http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49768
Summary: C99 style union initializations does not work as
expected with optimizations
Product: gcc
Version: 4.6.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: vrahk...@gmail.com
Created attachment 24779
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24779
Test code that fails with 4.6.1 and works with 4.5.3
It seems that union cannot be initialized with C99 style with 4.6.1. Attached
example code works with GCC 4.5.3 but fails on assert when compiled with GCC
4.6.1. The GCC 4.6.1 seems to generate only the assert failure i.e. example
compiled with -O3:
0000000000400500 <main>:
400500: 48 83 ec 08 sub $0x8,%rsp
400504: b9 2b 06 40 00 mov $0x40062b,%ecx
400509: ba 18 00 00 00 mov $0x18,%edx
40050e: be 0c 06 40 00 mov $0x40060c,%esi
400513: bf 13 06 40 00 mov $0x400613,%edi
400518: e8 cb fe ff ff callq 4003e8 <__assert_fail@plt>
40051d: 90 nop
40051e: 90 nop
40051f: 90 nop
It seems that using -fno-tree-pre, -fno-tree-ccp and -fno-tree-fre with -O3 the
code seems to work. Also setting the first bit in initialization seems to
generate working code i.e. rest of initialization works if it's set and also
setting the bits later seems to work too.
ps. I'm not totally sure that if this is a regression or meant to be that way
because the C-standard is not clear on this matter but following code works
with GCC 4.5.3 and the assert fails with GCC 4.6.1.
