[llvm-bugs] [Bug 30654] New: [LoopVectorizer/SCEV] induction with truncation prevents vectorization. Need runtime overflow test.

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=30654

            Bug ID: 30654
           Summary: [LoopVectorizer/SCEV] induction with truncation
                    prevents vectorization. Need runtime overflow test.
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Loop Optimizer
          Assignee: unassignedbugs at nondot.org
          Reporter: dorit.nuzman at intel.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Saw this missed optimization in a Geekbench workload:
We have a signed int index ‘w_ix’ which is incremented by an unsigned long
‘step’.
When compiling with -m32 all is well.
However when compiling with -m64 the result of the ulong addition may not fit
in back into the sint index and so we may have sint overflow (the index may
wrap).

for.body:
  %w_ix.014 = phi i64 [ %add3, %for.body ], [ 0, %for.body.preheader ]
  %sext = shl i64 %w_ix.014, 32
  %idxprom = ashr exact i64 %sext, 32
  %add3 = add i64 %idxprom, %step

As a result the loop vectorizer fails with
“LV: PHI is not a poly recurrence… Found an unidentified PHI”.
"loop not vectorized: value that could not be identified as reduction is used
outside the loop."

In order to guarantee that the induction behaves nicely we need to identify
this pattern (addition with 64-to-32-bit truncation), and generate a runtime
sint overflow check (e.g. check that step*loopTripCount is small enough).

This is a reduced testcase:

#include <stdlib.h>
float in[1000];
float out[1000];
void test(size_t out_start, size_t size, size_t step)
{
    int w_ix = 0;
    for (size_t out_offset = 0; out_offset < size; ++out_offset)
    {
        size_t out_ix = out_start + out_offset;
        float w = in[w_ix];
        out[out_ix] += w;
        w_ix += step;
    }
}

(I compiled it with -m64 -Ofast -static -march=core-avx2 ).

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