http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49064
Summary: [x86/x64]: broken alias analysis leads vectorizer to
emit poor code
Product: gcc
Version: 4.6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: piotr.wyder...@gmail.com
On an x86 capable of SSE2 or x64 (which has SSE2 by definition) GCC tries
to vectorize as much integer code as possible, but ends up witch code much
worse than without vectorization. The SSE2-based version unnecessarily
recomputes all the m_Data pointers, as demonstrated by the following C++
snippet. I guess the reason is unsophisticated alias analysis, but the
actual reason may in fact be different.
struct X {
__m128i* m_Data;
std::size_t m_Len;
void xor_all(const X& v1, const X& v2);
void xor_all2(const X& v1, const X& v2);
};
void X::xor_all(const X& v1, const X& v2) {
for(std::size_t i = 0; i != m_Len; ++i) {
m_Data[i] = v1.m_Data[i] ^ v2.m_Data[i];
}
}
void X::xor_all2(const X& v1, const X& v2) {
__m128i* p0 = m_Data;
__m128i* p1 = v1.m_Data;
__m128i* p2 = v2.m_Data;
for(std::size_t i = 0; i != m_Len; ++i) {
p0[i] = p1[i] ^ p2[i];
}
}
As can be seen, xor_all2 produces nice code and xor_all doesn't:
0000000000447c70 <_ZN1X7xor_allERKS_S1_>:
447c70: 48 83 7f 08 00 cmpq $0x0,0x8(%rdi)
447c75: 74 35 je 447cac
<_ZN1X7xor_allERKS_S1_+0x3c>
447c77: 31 c0 xor %eax,%eax
447c79: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
447c80: 4c 8b 12 mov (%rdx),%r10
447c83: 48 89 c1 mov %rax,%rcx
447c86: 48 83 c0 01 add $0x1,%rax
447c8a: 4c 8b 0e mov (%rsi),%r9
447c8d: 48 c1 e1 04 shl $0x4,%rcx
447c91: 4c 8b 07 mov (%rdi),%r8
447c94: 66 41 0f 6f 04 0a movdqa (%r10,%rcx,1),%xmm0
447c9a: 66 41 0f ef 04 09 pxor (%r9,%rcx,1),%xmm0
447ca0: 66 41 0f 7f 04 08 movdqa %xmm0,(%r8,%rcx,1)
447ca6: 48 39 47 08 cmp %rax,0x8(%rdi)
447caa: 75 d4 jne 447c80
<_ZN1X7xor_allERKS_S1_+0x10>
447cac: f3 c3 repz retq
0000000000447cb0 <_ZN1X8xor_all2ERKS_S1_>:
447cb0: 48 83 7f 08 00 cmpq $0x0,0x8(%rdi)
447cb5: 48 8b 0f mov (%rdi),%rcx
447cb8: 48 8b 36 mov (%rsi),%rsi
447cbb: 4c 8b 02 mov (%rdx),%r8
447cbe: 74 26 je 447ce6
<_ZN1X8xor_all2ERKS_S1_+0x36>
447cc0: 31 c0 xor %eax,%eax
447cc2: 31 d2 xor %edx,%edx
447cc4: 0f 1f 40 00 nopl 0x0(%rax)
447cc8: 66 41 0f 6f 04 00 movdqa (%r8,%rax,1),%xmm0
447cce: 48 83 c2 01 add $0x1,%rdx
447cd2: 66 0f ef 04 06 pxor (%rsi,%rax,1),%xmm0
447cd7: 66 0f 7f 04 01 movdqa %xmm0,(%rcx,%rax,1)
447cdc: 48 83 c0 10 add $0x10,%rax
447ce0: 48 39 57 08 cmp %rdx,0x8(%rdi)
447ce4: 75 e2 jne 447cc8
<_ZN1X8xor_all2ERKS_S1_+0x18>
447ce6: f3 c3 repz retq
