[InstCombine] dropRedundantMaskingOfLeftShiftInput(): propagate undef shift amounts

Summary:
When we do `ConstantExpr::getZExt()`, that "extends" `undef` to `0`,
which means that for patterns a/b we'd assume that we must not produce
any bits for that channel, while in reality we simply didn't care
about that channel - i.e. we don't need to mask it.

Reviewers: spatel

Reviewed By: spatel

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68239

llvm-svn: 373960

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
index 8ab4aeb..6675ab1 100644 (file)
--- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -117,6 +117,24 @@ reassociateShiftAmtsOfTwoSameDirectionShifts(BinaryOperator *Sh0,
   return Ret;
 }
 
+// Try to replace `undef` constants in C with Replacement.
+static Constant *replaceUndefsWith(Constant *C, Constant *Replacement) {
+  if (C && match(C, m_Undef()))
+    return Replacement;
+
+  if (auto *CV = dyn_cast<ConstantVector>(C)) {
+    llvm::SmallVector<Constant *, 32> NewOps(CV->getNumOperands());
+    for (unsigned i = 0, NumElts = NewOps.size(); i != NumElts; ++i) {
+      Constant *EltC = CV->getOperand(i);
+      NewOps[i] = EltC && match(EltC, m_Undef()) ? Replacement : EltC;
+    }
+    return ConstantVector::get(NewOps);
+  }
+
+  // Don't know how to deal with this constant.
+  return C;
+}
+
 // If we have some pattern that leaves only some low bits set, and then performs
 // left-shift of those bits, if none of the bits that are left after the final
 // shift are modified by the mask, we can omit the mask.
@@ -177,6 +195,14 @@ dropRedundantMaskingOfLeftShiftInput(BinaryOperator *OuterShift,
       // The mask must be computed in a type twice as wide to ensure
       // that no bits are lost if the sum-of-shifts is wider than the base type.
       Type *ExtendedTy = Ty->getExtendedType();
+      // An extend of an undef value becomes zero because the high bits are
+      // never completely unknown. Replace the the `undef` shift amounts with
+      // final shift bitwidth to ensure that the value remains undef when
+      // creating the subsequent shift op.
+      SumOfShAmts = replaceUndefsWith(
+          SumOfShAmts,
+          ConstantInt::get(SumOfShAmts->getType()->getScalarType(),
+                           ExtendedTy->getScalarType()->getScalarSizeInBits()));
       auto *ExtendedSumOfShAmts =
           ConstantExpr::getZExt(SumOfShAmts, ExtendedTy);
       // And compute the mask as usual: ~(-1 << (SumOfShAmts))
@@ -212,6 +238,13 @@ dropRedundantMaskingOfLeftShiftInput(BinaryOperator *OuterShift,
       // The mask must be computed in a type twice as wide to ensure
       // that no bits are lost if the sum-of-shifts is wider than the base type.
       Type *ExtendedTy = Ty->getExtendedType();
+      // An extend of an undef value becomes zero because the high bits are
+      // never completely unknown. Replace the the `undef` shift amounts with
+      // negated shift bitwidth to ensure that the value remains undef when
+      // creating the subsequent shift op.
+      ShAmtsDiff = replaceUndefsWith(
+          ShAmtsDiff,
+          ConstantInt::get(ShAmtsDiff->getType()->getScalarType(), -BitWidth));
       auto *ExtendedNumHighBitsToClear = ConstantExpr::getZExt(
           ConstantExpr::getAdd(
               ConstantExpr::getNeg(ShAmtsDiff),

diff --git a/llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-variant-a.ll b/llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-variant-a.ll
index bcaf644..205dcfb 100644 (file)
--- a/llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-variant-a.ll
+++ b/llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-variant-a.ll
@@ -82,7 +82,7 @@ define <8 x i32> @t1_vec_splat_undef(<8 x i32> %x, <8 x i32> %nbits) {
 ; CHECK-NEXT:    call void @use8xi32(<8 x i32> [[T2]])
 ; CHECK-NEXT:    call void @use8xi32(<8 x i32> [[T4]])
 ; CHECK-NEXT:    [[TMP1:%.*]] = shl <8 x i32> [[X:%.*]], [[T4]]
-; CHECK-NEXT:    [[T5:%.*]] = and <8 x i32> [[TMP1]], <i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 0, i32 2147483647>
+; CHECK-NEXT:    [[T5:%.*]] = and <8 x i32> [[TMP1]], <i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 undef, i32 2147483647>
 ; CHECK-NEXT:    ret <8 x i32> [[T5]]
 ;
   %t0 = add <8 x i32> %nbits, <i32 -1, i32 -1, i32 -1, i32 -1, i32 -1, i32 -1, i32 undef, i32 -1>

diff --git a/llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-variant-b.ll b/llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-variant-b.ll
index faf069c..4d3d543 100644 (file)
--- a/llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-variant-b.ll
+++ b/llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-variant-b.ll
@@ -82,7 +82,7 @@ define <8 x i32> @t1_vec_splat_undef(<8 x i32> %x, <8 x i32> %nbits) {
 ; CHECK-NEXT:    call void @use8xi32(<8 x i32> [[T2]])
 ; CHECK-NEXT:    call void @use8xi32(<8 x i32> [[T4]])
 ; CHECK-NEXT:    [[TMP1:%.*]] = shl <8 x i32> [[X:%.*]], [[T4]]
-; CHECK-NEXT:    [[T5:%.*]] = and <8 x i32> [[TMP1]], <i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 0, i32 2147483647>
+; CHECK-NEXT:    [[T5:%.*]] = and <8 x i32> [[TMP1]], <i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 undef, i32 2147483647>
 ; CHECK-NEXT:    ret <8 x i32> [[T5]]
 ;
   %t0 = add <8 x i32> %nbits, <i32 -1, i32 -1, i32 -1, i32 -1, i32 -1, i32 -1, i32 undef, i32 -1>

diff --git a/llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-variant-c.ll b/llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-variant-c.ll
index 2c6bb31..fc964f8 100644 (file)
--- a/llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-variant-c.ll
+++ b/llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-variant-c.ll
@@ -62,7 +62,7 @@ define <8 x i32> @t1_vec_splat_undef(<8 x i32> %x, <8 x i32> %nbits) {
 ; CHECK-NEXT:    call void @use8xi32(<8 x i32> [[T0]])
 ; CHECK-NEXT:    call void @use8xi32(<8 x i32> [[T2]])
 ; CHECK-NEXT:    [[TMP1:%.*]] = shl <8 x i32> [[X:%.*]], [[T2]]
-; CHECK-NEXT:    [[T3:%.*]] = and <8 x i32> [[TMP1]], <i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 -1, i32 2147483647>
+; CHECK-NEXT:    [[T3:%.*]] = and <8 x i32> [[TMP1]], <i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 undef, i32 2147483647>
 ; CHECK-NEXT:    ret <8 x i32> [[T3]]
 ;
   %t0 = lshr <8 x i32> <i32 -1, i32 -1, i32 -1, i32 -1, i32 -1, i32 -1, i32 undef, i32 -1>, %nbits

diff --git a/llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-variant-d.ll b/llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-variant-d.ll
index fcbb7eb..83c0df1 100644 (file)
--- a/llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-variant-d.ll
+++ b/llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-variant-d.ll
@@ -72,7 +72,7 @@ define <8 x i32> @t2_vec_splat_undef(<8 x i32> %x, <8 x i32> %nbits) {
 ; CHECK-NEXT:    call void @use8xi32(<8 x i32> [[T1]])
 ; CHECK-NEXT:    call void @use8xi32(<8 x i32> [[T3]])
 ; CHECK-NEXT:    [[TMP1:%.*]] = shl <8 x i32> [[X:%.*]], [[T3]]
-; CHECK-NEXT:    [[T4:%.*]] = and <8 x i32> [[TMP1]], <i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 -1, i32 2147483647>
+; CHECK-NEXT:    [[T4:%.*]] = and <8 x i32> [[TMP1]], <i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 undef, i32 2147483647>
 ; CHECK-NEXT:    ret <8 x i32> [[T4]]
 ;
   %t0 = shl <8 x i32> <i32 -1, i32 -1, i32 -1, i32 -1, i32 -1, i32 -1, i32 undef, i32 -1>, %nbits

diff --git a/llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-variant-e.ll b/llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-variant-e.ll
index 2b3fb50..200d414 100644 (file)
--- a/llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-variant-e.ll
+++ b/llvm/test/Transforms/InstCombine/partally-redundant-left-shift-input-masking-variant-e.ll
@@ -62,7 +62,7 @@ define <8 x i32> @t1_vec_splat_undef(<8 x i32> %x, <8 x i32> %nbits) {
 ; CHECK-NEXT:    call void @use8xi32(<8 x i32> [[T0]])
 ; CHECK-NEXT:    call void @use8xi32(<8 x i32> [[T2]])
 ; CHECK-NEXT:    [[TMP1:%.*]] = shl <8 x i32> [[X]], [[T2]]
-; CHECK-NEXT:    [[T3:%.*]] = and <8 x i32> [[TMP1]], <i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 -1, i32 2147483647>
+; CHECK-NEXT:    [[T3:%.*]] = and <8 x i32> [[TMP1]], <i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 2147483647, i32 undef, i32 2147483647>
 ; CHECK-NEXT:    ret <8 x i32> [[T3]]
 ;
   %t0 = shl <8 x i32> %x, %nbits