http://llvm.org/bugs/show_bug.cgi?id=16236
Bug ID: 16236
Summary: wrong code generated for union with optimization
Product: clang
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: LLVM Codegen
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
The following code produces 0 at -O0 and 1 at -O1 and above with clang trunk on
x86_64-linux. I believe the behavior is well-defined here, since the
assignment to the union member is through the union type. It is a change in
behavior from 2.8.
$ clang-trunk -v
clang version 3.4 (trunk 183303)
Target: x86_64-pc-linux-gnu
Thread model: posix
$ clang-trunk -O0 wrong.c
$ ./a.out
0
$ clang-2.8 -O1 wrong.c
$ ./a.out
0
$ clang-trunk -O1 wrong.c
$ ./a.out
1
$
----------------------------------
int printf(const char *, ...);
union
{
short f0;
int f1;
} u, *up = &u;
int a;
int main ()
{
for (; a <= 0; a++)
if ((u.f0 = 1))
up->f1 = 0;
printf ("%d\n", u.f1);
return 0;
}
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