From: Nicholas Piggin
> Sent: 20 April 2020 02:10
...
> >> Yes, but does it really matter to optimize this specific usage case
> >> for size? glibc, for instance, tries to leverage the syscall mechanism
> >> by adding some complex pre-processor asm directives. It optimizes
> >> the syscall code size in most cases. For instance, kill in static case
> >> generates on x86_64:
> >>
> >> 0000000000000000 <__kill>:
> >> 0: b8 3e 00 00 00 mov $0x3e,%eax
> >> 5: 0f 05 syscall
> >> 7: 48 3d 01 f0 ff ff cmp $0xfffffffffffff001,%rax
> >> d: 0f 83 00 00 00 00 jae 13 <__kill+0x13>
Hmmm... that cmp + jae is unnecessary here.
It is also a 32bit offset jump.
I also suspect it gets predicted very badly.
> >> 13: c3 retq
> >>
> >> While on musl:
> >>
> >> 0000000000000000 <kill>:
> >> 0: 48 83 ec 08 sub $0x8,%rsp
> >> 4: 48 63 ff movslq %edi,%rdi
> >> 7: 48 63 f6 movslq %esi,%rsi
> >> a: b8 3e 00 00 00 mov $0x3e,%eax
> >> f: 0f 05 syscall
> >> 11: 48 89 c7 mov %rax,%rdi
> >> 14: e8 00 00 00 00 callq 19 <kill+0x19>
> >> 19: 5a pop %rdx
> >> 1a: c3 retq
> >
> > Wow that's some extraordinarily bad codegen going on by gcc... The
> > sign-extension is semantically needed and I don't see a good way
> > around it (glibc's asm is kinda a hack taking advantage of kernel not
> > looking at high bits, I think), but the gratuitous stack adjustment
> > and refusal to generate a tail call isn't. I'll see if we can track
> > down what's going on and get it fixed.
A suitable cast might get rid of the sign extension.
Possibly just (unsigned int).
David
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