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* [Bug web/61054] New: Clearly note effect of memcpy optimization improvements in 4.9 changes @ 2014-05-04 12:57 harald at gigawatt dot nl 2014-05-04 13:52 ` [Bug web/61054] " redi at gcc dot gnu.org ` (2 more replies) 0 siblings, 3 replies; 4+ messages in thread From: harald at gigawatt dot nl @ 2014-05-04 12:57 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61054 Bug ID: 61054 Summary: Clearly note effect of memcpy optimization improvements in 4.9 changes Product: gcc Version: 4.9.0 Status: UNCONFIRMED Severity: enhancement Priority: P3 Component: web Assignee: unassigned at gcc dot gnu.org Reporter: harald at gigawatt dot nl Consider this reduced program: void *volatile null; volatile long zerolen; int main() { void *dst = null, *src = null; __builtin_memcpy (dst, src, zerolen); if (dst) __builtin_abort (); } This program is invalid, but until 4.8 it would compile and run without an issue. As of 4.9, it aborts. I'm not asking for GCC to change (the programs should be fixed instead), but it is not clear from the GCC 4.9 Changes page that such code no longer works. This affects real code using custom allocators and realloc-like functions, where some of those realloc-like functions don't bother to check for null pointers and assume zero-length memcpy is a no-op. When the old memory is then freed, and their free-like function performs a null pointer check, that check is optimised away if the function gets inlined. Could the GCC 4.9 Changes page get a mention of this? I do see "Better inlining of memcpy and memset that is aware of value ranges and produces shorter alignment prologues." in the IA-32/x86-64 section, but I cannot tell if it is related to that, or if it is actually arch-independent. ^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug web/61054] Clearly note effect of memcpy optimization improvements in 4.9 changes 2014-05-04 12:57 [Bug web/61054] New: Clearly note effect of memcpy optimization improvements in 4.9 changes harald at gigawatt dot nl @ 2014-05-04 13:52 ` redi at gcc dot gnu.org 2014-05-04 14:06 ` harald at gigawatt dot nl 2014-05-04 15:54 ` redi at gcc dot gnu.org 2 siblings, 0 replies; 4+ messages in thread From: redi at gcc dot gnu.org @ 2014-05-04 13:52 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61054 --- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> --- We already documented it, see "null pointer checks optimized away more aggressively" at http://gcc.gnu.org/gcc-4.9/porting_to.html ^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug web/61054] Clearly note effect of memcpy optimization improvements in 4.9 changes 2014-05-04 12:57 [Bug web/61054] New: Clearly note effect of memcpy optimization improvements in 4.9 changes harald at gigawatt dot nl 2014-05-04 13:52 ` [Bug web/61054] " redi at gcc dot gnu.org @ 2014-05-04 14:06 ` harald at gigawatt dot nl 2014-05-04 15:54 ` redi at gcc dot gnu.org 2 siblings, 0 replies; 4+ messages in thread From: harald at gigawatt dot nl @ 2014-05-04 14:06 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61054 Harald van Dijk <harald at gigawatt dot nl> changed: What |Removed |Added ---------------------------------------------------------------------------- Status|UNCONFIRMED |RESOLVED Resolution|--- |INVALID --- Comment #2 from Harald van Dijk <harald at gigawatt dot nl> --- You're right, thanks for the pointer, somehow I missed that page. ^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug web/61054] Clearly note effect of memcpy optimization improvements in 4.9 changes 2014-05-04 12:57 [Bug web/61054] New: Clearly note effect of memcpy optimization improvements in 4.9 changes harald at gigawatt dot nl 2014-05-04 13:52 ` [Bug web/61054] " redi at gcc dot gnu.org 2014-05-04 14:06 ` harald at gigawatt dot nl @ 2014-05-04 15:54 ` redi at gcc dot gnu.org 2 siblings, 0 replies; 4+ messages in thread From: redi at gcc dot gnu.org @ 2014-05-04 15:54 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61054 --- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> --- N.B. as well as custom allocators it affects the built-in delete[], which performs an implicit null check that can be optimized away in buggy code. We documented it on that page because of case where that happened. ^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2014-05-04 15:54 UTC | newest] Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2014-05-04 12:57 [Bug web/61054] New: Clearly note effect of memcpy optimization improvements in 4.9 changes harald at gigawatt dot nl 2014-05-04 13:52 ` [Bug web/61054] " redi at gcc dot gnu.org 2014-05-04 14:06 ` harald at gigawatt dot nl 2014-05-04 15:54 ` redi at gcc dot gnu.org
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