Bug 448897

Summary: including <vector> generates aliasing error on double __attribute__ ((may_alias))
Product: [Fedora] Fedora Reporter: Caolan McNamara <caolanm>
Component: gccAssignee: Jakub Jelinek <jakub>
Status: CLOSED UPSTREAM QA Contact: Fedora Extras Quality Assurance <extras-qa>
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Last Closed: 2008-05-29 14:16:06 UTC Type: ---
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Description Caolan McNamara 2008-05-29 12:35:31 UTC 
Description of problem:
gcc-4.3.0-8.i386

g++ -c -O2 -Werror=strict-aliasing -Wall may_alias.cxx
no errors, but

g++ -DINCLUDE_VECTOR -c -O2 -Werror=strict-aliasing -Wall may_alias.cxx
may_alias.cxx: In function ‘bool operator>>=(const Any&, long int&)’:
may_alias.cxx:30: error: dereferencing type-punned pointer will break
strict-aliasing rules
where INCLUDE_VECTOR just includes <vector>

strangely the effect is seen on the "double" but not on the "long" equivalent.
Demo attached
Comment 1 Caolan McNamara 2008-05-29 12:35:31 UTC 
Created attachment 307043 [details]
example
Comment 2 Jakub Jelinek 2008-05-29 14:16:06 UTC 
Avoid may_alias type for now, apparently it never actually worked.
You can use unions, or asm optimization barriers, unions being preferred.
Comment 3 Caolan McNamara 2008-05-29 15:05:39 UTC 
Yeah, but I got an alias from a void* to a class which has a constructor (!) and
that can't go into a union