[RISCV][GISel] Legalize G_SMULO/G_UMULO (#67635)

Update `LegalizerHelper::widenScalarMulo` to not create a mulo if we aren't going to use the overflow flag. This prevents needing to legalize the widened operation. This generates better code when we need to make a libcall for multiply.

Author: topperc

llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp

@@ -2141,8 +2141,20 @@ LegalizerHelper::widenScalarMulo(MachineInstr &MI, unsigned TypeIdx,
   auto LeftOperand = MIRBuilder.buildInstr(ExtOp, {WideTy}, {LHS});
   auto RightOperand = MIRBuilder.buildInstr(ExtOp, {WideTy}, {RHS});
 
-  auto Mulo = MIRBuilder.buildInstr(MI.getOpcode(), {WideTy, OverflowTy},
-                                    {LeftOperand, RightOperand});
+  // Multiplication cannot overflow if the WideTy is >= 2 * original width,
+  // so we don't need to check the overflow result of larger type Mulo.
+  bool WideMulCanOverflow = WideTy.getScalarSizeInBits() < 2 * SrcBitWidth;
+
+  unsigned MulOpc =
+      WideMulCanOverflow ? MI.getOpcode() : (unsigned)TargetOpcode::G_MUL;
+
+  MachineInstrBuilder Mulo;
+  if (WideMulCanOverflow)
+    Mulo = MIRBuilder.buildInstr(MulOpc, {WideTy, OverflowTy},
+                                 {LeftOperand, RightOperand});
+  else
+    Mulo = MIRBuilder.buildInstr(MulOpc, {WideTy}, {LeftOperand, RightOperand});
+
   auto Mul = Mulo->getOperand(0);
   MIRBuilder.buildTrunc(Result, Mul);
 
@@ -2160,9 +2172,7 @@ LegalizerHelper::widenScalarMulo(MachineInstr &MI, unsigned TypeIdx,
     ExtResult = MIRBuilder.buildZExtInReg(WideTy, Mul, SrcBitWidth);
   }
 
-  // Multiplication cannot overflow if the WideTy is >= 2 * original width,
-  // so we don't need to check the overflow result of larger type Mulo.
-  if (WideTy.getScalarSizeInBits() < 2 * SrcBitWidth) {
+  if (WideMulCanOverflow) {
     auto Overflow =
         MIRBuilder.buildICmp(CmpInst::ICMP_NE, OverflowTy, Mul, ExtResult);
     // Finally check if the multiplication in the larger type itself overflowed.

llvm/lib/Target/RISCV/GISel/RISCVLegalizerInfo.cpp

@@ -145,13 +145,31 @@ RISCVLegalizerInfo::RISCVLegalizerInfo(const RISCVSubtarget &ST) {
         .legalFor({XLenLLT})
         .lower();
     // clang-format on
+
+    getActionDefinitionsBuilder({G_SMULO, G_UMULO})
+        .minScalar(0, XLenLLT)
+        .lower();
   } else {
     getActionDefinitionsBuilder(G_MUL)
         .libcallFor({XLenLLT, DoubleXLenLLT})
         .widenScalarToNextPow2(0)
         .clampScalar(0, XLenLLT, DoubleXLenLLT);
 
     getActionDefinitionsBuilder({G_SMULH, G_UMULH}).lowerFor({XLenLLT});
+
+    getActionDefinitionsBuilder({G_SMULO, G_UMULO})
+        .minScalar(0, XLenLLT)
+        // Widen XLenLLT to DoubleXLenLLT so we can use a single libcall to get
+        // the low bits for the mul result and high bits to do the overflow
+        // check.
+        .widenScalarIf(
+            [=](const LegalityQuery &Query) {
+              return Query.Types[0] == XLenLLT;
+            },
+            [=](const LegalityQuery &Query) {
+              return std::make_pair(0, DoubleXLenLLT);
+            })
+        .lower();
   }
 
   if (ST.hasStdExtM()) {