[llvm] r286423 - [InstCombine] avoid infinite loop from shuffle-extract-insert sequence (PR30923)

[Sanjay Patel via llvm-commits]

Author: spatel
Date: Wed Nov  9 18:15:14 2016
New Revision: 286423

URL: http://llvm.org/viewvc/llvm-project?rev=286423&view=rev
Log:
[InstCombine] avoid infinite loop from shuffle-extract-insert sequence (PR30923)

Removing the limitation in visitInsertElementInst() causes several regressions
because we're not prepared to fold sequences of shuffles or inserts and extracts
separated by shuffles. Fixing that appears to be a difficult mission because we
are purposely trying to avoid creating shuffles with arbitrary shuffle masks
because some targets may choke on those.

https://llvm.org/bugs/show_bug.cgi?id=30923

Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
    llvm/trunk/test/Transforms/InstCombine/insert-extract-shuffle.ll

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineVectorOps.cpp?rev=286423&r1=286422&r2=286423&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineVectorOps.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineVectorOps.cpp Wed Nov  9 18:15:14 2016
@@ -413,6 +413,14 @@ static void replaceExtractElements(Inser
   if (InsertionBlock != InsElt->getParent())
     return;
 
+  // TODO: This restriction matches the check in visitInsertElementInst() and
+  // prevents an infinite loop caused by not turning the extract/insert pair
+  // into a shuffle. We really should not need either check, but we're lacking
+  // folds for shufflevectors because we're afraid to generate shuffle masks
+  // that the backend can't handle.
+  if (InsElt->hasOneUse() && isa<InsertElementInst>(InsElt->user_back()))
+    return;
+
   auto *WideVec = new ShuffleVectorInst(ExtVecOp, UndefValue::get(ExtVecType),
                                         ConstantVector::get(ExtendMask));
 

Modified: llvm/trunk/test/Transforms/InstCombine/insert-extract-shuffle.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/insert-extract-shuffle.ll?rev=286423&r1=286422&r2=286423&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/insert-extract-shuffle.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/insert-extract-shuffle.ll Wed Nov  9 18:15:14 2016
@@ -237,3 +237,30 @@ end:
   ret double %mu
 }
 
+; https://llvm.org/bugs/show_bug.cgi?id=30923
+; Delete the widening shuffle if we're not going to reduce the extract/insert to a shuffle.
+
+define <4 x float> @PR30923(<2 x float> %x) {
+; CHECK-LABEL: @PR30923(
+; CHECK-NEXT:  bb1:
+; CHECK-NEXT:    [[EXT1:%.*]] = extractelement <2 x float> %x, i32 1
+; CHECK-NEXT:    store float [[EXT1]], float* undef, align 4
+; CHECK-NEXT:    br label %bb2
+; CHECK:       bb2:
+; CHECK-NEXT:    [[EXT2:%.*]] = extractelement <2 x float> %x, i32 0
+; CHECK-NEXT:    [[INS1:%.*]] = insertelement <4 x float> <float 0.000000e+00, float 0.000000e+00, float undef, float undef>, float [[EXT2]], i32 2
+; CHECK-NEXT:    [[INS2:%.*]] = insertelement <4 x float> [[INS1]], float [[EXT1]], i32 3
+; CHECK-NEXT:    ret <4 x float> [[INS2]]
+;
+bb1:
+  %ext1 = extractelement <2 x float> %x, i32 1
+  store float %ext1, float* undef, align 4
+  br label %bb2
+
+bb2:
+  %widen = shufflevector <2 x float> %x, <2 x float> undef, <4 x i32> <i32 0, i32 1, i32 undef, i32 undef>
+  %ext2 = extractelement <4 x float> %widen, i32 0
+  %ins1 = insertelement <4 x float> <float 0.0, float 0.0, float undef, float undef>, float %ext2, i32 2
+  %ins2 = insertelement <4 x float> %ins1, float %ext1, i32 3
+  ret <4 x float> %ins2
+}