https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63599
Bug ID: 63599 Summary: "wrong" branch optimization with Ofast in a loop Product: gcc Version: 5.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: vincenzo.innocente at cern dot ch given this code #include <x86intrin.h> typedef float __attribute__( ( vector_size( 16 ) ) ) float32x4_t; inline float32x4_t atan(float32x4_t t) { constexpr float PIO4F = 0.7853981633974483096f; float32x4_t high = t > 0.4142135623730950f; auto z = t; float32x4_t ret={0.f,0.f,0.f,0.f}; // if all low no need to blend if ( _mm_movemask_ps(high) != 0) { z = ( t > 0.4142135623730950f ) ? (t-1.0f)/(t+1.0f) : t; ret = ( t > 0.4142135623730950f ) ? ret+PIO4F : ret; } /* polynomial removed */ return ret += z; } float32x4_t doAtan(float32x4_t z) { return atan(z);} float32x4_t va[1024]; float32x4_t vb[1024]; void computeV() { for (int i=0;i!=1024;++i) vb[i]=atan(va[i]); } compiled with -Ofast c++ -S -std=c++1y -Ofast bugmvmk.cc -march=nehalem; cat bugmvmk.s produces the following code where the "movmskps %xmm8, %edx" does not protect the code in the if block... __Z8computeVv: LFB2512: movaps LC0(%rip), %xmm4 xorl %eax, %eax movaps LC1(%rip), %xmm7 leaq _va(%rip), %rcx movaps LC2(%rip), %xmm6 movaps LC3(%rip), %xmm5 .align 4,0x90 L10: movaps (%rcx,%rax), %xmm2 movaps %xmm4, %xmm8 movaps %xmm2, %xmm3 cmpltps %xmm2, %xmm8 movaps %xmm2, %xmm1 addps %xmm6, %xmm3 addps %xmm7, %xmm1 movmskps %xmm8, %edx andps %xmm5, %xmm8 rcpps %xmm3, %xmm0 mulps %xmm0, %xmm3 mulps %xmm0, %xmm3 addps %xmm0, %xmm0 subps %xmm3, %xmm0 mulps %xmm0, %xmm1 movaps %xmm2, %xmm0 cmpleps %xmm4, %xmm0 blendvps %xmm0, %xmm2, %xmm1 pxor %xmm0, %xmm0 testl %edx, %edx je L7 movaps %xmm8, %xmm0 L7: testl %edx, %edx je L9 movaps %xmm1, %xmm2 L9: addps %xmm0, %xmm2 leaq _vb(%rip), %rdx movaps %xmm2, (%rdx,%rax) addq $16, %rax cmpq $16384, %rax jne L10 ret while with O2 is ok __Z8computeVv: LFB2512: movaps LC0(%rip), %xmm4 xorl %eax, %eax movaps LC1(%rip), %xmm7 leaq _va(%rip), %rsi movaps LC2(%rip), %xmm6 leaq _vb(%rip), %rcx movaps LC3(%rip), %xmm5 .align 4,0x90 L7: movaps (%rsi,%rax), %xmm1 movaps %xmm4, %xmm0 pxor %xmm2, %xmm2 cmpltps %xmm1, %xmm0 movmskps %xmm0, %edx testl %edx, %edx je L6 movaps %xmm1, %xmm3 movaps %xmm1, %xmm2 addps %xmm6, %xmm2 addps %xmm7, %xmm3 divps %xmm2, %xmm3 movaps %xmm0, %xmm2 andps %xmm5, %xmm2 blendvps %xmm0, %xmm3, %xmm1 L6: addps %xmm2, %xmm1 movaps %xmm1, (%rcx,%rax) addq $16, %rax cmpq $16384, %rax jne L7 ret note that the function not in the loop (doAtan) is ok with both O2 and Ofast