[llvm-bugs] [Bug 48167] New: If "#pragma clang loop interleave_count(3)" is specified for the loop, it is ignored.

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=48167

            Bug ID: 48167
           Summary: If "#pragma clang loop interleave_count(3)" is
                    specified for the loop, it is ignored.
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Loop Optimizer
          Assignee: unassignedbugs at nondot.org
          Reporter: fj8765ah at aa.jp.fujitsu.com
                CC: llvm-bugs at lists.llvm.org

"#pragma clang interleave_count(NUM)" appears to ignore NUM values that are not
powers of 2.
I think it is better to give priority to what is specified by pragma.
Does setting it to a value other than a power of 2 cause problems?

- interleave.c
void foo(double * restrict a,
         double * restrict b,
         double * restrict c,
         int n) {
#ifdef PRAGMA
#pragma clang loop interleave_count(_NUM) 
#endif
  for (int i=0;i<n;i++)
    c[i] = a[i] + b[i];

  return;
}

If "#pragma clang interleave_count(3)" is specified, the interleave is 2.


$ clang -O3 interleave.c -Rpass=loop-vectorize -S
interleave.c:8:3: remark: vectorized loop (vectorization width: 2, interleaved
count: 2) [-Rpass=loop-vectorize]
  for (int i=0;i<n;i++)
  ^
$ clang -O3 interleave.c -Rpass=loop-vectorize -S -DPRAGMA -D_NUM=1
interleave.c:8:3: remark: vectorized loop (vectorization width: 2, interleaved
count: 1) [-Rpass=loop-vectorize]
  for (int i=0;i<n;i++)
  ^
$ clang -O3 interleave.c -Rpass=loop-vectorize -S -DPRAGMA -D_NUM=2
interleave.c:8:3: remark: vectorized loop (vectorization width: 2, interleaved
count: 2) [-Rpass=loop-vectorize]
  for (int i=0;i<n;i++)
  ^
$ clang -O3 interleave.c -Rpass=loop-vectorize -S -DPRAGMA -D_NUM=3
interleave.c:8:3: remark: vectorized loop (vectorization width: 2, interleaved
count: 2) [-Rpass=loop-vectorize]
  for (int i=0;i<n;i++)
  ^


I think it is set in the following part of the
"llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp" file.
If the value is not a power of 2, the if statement on line 235 is false and
ignored.

    59  bool LoopVectorizeHints::Hint::validate(unsigned Val) {
    60    switch (Kind) {
    61    case HK_WIDTH:
    62      return isPowerOf2_32(Val) && Val <=
VectorizerParams::MaxVectorWidth;
    63    case HK_UNROLL:
    64      return isPowerOf2_32(Val) && Val <= MaxInterleaveFactor;
    65    case HK_FORCE:
    66      return (Val <= 1);
    67    case HK_ISVECTORIZED:
    68    case HK_PREDICATE:
    69      return (Val == 0 || Val == 1);
    70    }
    71    return false;
    72  }
   ...
   223  void LoopVectorizeHints::setHint(StringRef Name, Metadata *Arg) {
   224    if (!Name.startswith(Prefix()))
   225      return;
   226    Name = Name.substr(Prefix().size(), StringRef::npos);
   227
   228    const ConstantInt *C = mdconst::dyn_extract<ConstantInt>(Arg);
   229    if (!C)
   230      return;
   231    unsigned Val = C->getZExtValue();
   232
   233    Hint *Hints[] = {&Width, &Interleave, &Force, &IsVectorized,
&Predicate};
   234    for (auto H : Hints) {
   235      if (Name == H->Name) {
   236        if (H->validate(Val))
   237          H->Value = Val;
   238        else
   239          LLVM_DEBUG(dbgs() << "LV: ignoring invalid hint '" << Name <<
"'\n");
   240        break;
   241      }
   242    }
   243  }

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