[PATCH 5/7] jump_label: relax branch hinting restrictions

[Radim Krčmář]

2013-10-17 13:35-0400, Steven Rostedt:
> On Thu, 17 Oct 2013 12:10:28 +0200
> Radim Krčmář <rkrcmar at redhat.com> 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)