[GloalISel] Fold G_CTTZ if possible

This patch tries to fold `G_CTTZ` if possible.

Author: shiltian

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

@@ -308,10 +308,12 @@ std::optional<APFloat> ConstantFoldIntToFloat(unsigned Opcode, LLT DstTy,
                                               Register Src,
                                               const MachineRegisterInfo &MRI);
 
-/// Tries to constant fold a G_CTLZ operation on \p Src. If \p Src is a vector
-/// then it tries to do an element-wise constant fold.
+/// Tries to constant fold a counting-zero operation (G_CTLZ or G_CTTZ) on \p
+/// Src. If \p Src is a vector then it tries to do an element-wise constant
+/// fold.
 std::optional<SmallVector<unsigned>>
-ConstantFoldCTLZ(Register Src, const MachineRegisterInfo &MRI);
+ConstantFoldCountZeros(Register Src, const MachineRegisterInfo &MRI,
+                       std::function<unsigned(APInt)> CB);
 
 /// Test if the given value is known to have exactly one bit set. This differs
 /// from computeKnownBits in that it doesn't necessarily determine which bit is

llvm/lib/CodeGen/GlobalISel/CSEMIRBuilder.cpp

@@ -256,10 +256,16 @@ MachineInstrBuilder CSEMIRBuilder::buildInstr(unsigned Opc,
       return buildFConstant(DstOps[0], *Cst);
     break;
   }
-  case TargetOpcode::G_CTLZ: {
+  case TargetOpcode::G_CTLZ:
+  case TargetOpcode::G_CTTZ: {
     assert(SrcOps.size() == 1 && "Expected one source");
     assert(DstOps.size() == 1 && "Expected one dest");
-    auto MaybeCsts = ConstantFoldCTLZ(SrcOps[0].getReg(), *getMRI());
+    std::function<unsigned(APInt)> CB;
+    if (Opc == TargetOpcode::G_CTLZ)
+      CB = [](APInt V) -> unsigned { return V.countl_zero(); };
+    else
+      CB = [](APInt V) -> unsigned { return V.countTrailingZeros(); };
+    auto MaybeCsts = ConstantFoldCountZeros(SrcOps[0].getReg(), *getMRI(), CB);
     if (!MaybeCsts)
       break;
     if (MaybeCsts->size() == 1)

llvm/lib/CodeGen/GlobalISel/Utils.cpp

@@ -966,14 +966,15 @@ llvm::ConstantFoldIntToFloat(unsigned Opcode, LLT DstTy, Register Src,
 }
 
 std::optional<SmallVector<unsigned>>
-llvm::ConstantFoldCTLZ(Register Src, const MachineRegisterInfo &MRI) {
+llvm::ConstantFoldCountZeros(Register Src, const MachineRegisterInfo &MRI,
+                             std::function<unsigned(APInt)> CB) {
   LLT Ty = MRI.getType(Src);
   SmallVector<unsigned> FoldedCTLZs;
   auto tryFoldScalar = [&](Register R) -> std::optional<unsigned> {
     auto MaybeCst = getIConstantVRegVal(R, MRI);
     if (!MaybeCst)
       return std::nullopt;
-    return MaybeCst->countl_zero();
+    return CB(*MaybeCst);
   };
   if (Ty.isVector()) {
     // Try to constant fold each element.