John Meacham wrote:
On Wed, Jan 18, 2006 at 08:54:43PM +0300, Bulat Ziganshin wrote:
sorry, with the "gcc -O3 -ffast-math -fstrict-aliasing -funroll-loops"
the C version is 50 times faster than best Haskell one... it's the
loop from C version:
I believe something similar to what I noted here is the culprit:
http://www.haskell.org//pipermail/glasgow-haskell-users/2005-October/009174.html
it is fixable, but not without modifying ghc.
Ah, I see what you mean by indirect jumps. Those indirect jumps go away
if you compile with -optc-O2 or -fasm, they're droppings left by
inadequacies in gcc's standard -O optimisation.
Actually, -fasm does better by one instruction than gcc on this example:
.globl Test_zdwfac_info
Test_zdwfac_info:
movq (%rbp),%rax
cmpq $1,%rax
jne .LcmO
movq 8(%rbp),%r13
addq $16,%rbp
jmp *(%rbp)
.LcmO:
leaq -1(%rax),%rcx
imulq 8(%rbp),%rax
movq %rax,8(%rbp)
movq %rcx,(%rbp)
jmp Test_zdwfac_info
vs. gcc -O2:
Test_zdwfac_info:
.text
.align 8
movq (%rbp), %rdx
cmpq $1, %rdx
je .L6
.L3:
movq 8(%rbp), %rax
imulq %rdx, %rax
decq %rdx
movq %rdx, (%rbp)
movq %rax, 8(%rbp)
jmp Test_zdwfac_info
.p2align 4,,7
.L6:
movq 8(%rbp), %r13
addq $16, %rbp
jmp *(%rbp)
We should probably reverse the sense of that branch, like gcc does. The
memory accesses are still there, of course. Hopefully someday I'll get
around to trying to use more registers on x86_64 again.
Cheers,
Simon
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