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

[Masami Hiramatsu]

(2011/04/01 1:28), Peter Zijlstra wrote:
> On Thu, 2011-03-31 at 22:34 +0800, Lin Ming wrote:
>> 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

Hm, I think that is the easiest case (like as kprobes does in the kernel)
E.g. an indirect jump makes it hard to find where it jumps to.

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

Ah, that's reasonable :)

Thank you,

-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@xxxxxxxxxxx
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