Re: [RFC PATCH] perf report: add sort by file lines

[Peter Zijlstra]

On Thu, 2011-03-31 at 22:34 +0800, Lin Ming wrote:
> On Thu, 2011-03-31 at 22:01 +0800, Peter Zijlstra wrote:
> > On Thu, 2011-03-31 at 16:45 +0800, Lin Ming wrote:
> > > I am considering if it is possible to do "instruction unwind" to get a
> > > map from (temporarily used) register to a specific member of a data
> > > structure pointed by a pointer.
> > > 
> > > 4004a0:         movq    -8(%rbp), %rax    /* load foo arg from stack
> > > */
> > > 4004a4:         movq    24(%rax), %rax    /* load foo->bar */
> > > 4004a8:         movq    -16(%rbp), %rdx   /* load tmp arg from stack
> > > */
> > > 4004ac:         movl    32(%rdx), %edx    /* load tmp->blah */
> > > 4004af:         movl    %edx, 20(%rax)    /* store bar->fubar */ 
> > > 
> > > foo: -8(%rbp)
> > > tmp: -16(%rbp)
> > > 
> > > Assume we are now at ip 4004af, from the instruction decoder, we know
> > > it's a store operation, and we want to find out what %rax is.
> > > 
> > > 1. unwind to 4004ac
> > >    Ignore this, because it does not touch %rax
> > > 
> > > 2. unwind to 4004a8
> > >    Ignore this, because it does not touch %rax
> > > 
> > > 3. unwind to 4004a4
> > >    20(%rax) => 20(24(%rax)), continue to unwind because we still
> > >    have no idea what %rax is
> > > 
> > > 4. unwind to 4004a0
> > >    20(24(%rax)) => 20(24(-8(%rbp))), stop unwind, because we now know
> > >    -8(%rbp) is foo.
> > > 
> > > So the original 20(%rax) is replace as 20(24(-8(%rbp))), and it means
> > > foo->bar->fubar
> > > 
> > > Does this make sense? 
> > 
> > Yes and no, the problem is that you cannot unwind an x86 instruction
> > stream. Therefore its easier to start at the beginning of a function
> > where DWARF should be able to tell you everything you need and then do a
> > single fwd scan to propagate the information until you reach the point
> > of interest.
> 
> I'm afraid that fwd scan may not work, because of branch instruction.
> 
> void foo(struct foo *foo, struct tmp *tmp, int flag)
> { 
>         if (flag)
>                 foo->bar->fubar = tmp->blah;   
>         else    
>                 tmp->blah = foo->bar->fubar;
> }
> 
> ===>
> 
> void foo(struct foo *foo, struct tmp *tmp, int flag)
> {
>   400494:       55                      push   %rbp
>   400495:       48 89 e5                mov    %rsp,%rbp
>   400498:       48 89 7d f8             mov    %rdi,-0x8(%rbp)
>   40049c:       48 89 75 f0             mov    %rsi,-0x10(%rbp)
>   4004a0:       89 55 ec                mov    %edx,-0x14(%rbp)
>         if (flag)
>   4004a3:       83 7d ec 00             cmpl   $0x0,-0x14(%rbp)
>   4004a7:       74 14                   je     4004bd <foo+0x29>
>                 foo->bar->fubar = tmp->blah;
>   4004a9:       48 8b 45 f8             mov    -0x8(%rbp),%rax
>   4004ad:       48 8b 40 18             mov    0x18(%rax),%rax
>   4004b1:       48 8b 55 f0             mov    -0x10(%rbp),%rdx
>   4004b5:       8b 52 20                mov    0x20(%rdx),%edx
>   4004b8:       89 50 14                mov    %edx,0x14(%rax)
>   4004bb:       eb 12                   jmp    4004cf <foo+0x3b>
>         else
>                 tmp->blah = foo->bar->fubar;
>   4004bd:       48 8b 45 f8             mov    -0x8(%rbp),%rax
>   4004c1:       48 8b 40 18             mov    0x18(%rax),%rax
>   4004c5:       8b 50 14                mov    0x14(%rax),%edx
>   4004c8:       48 8b 45 f0             mov    -0x10(%rbp),%rax
>   4004cc:       89 50 20                mov    %edx,0x20(%rax)
> }
>   4004cf:       c9                      leaveq
>   4004d0:       c3                      retq
> 
> Assume we are at ip 4004c5, the fwd scan from the beginning of
> function(400494) to 4004c5 will not get what we want about %rax.

Conversely backwards scans can get confused if there's more places to
come from (intercal ftw!).

That said, your example above should not get confused about %rax if it
knows about the jumps, simply clone your context on any jump instruction
and follow both branches. That would then give you:

400494 -> 4004a7 -> 4004bb -> 4004cf
                 -> 4004bd

You could even first build the basic block tree and only follow those
branches that end up covering the region IP is in.



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