lkml.org 
[lkml]   [2011]   [Mar]   [31]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: [RFC PATCH] perf report: add sort by file lines
    From
    Date
    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.

    Lin Ming



    \
     
     \ /
      Last update: 2011-03-31 16:37    [W:0.026 / U:94.708 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site