On 01 November 2005 16:32, Florian Weimer wrote:
* Simon Marlow:
gcc started generating this rubbish around version 3.4, if I recall correctly. I've tried disabling various optimisations, but can't seem to convince gcc not to generate the extra jump. You don't get this from the native code generator, BTW.
But the comparison is present in the C code. What do you want GCC to do?
I didn't mean to sound overly critical of gcc. But here's what I was complaining about - the code generated by gcc (3.4.x) is as follows: Main_zdwfac_info: .text .align 8 .text movq (%rbp), %rdx cmpq $1, %rdx jne .L2 movq 8(%rbp), %r13 leaq 16(%rbp), %rbp movq (%rbp), %rax .L4: jmp *%rax .L2: movq %rdx, %rax imulq 8(%rbp), %rax movq %rax, 8(%rbp) leaq -1(%rdx), %rax movq %rax, (%rbp) movl $Main_zdwfac_info, %eax jmp .L4 there's an obvious simplification - the last two instructions should be replaced by just jmp Main_zdwfac_info eliminating one branch and a mov. This occurs all over the place in our code. Whenever a function has more than one computed goto, gcc insists on commoning up the jmp instructions even when it's a really bad idea, like above. Actually if I add -O2, then I get better code, so perhaps this isn't a real problem. Although gcc still generates this: Fac_zdwfac_info: .text .align 8 movq (%rbp), %rdx testq %rdx, %rdx jne .L3 movq 8(%rbp), %r13 addq $16, %rbp movq (%rbp), %rax jmp *%rax .p2align 4,,7 .L3: movq 8(%rbp), %rax imulq %rdx, %rax decq %rdx movq %rdx, (%rbp) movq %rax, 8(%rbp) movl $Fac_zdwfac_info, %eax jmp *%rax and fails to combine the movs with the jmp instruction (we do this simplification ourselves when post-processing the assembly code). I'll compile up gcc 4 and see what happens with that. Cheers, Simon