Messages in this thread | | | Subject | Re: [RFC PATCH] perf report: add sort by file lines | From | Peter Zijlstra <> | Date | Tue, 29 Mar 2011 11:54:17 +0200 |
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On Tue, 2011-03-29 at 17:32 +0800, Lin Ming wrote: > > Peter suggested to reverse map the reported IP (PEBS + fixup) > to a data access using dwarf info. > So I wrote this patch to see if the direction is right.
I'm not sure this is quite the same thing, I'm not arguing this might not be useful, but this is not about data access.
Suppose you have a line like:
foo->bar->fubar = tmp->blah;
There's 3 indirections there, a line number doesn't even get you close to knowing what data access triggered the event.
struct bar { int poekoe[5]; int fubar; };
struct foo { long poekoe[3]; struct bar *bar; };
struct tmp { long poekoe[4]; int blah; };
void foo(struct foo *foo, struct tmp *tmp) { foo->bar->fubar = tmp->blah; }
Which gives (somewhat simplified):
foo: .cfi_startproc pushq %rbp .cfi_def_cfa_offset 16 movq %rsp, %rbp .cfi_offset 6, -16 .cfi_def_cfa_register 6 movq %rdi, -8(%rbp) movq %rsi, -16(%rbp) movq -8(%rbp), %rax /* load foo arg from stack */ movq 24(%rax), %rax /* load foo->bar */ movq -16(%rbp), %rdx /* load tmp arg from stack */ movl 32(%rdx), %edx /* load tmp->blah */ movl %edx, 20(%rax) /* store bar->fubar */ leave ret .cfi_endproc
where I annotated the various moves with C comments.
Now depending on what exact IP you get using PEBS+fixup you could using DWARF bits generate similar deductions from the code as I did in those comments and thus know exactly what data member was accessed and how (read/write).
With that data you could then borrow some pahole code and annotate the various data structures to illustrate read/write distributions, which can then be used as input for data-reorder.
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