http://llvm.org/bugs/show_bug.cgi?id=9971
Summary: wrong code in union access
Product: clang
Version: trunk
Platform: PC
OS/Version: Linux
Status: NEW
Severity: normal
Priority: P
Component: -New Bugs
AssignedTo: [email protected]
ReportedBy: [email protected]
CC: [email protected]
The test case below (plain C) is miscompiled by clang when using -O2.
clang version 3.0 (trunk 131747)
Target: x86_64-unknown-linux-gnu
typedef union {
unsigned long long dword[2];
unsigned short half[8];
} vector_t;
void vrlh(vector_t *va, vector_t *vb, vector_t *vd)
{
vector_t t;
for (int i = 0; i < 8; i++) {
int sh = vb->half[7 - i] & 0xf;
t.half[7 - i] = va->half[7 - i] << sh | va->half[7 - i] >> (16 - sh);
}
vd->dword[1] = t.dword[1]; // These lines will be
vd->dword[0] = t.dword[0]; // ignored by the compiler.
}
I presume that clang should work like gcc with respects to unions (setting one
member and reading another defined in the punning way).
As you can see, the two last lines seem to have disappeared:
0000000000000000 <vrlh>:
0: ba 07 00 00 00 mov $0x7,%edx
5: 66 66 2e 0f 1f 84 00 nopw %cs:0x0(%rax,%rax,1)
c: 00 00 00 00
10: 0f b7 04 57 movzwl (%rdi,%rdx,2),%eax
14: 44 0f b7 04 56 movzwl (%rsi,%rdx,2),%r8d
19: 41 83 e0 0f and $0xf,%r8d
1d: 44 88 c1 mov %r8b,%cl
20: 41 89 c1 mov %eax,%r9d
23: 41 d3 e1 shl %cl,%r9d
26: b9 10 00 00 00 mov $0x10,%ecx
2b: 44 29 c1 sub %r8d,%ecx
2e: d3 e8 shr %cl,%eax
30: 44 09 c8 or %r9d,%eax
33: 66 89 44 54 f0 mov %ax,-0x10(%rsp,%rdx,2)
38: 48 85 d2 test %rdx,%rdx
3b: 48 8d 52 ff lea -0x1(%rdx),%rdx
3f: 75 cf jne 10 <vrlh+0x10>
41: c3 retq
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