[InstCombine] Fix miscompilation in PR83947 (#83993)

https://github.com/llvm/llvm-project/blob/762f762504967efbe159db5c737154b989afc9bb/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp#L394-L407

Comment from @topperc:
> This transforms assumes the mask is a non-zero splat. We only know its
a splat and not provably all 0s. The mask is a constexpr that includes
the address of the global variable. We can't resolve the constant
expression to an exact value.

Fixes #83947.

Author: dtcxzyw

llvm/include/llvm/Analysis/VectorUtils.h

@@ -406,6 +406,11 @@ bool maskIsAllZeroOrUndef(Value *Mask);
 /// lanes can be assumed active.
 bool maskIsAllOneOrUndef(Value *Mask);
 
+/// Given a mask vector of i1, Return true if any of the elements of this
+/// predicate mask are known to be true or undef.  That is, return true if at
+/// least one lane can be assumed active.
+bool maskContainsAllOneOrUndef(Value *Mask);
+
 /// Given a mask vector of the form <Y x i1>, return an APInt (of bitwidth Y)
 /// for each lane which may be active.
 APInt possiblyDemandedEltsInMask(Value *Mask);

llvm/lib/Analysis/VectorUtils.cpp

@@ -1012,6 +1012,31 @@ bool llvm::maskIsAllOneOrUndef(Value *Mask) {
   return true;
 }
 
+bool llvm::maskContainsAllOneOrUndef(Value *Mask) {
+  assert(isa<VectorType>(Mask->getType()) &&
+         isa<IntegerType>(Mask->getType()->getScalarType()) &&
+         cast<IntegerType>(Mask->getType()->getScalarType())->getBitWidth() ==
+             1 &&
+         "Mask must be a vector of i1");
+
+  auto *ConstMask = dyn_cast<Constant>(Mask);
+  if (!ConstMask)
+    return false;
+  if (ConstMask->isAllOnesValue() || isa<UndefValue>(ConstMask))
+    return true;
+  if (isa<ScalableVectorType>(ConstMask->getType()))
+    return false;
+  for (unsigned
+           I = 0,
+           E = cast<FixedVectorType>(ConstMask->getType())->getNumElements();
+       I != E; ++I) {
+    if (auto *MaskElt = ConstMask->getAggregateElement(I))
+      if (MaskElt->isAllOnesValue() || isa<UndefValue>(MaskElt))
+        return true;
+  }
+  return false;
+}
+
 /// TODO: This is a lot like known bits, but for
 /// vectors.  Is there something we can common this with?
 APInt llvm::possiblyDemandedEltsInMask(Value *Mask) {

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

@@ -399,11 +399,14 @@ Instruction *InstCombinerImpl::simplifyMaskedScatter(IntrinsicInst &II) {
   if (auto *SplatPtr = getSplatValue(II.getArgOperand(1))) {
     // scatter(splat(value), splat(ptr), non-zero-mask) -> store value, ptr
     if (auto *SplatValue = getSplatValue(II.getArgOperand(0))) {
-      Align Alignment = cast<ConstantInt>(II.getArgOperand(2))->getAlignValue();
-      StoreInst *S =
-          new StoreInst(SplatValue, SplatPtr, /*IsVolatile=*/false, Alignment);
-      S->copyMetadata(II);
-      return S;
+      if (maskContainsAllOneOrUndef(ConstMask)) {
+        Align Alignment =
+            cast<ConstantInt>(II.getArgOperand(2))->getAlignValue();
+        StoreInst *S = new StoreInst(SplatValue, SplatPtr, /*IsVolatile=*/false,
+                                     Alignment);
+        S->copyMetadata(II);
+        return S;
+      }
     }
     // scatter(vector, splat(ptr), splat(true)) -> store extract(vector,
     // lastlane), ptr

llvm/test/Transforms/InstCombine/pr83947.ll

@@ -0,0 +1,67 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
+; RUN: opt -S -passes=instcombine < %s | FileCheck %s
+
+@c = global i32 0, align 4
+@b = global i32 0, align 4
+
+define void @masked_scatter1() {
+; CHECK-LABEL: define void @masked_scatter1() {
+; CHECK-NEXT:    call void @llvm.masked.scatter.nxv4i32.nxv4p0(<vscale x 4 x i32> zeroinitializer, <vscale x 4 x ptr> shufflevector (<vscale x 4 x ptr> insertelement (<vscale x 4 x ptr> poison, ptr @c, i64 0), <vscale x 4 x ptr> poison, <vscale x 4 x i32> zeroinitializer), i32 4, <vscale x 4 x i1> shufflevector (<vscale x 4 x i1> insertelement (<vscale x 4 x i1> poison, i1 icmp eq (ptr getelementptr inbounds (i32, ptr @b, i64 1), ptr @c), i64 0), <vscale x 4 x i1> poison, <vscale x 4 x i32> zeroinitializer))
+; CHECK-NEXT:    ret void
+;
+  call void @llvm.masked.scatter.nxv4i32.nxv4p0(<vscale x 4 x i32> zeroinitializer, <vscale x 4 x ptr> splat (ptr @c), i32 4, <vscale x 4 x i1> splat (i1 icmp eq (ptr getelementptr (i32, ptr @b, i64 1), ptr @c)))
+  ret void
+}
+
+define void @masked_scatter2() {
+; CHECK-LABEL: define void @masked_scatter2() {
+; CHECK-NEXT:    store i32 0, ptr @c, align 4
+; CHECK-NEXT:    ret void
+;
+  call void @llvm.masked.scatter.v2i32.v2p0(<2 x i32> zeroinitializer, <2 x ptr> splat (ptr @c), i32 4, <2 x i1> splat (i1 true))
+  ret void
+}
+
+define void @masked_scatter3() {
+; CHECK-LABEL: define void @masked_scatter3() {
+; CHECK-NEXT:    store i32 0, ptr @c, align 4
+; CHECK-NEXT:    ret void
+;
+  call void @llvm.masked.scatter.v2i32.v2p0(<2 x i32> zeroinitializer, <2 x ptr> splat (ptr @c), i32 4, <2 x i1> undef)
+  ret void
+}
+
+define void @masked_scatter4() {
+; CHECK-LABEL: define void @masked_scatter4() {
+; CHECK-NEXT:    ret void
+;
+  call void @llvm.masked.scatter.v2i32.v2p0(<2 x i32> zeroinitializer, <2 x ptr> splat (ptr @c), i32 4, <2 x i1> splat (i1 false))
+  ret void
+}
+
+define void @masked_scatter5() {
+; CHECK-LABEL: define void @masked_scatter5() {
+; CHECK-NEXT:    store i32 0, ptr @c, align 4
+; CHECK-NEXT:    ret void
+;
+  call void @llvm.masked.scatter.v2i32.v2p0(<2 x i32> zeroinitializer, <2 x ptr> splat (ptr @c), i32 4, <2 x i1> <i1 true, i1 false>)
+  ret void
+}
+
+define void @masked_scatter6() {
+; CHECK-LABEL: define void @masked_scatter6() {
+; CHECK-NEXT:    store i32 0, ptr @c, align 4
+; CHECK-NEXT:    ret void
+;
+  call void @llvm.masked.scatter.v2i32.v2p0(<2 x i32> zeroinitializer, <2 x ptr> splat (ptr @c), i32 4, <2 x i1> <i1 undef, i1 false>)
+  ret void
+}
+
+define void @masked_scatter7() {
+; CHECK-LABEL: define void @masked_scatter7() {
+; CHECK-NEXT:    call void @llvm.masked.scatter.v2i32.v2p0(<2 x i32> zeroinitializer, <2 x ptr> <ptr @c, ptr @c>, i32 4, <2 x i1> <i1 icmp eq (ptr getelementptr inbounds (i32, ptr @b, i64 1), ptr @c), i1 icmp eq (ptr getelementptr inbounds (i32, ptr @b, i64 1), ptr @c)>)
+; CHECK-NEXT:    ret void
+;
+  call void @llvm.masked.scatter.v2i32.v2p0(<2 x i32> zeroinitializer, <2 x ptr> splat (ptr @c), i32 4, <2 x i1> splat (i1 icmp eq (ptr getelementptr (i32, ptr @b, i64 1), ptr @c)))
+  ret void
+}