[PATCH] Fix PR 23525 - Separate header mass propagation in irregular loops.

[Diego Novillo]

On Wed, Jun 10, 2015 at 12:28 PM, Xinliang David Li <xinliangli at gmail.com>
wrote:

>
>> > Secondly, I wonder whether this will regress cases we care about if you
>> > don't incorporate the entry frequency point.  E.g., your patch would
>> > make the frequencies *way* off for the following contrived example:
>> >
>> >     assert(n > 0);
>> >     int i = n - 2;
>> >     if (n % 2 == 0)
>> >       goto SeventyFive;
>> >
>> >     for (; i < n; ++i) {
>> >       if (i % n == 75) {
>> >     SeventyFive:
>> >         foo(i);
>> >       } else {
>> >         bar(i);
>> >       }
>> >     }
>> >
>> > whereas (I think) the current in-tree code would get it roughly correct
>> > (by fluke?).
>>
>> Actually, my patch addresses this problem in particular.  This code is
>> almost exactly the test case in PR 23525. The in-tree code gets it wrong
>> the same way it gets PR 23525 wrong. The call to foo(i) is almost never
>> taken vs the call to bar(i).
>>
>> We get the following frequencies for this code snippet (I wrapped it into
>> a function that I called 100 times).  The left column is current trunk.
>> The right column is trunk plus my patch.  I redacted the floating point
>> figures to make them more readable:
>>
>>
>
> Do you have a complete runnable example above?
>

Sure.

$ cat foo.c
#include <assert.h>
#include <stdio.h>

long X = 0;

void foo(int n) {
  assert(n > 0);
  int i = n - 2;
  if (n % 2 == 0)
    goto SeventyFive;

  for (; i < n; ++i) {
    if (i % n == 75) {
SeventyFive:
      X += i;
    } else {
      X *= i;
    }
  }
}

int main() {
  int i;
  for (i = 1; i < 100; i++) foo(i);
  printf("X = %ld\n", X);
  return 0;
}

$ bin/clang -O0 -fprofile-instr-generate -Wall foo.c -o foo
$ ./foo
X = 1502145969101944128
$ bin/llvm-profdata merge -o foo.prof default.profraw
$ bin/clang -O0 -fprofile-instr-use=foo.prof -emit-llvm -S foo.c
$ bin/opt -analyze -block-freq foo.ll
Printing analysis 'Block Frequency Analysis' for function 'foo':
block-frequency-info: foo
 - entry: float = 1.0, int = 8388608
 - cond.true: float = 0.9999990463, int = 8388600
 - cond.false: float = 0.0000009536743164, int = 8
 - : float = 0.0, int = 0
 - cond.end: float = 0.9999990463, int = 8388600
 - if.then: float = 0.4950490328, int = 4152772
 - if.end: float = 0.5049500135, int = 4235827
 - for.cond: float = 2.884612633, int = 24197884
 - for.body: float = 1.499998569, int = 12582899
 - if.then.5: float = 0.02980129608, int = 249991
 - SeventyFive: float = 0.03438611242, int = 288451
 - if.else: float = 1.470197273, int = 12332908
 - if.end.7: float = 4.580994102, int = 38428163
 - for.inc: float = 2.499997616, int = 20971499
 - for.end: float = 0.9999990463, int = 8388600

Printing analysis 'Block Frequency Analysis' for function 'main':
block-frequency-info: main
 - entry: float = 1.0, int = 8
 - for.cond: float = 51.0, int = 407
 - for.body: float = 50.0, int = 399
 - for.inc: float = 50.0, int = 399
 - for.end: float = 1.0, int = 8
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