there is a speed regression from gcc-4.4 with the following code:
void bench_3(float * out, float * in1, float in2, float slope, unsigned int n)
{
__m128 arg2 = _mm_set_ps(in2, in2+slope, in2+slope+slope,
in2+slope+slope+slope);
const __m128 vslope = _mm_set_ps1(slope+slope+slope+slope);
std::size_t loops = n / 4;
do {
__m128 arg1 = _mm_load_ps(in1);
__m128 result = _mm_add_ps(arg1, arg2);
arg2 = _mm_add_ps(arg2, vslope);
_mm_store_ps(out, result);
in1+=4;
out+=4;
} while (--loops);
}
gcc-4.3 generates the code:
0000000000400f00 <bench_3(float*, float*, float, float, unsigned int)>:
400f00: 0f 28 e1 movaps %xmm1,%xmm4
400f03: c1 ea 02 shr $0x2,%edx
400f06: f3 0f 58 e0 addss %xmm0,%xmm4
400f0a: 89 d2 mov %edx,%edx
400f0c: 0f 28 dc movaps %xmm4,%xmm3
400f0f: 31 c0 xor %eax,%eax
400f11: f3 0f 58 d9 addss %xmm1,%xmm3
400f15: 0f 14 e0 unpcklps %xmm0,%xmm4
400f18: 0f 28 d3 movaps %xmm3,%xmm2
400f1b: f3 0f 58 d1 addss %xmm1,%xmm2
400f1f: f3 0f 59 0d 79 17 00 mulss 0x1779(%rip),%xmm1 #
4026a0 <_IO_stdin_used+0xa0>
400f26: 00
400f27: 0f 14 d3 unpcklps %xmm3,%xmm2
400f2a: 0f c6 c9 00 shufps $0x0,%xmm1,%xmm1
400f2e: 0f 16 d4 movlhps %xmm4,%xmm2
400f31: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
400f38: 0f 28 04 06 movaps (%rsi,%rax,1),%xmm0
400f3c: 0f 58 c2 addps %xmm2,%xmm0
400f3f: 0f 58 d1 addps %xmm1,%xmm2
400f42: 0f 29 04 07 movaps %xmm0,(%rdi,%rax,1)
400f46: 48 83 c0 10 add $0x10,%rax
400f4a: 48 ff ca dec %rdx
400f4d: 75 e9 jne 400f38 <bench_3(float*, float*,
float, float, unsigned int)+0x38>
400f4f: f3 c3 repz retq
400f51: 66 66 66 66 66 66 2e nopw %cs:0x0(%rax,%rax,1)
400f58: 0f 1f 84 00 00 00 00
400f5f: 00
while gcc-4.4 generates:
0000000000400ea0 <bench_3(float*, float*, float, float, unsigned int)>:
400ea0: 0f 28 d9 movaps %xmm1,%xmm3
400ea3: c1 ea 02 shr $0x2,%edx
400ea6: f3 0f 58 d8 addss %xmm0,%xmm3
400eaa: 89 d2 mov %edx,%edx
400eac: 0f 28 d3 movaps %xmm3,%xmm2
400eaf: 31 c0 xor %eax,%eax
400eb1: f3 0f 58 d1 addss %xmm1,%xmm2
400eb5: 0f 14 d8 unpcklps %xmm0,%xmm3
400eb8: 0f 28 e2 movaps %xmm2,%xmm4
400ebb: f3 0f 58 e1 addss %xmm1,%xmm4
400ebf: f3 0f 59 0d 39 1e 00 mulss 0x1e39(%rip),%xmm1 #
402d00 <_IO_stdin_used+0xa0>
400ec6: 00
400ec7: 0f 28 c4 movaps %xmm4,%xmm0
400eca: 0f c6 c9 00 shufps $0x0,%xmm1,%xmm1
400ece: 0f 14 c2 unpcklps %xmm2,%xmm0
400ed1: 0f 16 c3 movlhps %xmm3,%xmm0
400ed4: 0f 1f 40 00 nopl 0x0(%rax)
400ed8: 0f 28 d0 movaps %xmm0,%xmm2
400edb: 0f 58 c1 addps %xmm1,%xmm0
400ede: 0f 58 14 06 addps (%rsi,%rax,1),%xmm2
400ee2: 0f 29 14 07 movaps %xmm2,(%rdi,%rax,1)
400ee6: 48 83 c0 10 add $0x10,%rax
400eea: 48 ff ca dec %rdx
400eed: 75 e9 jne 400ed8 <bench_3(float*, float*,
float, float, unsigned int)+0x38>
400eef: f3 c3 repz retq
400ef1: 66 66 66 66 66 66 2e nopw %cs:0x0(%rax,%rax,1)
400ef8: 0f 1f 84 00 00 00 00
400eff: 00
the movaps in 400ec7 is not generated by gcc-4.3 ... the code generated by
gcc-4.4 is running about 7% slower on a core2 (x86_64).
gcc -v:
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../gcc-4.4-20081226/configure -v
--with-bugurl=file:///usr/share/doc/gcc-snapshot/README.Bugs
--enable-languages=c,c++ --prefix=/usr/local/lib/gcc-snapshot --enable-shared
--with-system-zlib --disable-nls --enable-clocale=gnu --enable-libstdcxx-debug
--enable-gtk-cairo --disable-plugin --enable-objc-gc --enable-mpfr
--disable-werror --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 4.4.0 20081226 (experimental) (GCC)
--
Summary: [4.4 Regression] speed regression with sse intrinsics
and -ffast-math
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: tree-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: tim at klingt dot org
GCC target triplet: x86_64-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38682
