threads / patch / 38567

patchhex.c: reduce memory footprint of sha1_to_hex static buffers

Subject: [PATCH] hex.c: reduce memory footprint of sha1_to_hex static buffers

## tl;dr

5 messages between Feb 13, 2015 and Feb 13, 2015. Diffs are folded; open one to read it.

replies: 4people: 3as markdown or json

Stefan Beller· Feb 13, 2015, 21:18 UTC · lore

41 bytes is the exact number of bytes needed for having the returned hex string represented. 50 seems to be an arbitrary number, such that there are no benefits from alignment to certain address boundaries.

Signed-off-by: Stefan Beller <sbeller@google.com>
---
 hex.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
Show changes to hex.c +1 −1
diff --git a/hex.c b/hex.c
index 9ebc050..cfd9d72 100644
--- a/hex.c
+++ b/hex.c
@@ -59,7 +59,7 @@ int get_sha1_hex(const char *hex, unsigned char *sha1)
 char *sha1_to_hex(const unsigned char *sha1)
 {
 	static int bufno;
-	static char hexbuffer[4][50];
+	static char hexbuffer[4][41];
 	static const char hex[] = "0123456789abcdef";
 	char *buffer = hexbuffer[3 & ++bufno], *buf = buffer;
 	int i;

-- 
2.3.0.81.gc37f363
Junio C Hamano· Feb 13, 2015, 21:41 UTC · re: Stefan Beller · lore

Re: [PATCH] hex.c: reduce memory footprint of sha1_to_hex static buffers

Stefan Beller <sbeller@google.com> writes:
> 41 bytes is the exact number of bytes needed for having the returned
> hex string represented. 50 seems to be an arbitrary number, such
> that there are no benefits from alignment to certain address boundaries.
Yes, with s/seems to be/is/;

This comes from e83c5163 (Initial revision of "git", the information manager from hell, 2005-04-07), and when dcb3450f (sha1_to_hex() usage cleanup, 2006-05-03) introduced the "4 recycled buffers" on top, the underlying array was left at 50 bytes long.

You can now have "I fixed Linus's bug" badge ;-)
Show 18 quoted lines
> Signed-off-by: Stefan Beller <sbeller@google.com>
> ---
>  hex.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/hex.c b/hex.c
> index 9ebc050..cfd9d72 100644
> --- a/hex.c
> +++ b/hex.c
> @@ -59,7 +59,7 @@ int get_sha1_hex(const char *hex, unsigned char *sha1)
>  char *sha1_to_hex(const unsigned char *sha1)
>  {
>  	static int bufno;
> -	static char hexbuffer[4][50];
> +	static char hexbuffer[4][41];
>  	static const char hex[] = "0123456789abcdef";
>  	char *buffer = hexbuffer[3 & ++bufno], *buf = buffer;
>  	int i;
Stefan Beller· Feb 13, 2015, 21:56 UTC · re: Junio C Hamano · lore

Re: [PATCH] hex.c: reduce memory footprint of sha1_to_hex static buffers

On Fri, Feb 13, 2015 at 1:41 PM, Junio C Hamano <gitster@pobox.com> wrote:
Show 14 quoted lines
> Stefan Beller <sbeller@google.com> writes:
>
>> 41 bytes is the exact number of bytes needed for having the returned
>> hex string represented. 50 seems to be an arbitrary number, such
>> that there are no benefits from alignment to certain address boundaries.
>
> Yes, with s/seems to be/is/;
>
> This comes from e83c5163 (Initial revision of "git", the information
> manager from hell, 2005-04-07), and when dcb3450f (sha1_to_hex()
> usage cleanup, 2006-05-03) introduced the "4 recycled buffers" on
> top, the underlying array was left at 50 bytes long.
>
> You can now have "I fixed Linus's bug" badge ;-)
I don't think it's a bug, it's just wasting memory?

As I could not find any documentation on the magical 50 in the early days, I cc'd Linus in case there is something I did not think of yet.

Linus Torvalds· Feb 13, 2015, 21:58 UTC · re: Stefan Beller · lore

Re: [PATCH] hex.c: reduce memory footprint of sha1_to_hex static buffers

On Fri, Feb 13, 2015 at 1:56 PM, Stefan Beller <sbeller@google.com> wrote:
>
> As I could not find any documentation on the
> magical 50 in the early days, I cc'd Linus
> in case there is something I did not think of yet.
Nothing magical, it's just "rounded up from 40 + NUL character".
                        Linus
Junio C Hamano· Feb 13, 2015, 23:45 UTC · re: Stefan Beller · lore

Re: [PATCH] hex.c: reduce memory footprint of sha1_to_hex static buffers

Stefan Beller <sbeller@google.com> writes:
Show 17 quoted lines
> On Fri, Feb 13, 2015 at 1:41 PM, Junio C Hamano <gitster@pobox.com> wrote:
>> Stefan Beller <sbeller@google.com> writes:
>>
>>> 41 bytes is the exact number of bytes needed for having the returned
>>> hex string represented. 50 seems to be an arbitrary number, such
>>> that there are no benefits from alignment to certain address boundaries.
>>
>> Yes, with s/seems to be/is/;
>>
>> This comes from e83c5163 (Initial revision of "git", the information
>> manager from hell, 2005-04-07), and when dcb3450f (sha1_to_hex()
>> usage cleanup, 2006-05-03) introduced the "4 recycled buffers" on
>> top, the underlying array was left at 50 bytes long.
>>
>> You can now have "I fixed Linus's bug" badge ;-)
>
> I don't think it's a bug, it's just wasting memory?

Yes and no ;-) As I already said above, 50 "is" just an arbitrary number that is round and enough to hold 40 bytes with trailing NUL, and the waste does not lead to behaviour that is different from what was intended, of course, so it would not crash.

However, the wastage was bothersome enough to make you send a patch, so you can call it a "bug". It was wasting readers time ;-)

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