Messages in this thread | | | Subject | Re: [RFC PATCH] perf report: add sort by file lines | From | Lin Ming <> | Date | Thu, 31 Mar 2011 22:34:16 +0800 |
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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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