[llvm-bugs] [Bug 28374] New: -Rpass-missed=loop-vectorize says nothing at all for outer loops

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=28374

            Bug ID: 28374
           Summary: -Rpass-missed=loop-vectorize says nothing at all for
                    outer loops
           Product: new-bugs
           Version: trunk
          Hardware: Macintosh
                OS: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: sean at rogue-research.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Created attachment 16662
  --> https://llvm.org/bugs/attachment.cgi?id=16662&action=edit
Repro case

Consider:

--------test.c-------
#include <stdio.h>

int foo(int *A, int *B, int n) {

#pragma clang loop vectorize(enable)
  for (int k = 0; k < n; ++k)
  {
    unsigned sum = 0;

#pragma clang loop vectorize(enable)
    for (int i = 0; i < n; ++i)
      sum += A[i] + 5;

    printf("%i", sum);
  }

 return 0;
}
---------------

$ clang --version
clang version 3.9.0 (trunk 274259)

$ clang -S -fvectorize -O3 -Rpass=loop-vectorize -Rpass-missed=loop-vectorize
-Rpass-analysis=loop-vectorize ~/Desktop/test.c
/Users/sean/Desktop/test.c:11:4: remark: vectorized loop (vectorization width:
4, interleaved count: 2) [-Rpass=loop-vectorize]
          for (int i = 0; i < n; ++i)
          ^


It helpfully says it vectorized the inner loop, but says *nothing* about the
outer loop (despite the #pragma even).

Adam Nemet said on cfe-dev that "the vectorizer does not visit outer loops and
the diagnostics is emitted in the vectorizer so we don’t get a chance to report
anything."

For a user trying to audit/tweak his loops to be vectorization-friendly, it's
frustrating that the flags that are supposed to report if/why a loop has/has
not been vectorized are not behaving reliably.

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