lkml.org 
[lkml]   [2011]   [Apr]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [RFC PATCH] perf report: add sort by file lines
    (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@hitachi.com


    \
     
     \ /
      Last update: 2011-04-01 12:47    [W:0.038 / U:59.796 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site