[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.
diff --git a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
index 196da03..1087684 100644
--- a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -2141,8 +2141,20 @@
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 @@
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.