GlobalISel: expand mul-with-overflow into mul-hi on AArch64.

AArch64 has specific instructions to multiply two numbers at double the width
and produce the high part of the result. These can be used to implement LLVM's
mul.with.overflow instructions fairly simply. Helps with C++ operator new[].

llvm-svn: 294519

1 files changed, 27 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
index aeb89a41aa9..f41af0547a3 100644
--- a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -519,6 +519,33 @@ LegalizerHelper::lower(MachineInstr &MI, unsigned TypeIdx, LLT Ty) {
     MI.eraseFromParent();
     return Legalized;
   }
+  case TargetOpcode::G_SMULO:
+  case TargetOpcode::G_UMULO: {
+    // Generate G_UMULH/G_SMULH to check for overflow and a normal G_MUL for the
+    // result.
+    unsigned Res = MI.getOperand(0).getReg();
+    unsigned Overflow = MI.getOperand(1).getReg();
+    unsigned LHS = MI.getOperand(2).getReg();
+    unsigned RHS = MI.getOperand(3).getReg();
+
+    MIRBuilder.buildMul(Res, LHS, RHS);
+
+    unsigned Opcode = MI.getOpcode() == TargetOpcode::G_SMULO
+                          ? TargetOpcode::G_SMULH
+                          : TargetOpcode::G_UMULH;
+
+    unsigned HiPart = MRI.createGenericVirtualRegister(Ty);
+    MIRBuilder.buildInstr(Opcode)
+      .addDef(HiPart)
+      .addUse(LHS)
+      .addUse(RHS);
+
+    unsigned Zero = MRI.createGenericVirtualRegister(Ty);
+    MIRBuilder.buildConstant(Zero, 0);
+    MIRBuilder.buildICmp(CmpInst::ICMP_NE, Overflow, HiPart, Zero);
+    MI.eraseFromParent();
+    return Legalized;
+  }
   }
 }