https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64410
Bug ID: 64410
Summary: gcc 25% slower than clang 3.5 for adding complex
numbers
Product: gcc
Version: 4.9.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: conradsand.arma at gmail dot com
Created attachment 34336
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34336&action=edit
cxaddspeed.cpp
gcc 4.9.2 has worse performance than clang 3.5 when dealing with complex
numbers.
Attached is a simple program which adds two vectors with complex numbers.
Compiled with -O3 on x86-64 (i7), Fedora 21, gcc 4.9.2 and clang 3.5.0.
$ time ./cxaddspeed_gcc 5000 1000000
5.364u 0.002s 0:05.36 100.0%
$ time ./cxaddspeed_clang 5000 1000000
4.417u 0.001s 0:04.41 100.0%
ie. gcc is about 25% slower.
inner loop produced by gcc:
.L52:
movsd (%r15,%rax), %xmm1
movsd 8(%r15,%rax), %xmm0
addsd 0(%rbp,%rax), %xmm1
addsd 8(%rbp,%rax), %xmm0
movsd %xmm1, (%rbx,%rax)
movsd %xmm0, 8(%rbx,%rax)
addq $16, %rax
cmpq %rsi, %rax
jne .L52
inner loop produced by clang:
.LBB0_145:
movupd -16(%rbx), %xmm0
movupd -16(%rax), %xmm1
addpd %xmm0, %xmm1
movupd %xmm1, -16(%rdi)
movupd (%rbx), %xmm0
movupd (%rax), %xmm1
addpd %xmm0, %xmm1
movupd %xmm1, (%rdi)
addq $2, %rbp
addq $32, %rbx
addq $32, %rax
addq $32, %rdi
addl $-2, %ecx
jne .LBB0_145
