $A 0	fild" &&
+		echo "100644 $O 1	file" &&
+		echo "100644 $A 2	file" &&
+		echo "100644 $B 3	file" &&
+		echo "100644 $A 0	filf"
+	) | git update-index --index-info &&
+	echo "none of the above" >sample &&
+	echo ourside >expect &&
+	cat sample >fild &&
+	cat sample >file &&
+	cat sample >filf &&
+	git checkout -f fild file filf &&
+	test_cmp expect fild &&
+	test_cmp expect filf &&
+	test_cmp sample file
+'
 
+test_done

builtin-checkout.c, t/t7201-co.sh

diff --git a/builtin-checkout.c b/builtin-checkout.c
index 49c43d9..bdfdb65 100644
--- a/builtin-checkout.c
+++ b/builtin-checkout.c
@@ -24,6 +24,7 @@ struct checkout_opts {
 	int quiet;
 	int merge;
 	int force;
+	int writeout_stage;
 	int writeout_error;
 
 	const char *new_branch;
@@ -95,6 +96,32 @@ static int skip_same_name(struct cache_entry *ce, int pos)
 	return pos;
 }
 
+static int check_stage(int stage, struct cache_entry *ce, int pos)
+{
+	while (pos < active_nr &&
+	       !strcmp(active_cache[pos]->name, ce->name)) {
+		if (ce_stage(active_cache[pos]) == stage)
+			return 0;
+		pos++;
+	}
+	return error("path '%s' does not have %s version",
+		     ce->name,
+		     (stage == 2) ? "our" : "their");
+}
+
+static int checkout_stage(int stage, struct cache_entry *ce, int pos,
+			  struct checkout *state)
+{
+	while (pos < active_nr &&
+	       !strcmp(active_cache[pos]->name, ce->name)) {
+		if (ce_stage(active_cache[pos]) == stage)
+			return checkout_entry(active_cache[pos], state, NULL);
+		pos++;
+	}
+	return error("path '%s' does not have %s version",
+		     ce->name,
+		     (stage == 2) ? "our" : "their");
+}
 
 static int checkout_paths(struct tree *source_tree, const char **pathspec,
 			  struct checkout_opts *opts)
@@ -106,6 +133,7 @@ static int checkout_paths(struct tree *source_tree, const char **pathspec,
 	int flag;
 	struct commit *head;
 	int errs = 0;
+	int stage = opts->writeout_stage;
 
 	int newfd;
 	struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
@@ -131,13 +159,16 @@ static int checkout_paths(struct tree *source_tree, const char **pathspec,
 	/* Any unmerged paths? */
 	for (pos = 0; pos < active_nr; pos++) {
 		struct cache_entry *ce = active_cache[pos];
-		if (pathspec_match(pathspec, NULL, ce->name, 0) &&
-		    ce_stage(ce)) {
-			if (!opts->force) {
+		if (pathspec_match(pathspec, NULL, ce->name, 0)) {
+			if (!ce_stage(ce))
+				continue;
+			if (stage) {
+				errs |= check_stage(stage, ce, pos);
+			} else if (opts->force) {
+				warning("path '%s' is unmerged", ce->name);
+			} else {
 				errs = 1;
 				error("path '%s' is unmerged", ce->name);
-			} else {
-				warning("path '%s' is unmerged", ce->name);
 			}
 			pos = skip_same_name(ce, pos) - 1;
 			continue;
@@ -154,6 +185,8 @@ static int checkout_paths(struct tree *source_tree, const char **pathspec,
 		struct cache_entry *ce = active_cache[pos];
 		if (pathspec_match(pathspec, NULL, ce->name, 0)) {
 			if (ce_stage(ce)) {
+				if (stage)
+					errs |= checkout_stage(stage, ce, pos, &state);
 				pos = skip_same_name(ce, pos) - 1;
 				continue;
 			}
@@ -458,6 +491,10 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 		OPT_BOOLEAN('l', NULL, &opts.new_branch_log, "log for new branch"),
 		OPT_SET_INT('t', "track",  &opts.track, "track",
 			BRANCH_TRACK_EXPLICIT),
+		OPT_SET_INT('2', "ours", &opts.writeout_stage, "stage",
+			    2),
+		OPT_SET_INT('3', "theirs", &opts.writeout_stage, "stage",
+			    3),
 		OPT_BOOLEAN('f', NULL, &opts.force, "force"),
 		OPT_BOOLEAN('m', NULL, &opts.merge, "merge"),
 		OPT_END(),
diff --git a/t/t7201-co.sh b/t/t7201-co.sh
index b7274eb..85c792c 100755
--- a/t/t7201-co.sh
+++ b/t/t7201-co.sh
@@ -414,4 +414,29 @@ test_expect_success 'checkout with an unmerged path can be ignored' '
 	test_cmp sample file
 '
 
+test_expect_success 'checkout unmerged stage' '
+	rm -f .git/index &&
+	O=$(echo original | git hash-object -w --stdin) &&
+	A=$(echo ourside | git hash-object -w --stdin) &&
+	B=$(echo theirside | git hash-object -w --stdin) &&
+	(
+		echo "100644 $A 0	fild" &&
+		echo "100644 $O 1	file" &&
+		echo "100644 $A 2	file" &&
+		echo "100644 $B 3	file" &&
+		echo "100644 $A 0	filf"
+	) | git update-index --index-info &&
+	echo "none of the above" >sample &&
+	echo ourside >expect &&
+	cat sample >fild &&
+	cat sample >file &&
+	cat sample >filf &&
+	git checkout --ours . &&
+	test_cmp expect fild &&
+	test_cmp expect filf &&
+	test_cmp expect file &&
+	git checkout --theirs file &&
+	test ztheirside = "z$(cat file)"
+'
+
 test_done

builtin-checkout.c, t/t7201-co.sh

diff --git a/builtin-checkout.c b/builtin-checkout.c
index bdfdb65..8b5da7f 100644
--- a/builtin-checkout.c
+++ b/builtin-checkout.c
@@ -13,6 +13,9 @@
 #include "diff.h"
 #include "revision.h"
 #include "remote.h"
+#include "blob.h"
+#include "xdiff-interface.h"
+#include "ll-merge.h"
 
 static const char * const checkout_usage[] = {
 	"git checkout [options] <branch>",
@@ -109,6 +112,19 @@ static int check_stage(int stage, struct cache_entry *ce, int pos)
 		     (stage == 2) ? "our" : "their");
 }
 
+static int check_all_stage(struct cache_entry *ce, int pos)
+{
+	if (ce_stage(ce) != 1 ||
+	    active_nr <= pos + 2 ||
+	    strcmp(active_cache[pos+1]->name, ce->name) ||
+	    ce_stage(active_cache[pos+1]) != 2 ||
+	    strcmp(active_cache[pos+2]->name, ce->name) ||
+	    ce_stage(active_cache[pos+2]) != 3)
+		return error("path '%s' does not have all three versions",
+			     ce->name);
+	return 0;
+}
+
 static int checkout_stage(int stage, struct cache_entry *ce, int pos,
 			  struct checkout *state)
 {
@@ -123,6 +139,77 @@ static int checkout_stage(int stage, struct cache_entry *ce, int pos,
 		     (stage == 2) ? "our" : "their");
 }
 
+/* NEEDSWORK: share with merge-recursive */
+static void fill_mm(const unsigned char *sha1, mmfile_t *mm)
+{
+	unsigned long size;
+	enum object_type type;
+
+	if (!hashcmp(sha1, null_sha1)) {
+		mm->ptr = xstrdup("");
+		mm->size = 0;
+		return;
+	}
+
+	mm->ptr = read_sha1_file(sha1, &type, &size);
+	if (!mm->ptr || type != OBJ_BLOB)
+		die("unable to read blob object %s", sha1_to_hex(sha1));
+	mm->size = size;
+}
+
+static int checkout_merged(int pos, struct checkout *state)
+{
+	struct cache_entry *ce = active_cache[pos];
+	const char *path = ce->name;
+	mmfile_t ancestor, ours, theirs;
+	int status;
+	unsigned char sha1[20];
+	mmbuffer_t result_buf;
+
+	if (ce_stage(ce) != 1 ||
+	    active_nr <= pos + 2 ||
+	    strcmp(active_cache[pos+1]->name, path) ||
+	    ce_stage(active_cache[pos+1]) != 2 ||
+	    strcmp(active_cache[pos+2]->name, path) ||
+	    ce_stage(active_cache[pos+2]) != 3)
+		return error("path '%s' does not have all 3 versions", path);
+
+	fill_mm(active_cache[pos]->sha1, &ancestor);
+	fill_mm(active_cache[pos+1]->sha1, &ours);
+	fill_mm(active_cache[pos+2]->sha1, &theirs);
+
+	status = ll_merge(&result_buf, path, &ancestor,
+			  &ours, "ours", &theirs, "theirs", 1);
+	free(ancestor.ptr);
+	free(ours.ptr);
+	free(theirs.ptr);
+	if (status < 0 || !result_buf.ptr) {
+		free(result_buf.ptr);
+		return error("path '%s': cannot merge", path);
+	}
+
+	/*
+	 * NEEDSWORK:
+	 * There is absolutely no reason to write this as a blob object
+	 * and create a phoney cache entry just to leak.  This hack is
+	 * primarily to get to the write_entry() machinery that massages
+	 * the contents to work-tree format and writes out which only
+	 * allows it for a cache entry.  The code in write_entry() needs
+	 * to be refactored to allow us to feed a <buffer, size, mode>
+	 * instead of a cache entry.  Such a refactoring would help
+	 * merge_recursive as well (it also writes the merge result to the
+	 * object database even when it may contain conflicts).
+	 */
+	if (write_sha1_file(result_buf.ptr, result_buf.size,
+			    blob_type, sha1))
+		die("Unable to add merge result for '%s'", path);
+	ce = make_cache_entry(create_ce_mode(active_cache[pos+1]->ce_mode),
+			      sha1,
+			      path, 2, 0);
+	status = checkout_entry(ce, state, NULL);
+	return status;
+}
+
 static int checkout_paths(struct tree *source_tree, const char **pathspec,
 			  struct checkout_opts *opts)
 {
@@ -134,7 +221,7 @@ static int checkout_paths(struct tree *source_tree, const char **pathspec,
 	struct commit *head;
 	int errs = 0;
 	int stage = opts->writeout_stage;
-
+	int merge = opts->merge;
 	int newfd;
 	struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
 
@@ -166,6 +253,8 @@ static int checkout_paths(struct tree *source_tree, const char **pathspec,
 				errs |= check_stage(stage, ce, pos);
 			} else if (opts->force) {
 				warning("path '%s' is unmerged", ce->name);
+			} else if (opts->merge) {
+				errs |= check_all_stage(ce, pos);
 			} else {
 				errs = 1;
 				error("path '%s' is unmerged", ce->name);
@@ -184,13 +273,15 @@ static int checkout_paths(struct tree *source_tree, const char **pathspec,
 	for (pos = 0; pos < active_nr; pos++) {
 		struct cache_entry *ce = active_cache[pos];
 		if (pathspec_match(pathspec, NULL, ce->name, 0)) {
-			if (ce_stage(ce)) {
-				if (stage)
-					errs |= checkout_stage(stage, ce, pos, &state);
-				pos = skip_same_name(ce, pos) - 1;
+			if (!ce_stage(ce)) {
+				errs |= checkout_entry(ce, &state, NULL);
 				continue;
 			}
-			errs |= checkout_entry(ce, &state, NULL);
+			if (stage)
+				errs |= checkout_stage(stage, ce, pos, &state);
+			else if (merge)
+				errs |= checkout_merged(pos, &state);
+			pos = skip_same_name(ce, pos) - 1;
 		}
 	}
 
@@ -478,6 +569,11 @@ static int switch_branches(struct checkout_opts *opts, struct branch_info *new)
 	return ret || opts->writeout_error;
 }
 
+static int git_checkout_config(const char *var, const char *value, void *cb)
+{
+	return git_xmerge_config(var, value, cb);
+}
+
 int cmd_checkout(int argc, const char **argv, const char *prefix)
 {
 	struct checkout_opts opts;
@@ -504,7 +600,7 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 	memset(&opts, 0, sizeof(opts));
 	memset(&new, 0, sizeof(new));
 
-	git_config(git_default_config, NULL);
+	git_config(git_checkout_config, NULL);
 
 	opts.track = BRANCH_TRACK_UNSPECIFIED;
 
@@ -611,7 +707,7 @@ no_reference:
 			die("invalid path specification");
 
 		/* Checkout paths */
-		if (opts.new_branch || opts.merge) {
+		if (opts.new_branch) {
 			if (argc == 1) {
 				die("git checkout: updating paths is incompatible with switching branches.\nDid you intend to checkout '%s' which can not be resolved as commit?", argv[0]);
 			} else {
@@ -619,6 +715,9 @@ no_reference:
 			}
 		}
 
+		if (1 < !!opts.writeout_stage + !!opts.force + !!opts.merge)
+			die("git checkout: --ours/--theirs, --force and --merge are incompatible when\nchecking out of the index.");
+
 		return checkout_paths(source_tree, pathspec, &opts);
 	}
 
diff --git a/t/t7201-co.sh b/t/t7201-co.sh
index 85c792c..6016915 100755
--- a/t/t7201-co.sh
+++ b/t/t7201-co.sh
@@ -439,4 +439,67 @@ test_expect_success 'checkout unmerged stage' '
 	test ztheirside = "z$(cat file)"
 '
 
+test_expect_success 'checkout with --merge' '
+	rm -f .git/index &&
+	O=$(echo original | git hash-object -w --stdin) &&
+	A=$(echo ourside | git hash-object -w --stdin) &&
+	B=$(echo theirside | git hash-object -w --stdin) &&
+	(
+		echo "100644 $A 0	fild" &&
+		echo "100644 $O 1	file" &&
+		echo "100644 $A 2	file" &&
+		echo "100644 $B 3	file" &&
+		echo "100644 $A 0	filf"
+	) | git update-index --index-info &&
+	echo "none of the above" >sample &&
+	echo ourside >expect &&
+	cat sample >fild &&
+	cat sample >file &&
+	cat sample >filf &&
+	git checkout -m -- fild file filf &&
+	(
+		echo "<<<<<<< ours"
+		echo ourside
+		echo "======="
+		echo theirside
+		echo ">>>>>>> theirs"
+	) >merged &&
+	test_cmp expect fild &&
+	test_cmp expect filf &&
+	test_cmp merged file
+'
+
+test_expect_success 'checkout with --merge, in diff3 -m style' '
+	git config merge.conflictstyle diff3 &&
+	rm -f .git/index &&
+	O=$(echo original | git hash-object -w --stdin) &&
+	A=$(echo ourside | git hash-object -w --stdin) &&
+	B=$(echo theirside | git hash-object -w --stdin) &&
+	(
+		echo "100644 $A 0	fild" &&
+		echo "100644 $O 1	file" &&
+		echo "100644 $A 2	file" &&
+		echo "100644 $B 3	file" &&
+		echo "100644 $A 0	filf"
+	) | git update-index --index-info &&
+	echo "none of the above" >sample &&
+	echo ourside >expect &&
+	cat sample >fild &&
+	cat sample >file &&
+	cat sample >filf &&
+	git checkout -m -- fild file filf &&
+	(
+		echo "<<<<<<< ours"
+		echo ourside
+		echo "|||||||"
+		echo original
+		echo "======="
+		echo theirside
+		echo ">>>>>>> theirs"
+	) >merged &&
+	test_cmp expect fild &&
+	test_cmp expect filf &&
+	test_cmp merged file
+'
+
 test_done