[GlobalISel] Add isConstFalseVal helper to Utils

Add a utility function which returns true if the given value is a constant
false value.

This is necessary to port one of the compare simplifications in
TargetLowering::SimplifySetCC.

Differential Revision: https://reviews.llvm.org/D91754

llvm/include/llvm/CodeGen/GlobalISel/Utils.h

@@ -488,6 +488,10 @@ bool matchUnaryPredicate(const MachineRegisterInfo &MRI, Register Reg,
 /// the value \p Val contains a true value.
 bool isConstTrueVal(const TargetLowering &TLI, int64_t Val, bool IsVector,
                     bool IsFP);
+/// \returns true if given the TargetLowering's boolean contents information,
+/// the value \p Val contains a false value.
+bool isConstFalseVal(const TargetLowering &TLI, int64_t Val, bool IsVector,
+                    bool IsFP);
 
 /// Returns an integer representing true, as defined by the
 /// TargetBooleanContents.

llvm/lib/CodeGen/GlobalISel/Utils.cpp

@@ -1278,6 +1278,18 @@ bool llvm::isConstTrueVal(const TargetLowering &TLI, int64_t Val, bool IsVector,
   llvm_unreachable("Invalid boolean contents");
 }
 
+bool llvm::isConstFalseVal(const TargetLowering &TLI, int64_t Val,
+                           bool IsVector, bool IsFP) {
+  switch (TLI.getBooleanContents(IsVector, IsFP)) {
+  case TargetLowering::UndefinedBooleanContent:
+    return ~Val & 0x1;
+  case TargetLowering::ZeroOrOneBooleanContent:
+  case TargetLowering::ZeroOrNegativeOneBooleanContent:
+    return Val == 0;
+  }
+  llvm_unreachable("Invalid boolean contents");
+}
+
 int64_t llvm::getICmpTrueVal(const TargetLowering &TLI, bool IsVector,
                              bool IsFP) {
   switch (TLI.getBooleanContents(IsVector, IsFP)) {

llvm/unittests/CodeGen/GlobalISel/GISelUtilsTest.cpp

@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "GISelMITest.h"
 #include "llvm/CodeGen/GlobalISel/Utils.h"
 #include "gtest/gtest.h"
 
@@ -245,4 +246,23 @@ TEST(GISelUtilsTest, getLCMType) {
   EXPECT_EQ(V4P1, getLCMType(P1, V2S64));
 }
 
+TEST_F(AArch64GISelMITest, ConstFalseTest) {
+  setUp();
+  if (!TM)
+    return;
+  const auto &TLI = *B.getMF().getSubtarget().getTargetLowering();
+  bool BooleanChoices[2] = {true, false};
+
+  // AArch64 uses ZeroOrOneBooleanContent for scalars, and
+  // ZeroOrNegativeOneBooleanContent for vectors.
+  for (auto IsVec : BooleanChoices) {
+    for (auto IsFP : BooleanChoices) {
+      EXPECT_TRUE(isConstFalseVal(TLI, 0, IsVec, IsFP));
+      EXPECT_FALSE(isConstFalseVal(TLI, 1, IsVec, IsFP));
+
+      // This would be true with UndefinedBooleanContent.
+      EXPECT_FALSE(isConstFalseVal(TLI, 2, IsVec, IsFP));
+    }
+  }
+}
 }