[llvm-dev] Interest in integrating a linux perf JITEventListener?

[Andres Freund via llvm-dev]

Hi,

On 2016-12-29 13:17:50 -0800, Philip Reames wrote:
> Having something like this available in tree would definitely be
> useful.

Cool.

> For simplicity, why don't we start with support for the second style?  This
> is the long term useful one and would be a good starting point for getting
> the code in tree.

Works for me.


> Can you give a pointer to the patch so that I can assess the rough
> complexity?  If it's simple enough, I'd be happy to help get it
> reviewed and in.  If it's more complicated, I probably won't have the
> time to assist.

Patch (and a prerequisite) attached. Took me a while to get it cleaned
up to some degree - I'm a C programmer these days, and a lot of my C++
knowledge has been replaced by other things...  It's still not super
clean, but I think in a good enough state for you to estimate
complexity.

What do you think?  I've below included some example output to show
what's going on.

Regards,

Andres

A random example (source c file also attached):
# generate some IR, with debug info
clang -ggdb -S -c -emit-llvm expensive_loop.c -o tmp/expensive_loop.ll
# record profile ('-k1; is the clocksource, -g hierarchical)
perf record -g -k 1 lli -jit-kind=mcjit /tmp/expensive_loop.ll 1
# enrich profile with JIT information emitted due to patch
perf inject --jit -i perf.data -o perf.jit.data
# and show information
perf report -i perf.jit.data

Example output:
Samples: 3K of event 'cycles:ppp', Event count (approx.): 3127026392
  Overhead  Command  Shared Object                      Symbol
-   93.41%  lli      jitted-27248-2.so                  [.] stupid_isprime
     stupid_isprime
     main
     llvm::MCJIT::runFunction
     llvm::ExecutionEngine::runFunctionAsMain
     main
     __libc_start_main
     0xec26258d4c544155
+    0.55%  lli      ld-2.24.so                         [.] do_lookup_x
+    0.22%  lli      ld-2.24.so                         [.] _dl_lookup_symbol_x
+    0.17%  lli      [kernel.vmlinux]                   [k] unmap_page_range
+    0.16%  lli      ld-2.24.so                         [.] _dl_fixup


Instruction level view:

       │      Disassembly of section .text:
       │
       │      0000000000000040 <stupid_isprime>:
       │      stupid_isprime():
       │      #include <stdint.h>
       │      #include <stdbool.h>
       │
       │      bool stupid_isprime(uint64_t num)
       │      {
       │        push   %rbp
       │        mov    %rsp,%rbp
       │        mov    %rdi,-0x10(%rbp)
       │              if (num == 2)
       │        cmp    $0x2,%rdi
       │      ↓ jne    14
       │1  e:┌─→movb   $0x1,-0x1(%rbp)
       │     │↓ jmp    55
       │     │                return true;
       │     │        if (num < 1 || num % 2 == 0)
       │1 14:│  cmpq   $0x0,-0x10(%rbp)
       │     │↓ je     51
       │     │  testb  $0x1,-0x10(%rbp)
       │     │↓ je     51
       │     │                return false;
       │     │        for(uint64_t i = 3; i < num / 2; i+= 2) {
       │     │  movq   $0x3,-0x18(%rbp)
       │     │↓ jmp    35
       │     │  nop
       │1 30:│  addq   $0x2,-0x18(%rbp)
  4.03 │1 35:│  mov    -0x10(%rbp),%rax
  0.06 │     │  shr    %rax
       │     │  cmp    %rax,-0x18(%rbp)
       │     └──jae    e
       │                      if (num % i == 0)
  3.74 │        mov    -0x10(%rbp),%rax
  0.09 │        xor    %edx,%edx
 91.82 │        divq   -0x18(%rbp)
       │        test   %rdx,%rdx
  0.23 │      ↑ jne    30
  0.03 │2 51:   movb   $0x0,-0x1(%rbp)
       │1 55:   mov    -0x1(%rbp),%al
       │        pop    %rbp
       │      ← retq

(the missing colors make it harder to see what's going on)
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#include <stdint.h>
#include <stdbool.h>

bool stupid_isprime(uint64_t num)
{
	if (num == 2)
		return true;
	if (num < 1 || num % 2 == 0)
		return false;
	for(uint64_t i = 3; i < num / 2; i+= 2) {
		if (num % i == 0)
			return false;
	}
	return true;
}

int main(int argc, char **argv)
{
	int numprimes = 0;

	for (uint64_t num = argc; num < 100000; num++)
	{
		if (stupid_isprime(num))
			numprimes++;
	}

	return numprimes;
}