On Thu, Jul 24, 2014 at 6:25 PM, Michel Dänzer <mic...@daenzer.net> wrote:
>
> Attached is fair.s from Debian gcc 4.8.3-5. Does that look better? I'm
> going to try reproducing the problem with a kernel built by that now.
This looks better. For roughly that same code sequence it does
(ignoring the debug line and cfi information):
subq $184, %rsp #,
movq (%r12), %rax # sd_22(D)->parent, sd_parent
movl %edi, -156(%rbp) # this_cpu, %sfp
movl %ecx, -160(%rbp) # idle, %sfp
movq %r8, -184(%rbp) # continue_balancing, %sfp
movq %rax, -176(%rbp) # sd_parent, %sfp
movq $load_balance_mask, %rax #, tcp_ptr__
#APP
add %gs:this_cpu_off, %rax # this_cpu_off, tcp_ptr__
#NO_APP
so it updates the stack pointer before any spills, and it also doesn't
spill that constant value.
I still have no idea why it does the 4-byte rep stosl/movsl thing, but
that's a whole separate guessing game and might have something to do
with the fact that you do CONFIG_CC_OPTIMIZE_FOR_SIZE and the 4-byte
form is one byte smaller.
I'm a big believer in not blowing up the I$ footprint, and I have to
admit to pushing that myself a few years ago, but gcc does some rather
bad things with '-Os', so it's not actually suggested for the kernel
any more. I wish there was some middle ground model that cared about
size, but not to exclusion of everything else. The string instructions
are not good for performance when it's a compile-time known small
size.
Linus
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