2013-10-17 13:35-0400, Steven Rostedt:
> On Thu, 17 Oct 2013 12:10:28 +0200
> Radim Krčmář <rkrcmar@xxxxxxxxxx> wrote:
>
> > We implemented the optimized branch selection in higher levels of api.
> > That made static_keys very unintuitive, so this patch introduces another
> > element to jump_table, carrying one bit that tells the underlying code
> > which branch to optimize.
> >
> > It is now possible to select optimized branch for every jump_entry.
> >
> > Current side effect is 1/3 increase increase in space, we could:
> > * use bitmasks and selectors on 2+ aligned code/struct.
> > - aligning jump target is easy, but because it is not done by default
> > and few bytes in .text are much worse that few kilos in .data,
> > I chose not to
> > - data is probably aligned by default on all current architectures,
> > but programmer can force misalignment of static_key
> > * optimize each architecture independently
> > - I can't test everything and this patch shouldn't break anything, so
> > others can contribute in the future
> > * choose something worse, like packing or splitting
> > * ignore
> >
> > proof: example & x86_64 disassembly: (F = ffffffff)
> >
> > struct static_key flexible_feature;
> > noinline void jump_label_experiment(void) {
> > if ( static_key_false(&flexible_feature))
> > asm ("push 0xa1");
> > else asm ("push 0xa0");
> > if (!static_key_false(&flexible_feature))
> > asm ("push 0xb0");
> > else asm ("push 0xb1");
> > if ( static_key_true(&flexible_feature))
> > asm ("push 0xc1");
> > else asm ("push 0xc0");
> > if (!static_key_true(&flexible_feature))
> > asm ("push 0xd0");
> > else asm ("push 0xd1");
> > }
> >
> > Disassembly of section .text: (push marked by "->")
> >
> > F81002000 <jump_label_experiment>:
> > F81002000: e8 7b 29 75 00 callq F81754980 <__fentry__>
> > F81002005: 55 push %rbp
> > F81002006: 48 89 e5 mov %rsp,%rbp
> > F81002009: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
> > F8100200e: -> ff 34 25 a0 00 00 00 pushq 0xa0
> > F81002015: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
> > F8100201a: -> ff 34 25 b0 00 00 00 pushq 0xb0
> > F81002021: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
> > F81002026: -> ff 34 25 c1 00 00 00 pushq 0xc1
> > F8100202d: 0f 1f 00 nopl (%rax)
> > F81002030: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
> > F81002035: -> ff 34 25 d1 00 00 00 pushq 0xd1
> > F8100203c: 5d pop %rbp
> > F8100203d: 0f 1f 00 nopl (%rax)
> > F81002040: c3 retq
>
> This looks exactly like what we want. I take it this is with your
> patch. What was the result before the patch?
Yes, this is after the patch.
The branches would (should) be the same without patch, but
static_key_true() was defined as !static_key_false(), so this piece of
code was invalid before, because half of them would be patched to use
the wrong branch.
> -- Steve
>
> > F81002041: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
> > F81002048: -> ff 34 25 d0 00 00 00 pushq 0xd0
> > F8100204f: 5d pop %rbp
> > F81002050: c3 retq
> > F81002051: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
> > F81002058: -> ff 34 25 c0 00 00 00 pushq 0xc0
> > F8100205f: 90 nop
> > F81002060: eb cb jmp F8100202d <[...]+0x2d>
> > F81002062: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
> > F81002068: -> ff 34 25 b1 00 00 00 pushq 0xb1
> > F8100206f: 90 nop
> > F81002070: eb af jmp F81002021 <[...]+0x21>
> > F81002072: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
> > F81002078: -> ff 34 25 a1 00 00 00 pushq 0xa1
> > F8100207f: 90 nop
> > F81002080: eb 93 jmp F81002015 <[...]+0x15>
> > F81002082: 66 66 66 66 66 2e 0f [...]
> > F81002089: 1f 84 00 00 00 00 00
> >
> > Contents of section .data: (relevant part of embedded __jump_table)
