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



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