[LV] Fix crash when computing max VF too early

[c-rhodes]

Summary

D90687 introduced a crash:

llvm::LoopVectorizationCostModel::computeMaxVF(llvm::ElementCount, unsigned int): Assertion `WideningDecisions.empty() && Uniforms.empty() && Scalars.empty() && "No decisions should have been taken at this point"' failed.

when compiling the following C code:

typedef struct {
char a;
} b;

b *c;
int d, e;

int f() {
  int g = 0;
  for (; d; d++) {
    e = 0;
    for (; e < c[d].a; e++)
      g++;
  }
  return g;
}

with:

clang -Os -target hexagon -mhvx -fvectorize -mv67 testcase.c -S -o -

This occurred since prior to D90687 computeFeasibleMaxVF would only be
called in computeMaxVF when a scalar epilogue was allowed, but now it's
always called. This causes the assert above since computeFeasibleMaxVF
collects all viable VFs larger than the default MaxVF, and for each VF
calculates the register usage which results in analysis being done the
assert above guards against. This can occur in computeFeasibleMaxVF if
TTI.shouldMaximizeVectorBandwidth and this target hook is implemented in
the hexagon backend to always return true.

Reported by @iajbar.

[c-rhodes]

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

5558–5559

@fhahn Any thoughts on this? This assert is firing since D90687, details above. I'm just wondering having looked at your patch D78298 if there's a more sensible fix here, maybe to get rid of the assert and call invalidateCostModelingDecisions?

[fhahn]

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

5558–5559

I think it is still preferable to avoid spending time computing cost-model decisions unnecessarily.

Duplicating the MaxVF computation at a few places to ensure we only do it when needed looks good to me.

llvm/test/Transforms/LoopVectorize/Hexagon/maximum-vf-crash.ll

2

please add some check lines to make sure something sensible happens, besides not crashing.

15

Personally I think the C source code mostly just adds clutter. Ideally the IR source would be concise and with descriptive variable names, so it should be relatively easy to see what's going on without C source; especially if the C source looks like something auto-generated/C-reduced. Also, there's no guarantee that Clang will generate the same IR in future versions.

45

can this function be a bit more simplified & cleaned up? I'll leave some suggestions below & I think the basic block names could be improved & shortened,

47

none of this should be needed to reproduce the failure, you should be able to just use a constant instead of %.pr as incoming value below.

52

can we instead just pass a pointer argument?

58

instead of using a struct, can this just be plain pointer to i8 or something like that?

62

are all those compares/extensions/selects needed?

73

not needed?

83

not needed?

[c-rhodes]

@fhahn I've simplified the test, thanks for the comments!

[fhahn]

LGTM, thanks! This should probably also go onto the 12.x release branch.

[c-rhodes]

In D94869#2533313, @fhahn wrote:

LGTM, thanks! This should probably also go onto the 12.x release branch.

Landed, cheers!

I've created https://bugs.llvm.org/show_bug.cgi?id=48989 to get it cherry-picked onto the 12.x release branch.

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
   LLVM_DEBUG(dbgs() << "LV: Found trip count: " << TC << '\n');
   if (TC == 1) {
     reportVectorizationFailure("Single iteration (non) loop",
         "loop trip count is one, irrelevant for vectorization",
         "SingleIterationLoop", ORE, TheLoop);
     return None;
   }
 
-  ElementCount MaxVF = computeFeasibleMaxVF(TC, UserVF);
-
   switch (ScalarEpilogueStatus) {
   case CM_ScalarEpilogueAllowed:
-    return MaxVF;
+    return computeFeasibleMaxVF(TC, UserVF);
   case CM_ScalarEpilogueNotAllowedUsePredicate:
     LLVM_FALLTHROUGH;
   case CM_ScalarEpilogueNotNeededUsePredicate:
     LLVM_DEBUG(
         dbgs() << "LV: vector predicate hint/switch found.\n"
                << "LV: Not allowing scalar epilogue, creating predicated "
                << "vector loop.\n");
     break;
LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
   // require a lane mask which varies through the vector loop body.  (TODO)
   if (TheLoop->getExitingBlock() != TheLoop->getLoopLatch()) {
     // If there was a tail-folding hint/switch, but we can't fold the tail by
     // masking, fallback to a vectorization with a scalar epilogue.
     if (ScalarEpilogueStatus == CM_ScalarEpilogueNotNeededUsePredicate) {
       LLVM_DEBUG(dbgs() << "LV: Cannot fold tail by masking: vectorize with a "
                            "scalar epilogue instead.\n");
       ScalarEpilogueStatus = CM_ScalarEpilogueAllowed;
-      return MaxVF;
+      return computeFeasibleMaxVF(TC, UserVF);
     }
     return None;
   }
 
   // Now try the tail folding
 
   // Invalidate interleave groups that require an epilogue if we can't mask
   // the interleave-group.
   if (!useMaskedInterleavedAccesses(TTI)) {
     assert(WideningDecisions.empty() && Uniforms.empty() && Scalars.empty() &&
            "No decisions should have been taken at this point");
     // Note: There is no need to invalidate any cost modeling decisions here, as
     // non where taken so far.
     InterleaveInfo.invalidateGroupsRequiringScalarEpilogue();
   }
 
+  ElementCount MaxVF = computeFeasibleMaxVF(TC, UserVF);
   assert(!MaxVF.isScalable() &&
          "Scalable vectors do not yet support tail folding");
   assert((UserVF.isNonZero() || isPowerOf2_32(MaxVF.getFixedValue())) &&
          "MaxVF must be a power of 2");
   unsigned MaxVFtimesIC =
       UserIC ? MaxVF.getFixedValue() * UserIC : MaxVF.getFixedValue();
   // Avoid tail folding if the trip count is known to be a multiple of any VF we
   // chose.

llvm/test/Transforms/LoopVectorize/Hexagon/maximum-vf-crash.ll

+; RUN: opt -march=hexagon -hexagon-autohvx -loop-vectorize -S < %s 2>&1 | FileCheck %s
+
+; Check that we don't crash.
+
+; CHECK-LABEL: @f
+; CHECK: vector.body
+
+target datalayout = "e-m:e-p:32:32:32-a:0-n16:32-i64:64:64-i32:32:32-i16:16:16-i1:8:8-f32:32:32-f64:64:64-v32:32:32-v64:64:64-v512:512:512-v1024:1024:1024-v2048:2048:2048"
+target triple = "hexagon"
+
+; Function Attrs: optsize
+define i32 @f() #0 {
+entry:
+  br label %loop
+
+loop:
+  %g.016 = phi i32 [ 0, %entry ], [ %g.1.lcssa, %loop ]
+  %iv = phi i32 [ 0, %entry ], [ %iv.next, %loop ]
+  %0 = load i8, i8* undef, align 1
+  %g.1.lcssa = add i32 %g.016, undef
+  %iv.next = add nsw i32 %iv, 1
+  %exitcond = icmp eq i32 %iv.next, 0
+  br i1 %exitcond, label %exit, label %loop
+
+exit:
+  ret i32 %g.1.lcssa
+}
+
+attributes #0 = { optsize "target-features"="+hvx-length128b" }