git/list[1] front-page[2] threads[3] people[4] search[5] about
 

Re: [PATCH/RFC] reflog: silence -O3 -Wuninitialized warning

From
Jonathan Nieder <jrnieder@gmail.com>
Date
Mar 16, 2011, 06:28 UTC
Message-ID
<20110316062831.GA5252@elie>
In-Reply-To
<7vfwqnabbi.fsf@alter.siamese.dyndns.org>
Junio C Hamano wrote:
Show 7 quoted lines
> We could obviously do (2) or (3), but the thing is, I don't think we can
> have much confidence on -Wuninitialized warnings from this compiler once
> we go down that route.  Is it _guaranteed_ that the compiler bug _always_
> err on the false-positive side?
>
> IOW, I'd very much prefer (1) for this particular case and if somebody
> really cares (2).
*nod*.  The tricky heuristics here are that (A) a call to the function
 extern void foo(int *var);

is assumed to initialize *var (to support idioms like strbuf_init), while (B) no assumption is made about the return value of a call to the function

 extern int bar(void);

The assumption (B) is the scourge of -Wuninitialized users. It is always going produce a lot of false positives --- the compiler simply doesn't have enough information to completely analyze the flow through a function. (Even if it did have enough information, completely solving the problem is Turing-complete.)

Problems from (A) are more rare. -Wuninitialized will produce _some_ false negatives, but problems it misses would typically be due to failure to check for errors and return early, which is not what -Wuninitialized is about.

I don't mind the warning, especially since it only appears with -O3. What makes this interesting to me is that it is a reminder that the compiler has very little information about the flow of control. A simple

 #define error(...) (report_error(__VA_ARGS__), -1)
could open the door to some nice micro-optimizations.
Previous: Junio C HamanoNext: Johannes Sixt
Message 6 of 34 in “reflog: silence -O3 -Wuninitialized warning”
  1. reflog: silence -O3 -Wuninitialized warningJonathan Nieder, Mar 16, 2011
  2. declare 1-bit bitfields to be unsignedJonathan Nieder, Mar 16, 2011
  3. Junio C HamanoMar 16, 2011
  4. Nguyen Thai Ngoc DuyMar 16, 2011
  5. Junio C HamanoMar 16, 2011
  6. Jonathan NiederMar 16, 2011
  7. Johannes SixtMar 16, 2011
  8. Jonathan NiederMar 16, 2011
  9. Johannes SixtMar 16, 2011
  10. Jonathan NiederMar 16, 2011
  11. 0/6 silence -Wuninitialized warnings that previously used the a = a trickJonathan Nieder, Mar 16, 2011
  12. 1/6 match-trees: kill off remaining -Wuninitialized warningJonathan Nieder, Mar 16, 2011
  13. 2/6 run-command: initialize failed_errno to 0Jonathan Nieder, Mar 16, 2011
  14. 3/6 diff --submodule: suppress -Wuninitialized warning by initializing to NULLJonathan Nieder, Mar 16, 2011
  15. 4/6 rsync transport: clarify insert_packed_refsJonathan Nieder, Mar 16, 2011
  16. 5/6 wt-status: protect against invalid change_typeJonathan Nieder, Mar 16, 2011
  17. 6/6 fast-import: suppress -Wuninitialized warning by initializing to NULLJonathan Nieder, Mar 16, 2011
  18. 0/8 more warnings and cleanupsJonathan Nieder, Mar 16, 2011
  19. 1/8 enums: omit trailing comma for portabilityJonathan Nieder, Mar 16, 2011
  20. 2/8 compat: make gcc bswap an inline functionJonathan Nieder, Mar 16, 2011
  21. Johannes SixtMar 16, 2011
  22. Jonathan NiederMar 16, 2011
  23. Junio C HamanoMar 16, 2011
  24. 3/8 svn-fe: do not use "return" for tail call returning voidJonathan Nieder, Mar 16, 2011
  25. 4/8 vcs-svn: remove spurious semicolonsJonathan Nieder, Mar 16, 2011
  26. Junio C HamanoMar 16, 2011
  27. Jonathan NiederMar 16, 2011
  28. 5/8 standardize brace placement in struct definitionsJonathan Nieder, Mar 16, 2011
  29. Junio C HamanoMar 18, 2011
  30. 6/8 branch: split off function that writes tracking info and commit subjectJonathan Nieder, Mar 16, 2011
  31. 7/8 cherry: split off function to print output linesJonathan Nieder, Mar 16, 2011
  32. 8/8 diff --submodule: split into bite-sized piecesJonathan Nieder, Mar 16, 2011
  33. Jens LehmannMar 16, 2011
  34. Jonathan NiederMar 16, 2011

Read the whole thread, see it on lore, or plain text.

$ cat FOOTERMessages come from the public archive at lore.kernel.org/git, fetched every hour. The front page is chosen and written each morning by an AI editor and can be wrong; the threads themselves are the record. About and API. For agents: an MCP server at https://gitlist.dev/mcp, and any thread, story or person page as Markdown by adding .md to its URL (or sending Accept: text/markdown). Details in /llms.txt.