[llvm] [GlobalIsel] Combine selects with constants (PR #76089)

[Thorsten Schütt via llvm-commits]

================
@@ -6318,3 +6262,308 @@ void CombinerHelper::applyCommuteBinOpOperands(MachineInstr &MI) {
   MI.getOperand(2).setReg(LHSReg);
   Observer.changedInstr(MI);
 }
+
+bool CombinerHelper::isOneOrOneSplat(Register Src, bool AllowUndefs) {
+  LLT SrcTy = MRI.getType(Src);
+  if (SrcTy.isFixedVector())
+    return isConstantSplatVector(Src, 1, AllowUndefs);
+  if (SrcTy.isScalar()) {
+    if (AllowUndefs && getOpcodeDef<GImplicitDef>(Src, MRI) != nullptr)
+      return true;
+    auto IConstant = getIConstantVRegValWithLookThrough(Src, MRI);
+    return IConstant && IConstant->Value == 1;
+  }
+  return false; // scalable vector
+}
+
+bool CombinerHelper::isZeroOrZeroSplat(Register Src, bool AllowUndefs) {
+  LLT SrcTy = MRI.getType(Src);
+  if (SrcTy.isFixedVector())
+    return isConstantSplatVector(Src, 0, AllowUndefs);
+  if (SrcTy.isScalar()) {
+    if (AllowUndefs && getOpcodeDef<GImplicitDef>(Src, MRI) != nullptr)
+      return true;
+    auto IConstant = getIConstantVRegValWithLookThrough(Src, MRI);
+    return IConstant && IConstant->Value == 0;
+  }
+  return false; // scalable vector
+}
+
+// Ignores COPYs during conformance checks.
+// FIXME scalable vectors.
+bool CombinerHelper::isConstantSplatVector(Register Src, int64_t SplatValue,
+                                           bool AllowUndefs) {
+  GBuildVector *BuildVector = getOpcodeDef<GBuildVector>(Src, MRI);
+  if (!BuildVector)
+    return false;
+  unsigned NumSources = BuildVector->getNumSources();
+
+  for (unsigned I = 0; I < NumSources; ++I) {
+    GImplicitDef *ImplicitDef =
+        getOpcodeDef<GImplicitDef>(BuildVector->getSourceReg(I), MRI);
+    if (ImplicitDef && AllowUndefs)
+      continue;
+    if (ImplicitDef && !AllowUndefs)
+      return false;
+    std::optional<ValueAndVReg> IConstant =
+        getIConstantVRegValWithLookThrough(BuildVector->getSourceReg(I), MRI);
+    if (IConstant && IConstant->Value == SplatValue)
+      continue;
+    return false;
+  }
+  return true;
+}
+
+// Ignores COPYs during lookups.
+// FIXME scalable vectors
+std::optional<APInt>
+CombinerHelper::getConstantOrConstantSplatVector(Register Src) {
+  auto IConstant = getIConstantVRegValWithLookThrough(Src, MRI);
+  if (IConstant)
+    return IConstant->Value;
+
+  GBuildVector *BuildVector = getOpcodeDef<GBuildVector>(Src, MRI);
+  if (!BuildVector)
+    return std::nullopt;
+  unsigned NumSources = BuildVector->getNumSources();
+
+  std::optional<APInt> Value = std::nullopt;
+  for (unsigned I = 0; I < NumSources; ++I) {
+    std::optional<ValueAndVReg> IConstant =
+        getIConstantVRegValWithLookThrough(BuildVector->getSourceReg(I), MRI);
+    if (!IConstant)
+      return std::nullopt;
+    if (!Value)
+      Value = IConstant->Value;
+    else if (*Value != IConstant->Value)
+      return std::nullopt;
+  }
+  return Value;
+}
+
+// TODO: use knownbits to determine zeros
+bool CombinerHelper::tryFoldSelectOfConstants(GSelect *Select,
----------------
tschuett wrote:

This is a partial port of foldSelectOfConstants from the DAG.

https://github.com/llvm/llvm-project/pull/76089