[GlobalISel]: Allow targets to override how to widen constants during legalization

https://reviews.llvm.org/D70922

This adds a hook to allow targets to define exactly what extension
operation should be performed for widening constants. This handles cases
like widening i1 true which would end up becoming -1 which affects code
quality during combines.
Additionally, in order to stay consistent with how DAG is promoting
constants, we now signextend for byte sized types and zero extend
otherwise (by default). Targets can of course override this if
necessary.

2 files changed, 13 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
index d8bcc59c765..a9af31f882e 100644
--- a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -1675,7 +1675,15 @@ LegalizerHelper::widenScalar(MachineInstr &MI, unsigned TypeIdx, LLT WideTy) {
   case TargetOpcode::G_CONSTANT: {
     MachineOperand &SrcMO = MI.getOperand(1);
     LLVMContext &Ctx = MIRBuilder.getMF().getFunction().getContext();
-    const APInt &Val = SrcMO.getCImm()->getValue().sext(WideTy.getSizeInBits());
+    unsigned ExtOpc = LI.getExtOpcodeForWideningConstant(
+        MRI.getType(MI.getOperand(0).getReg()));
+    assert((ExtOpc == TargetOpcode::G_ZEXT || ExtOpc == TargetOpcode::G_SEXT ||
+            ExtOpc == TargetOpcode::G_ANYEXT) &&
+           "Illegal Extend");
+    const APInt &SrcVal = SrcMO.getCImm()->getValue();
+    const APInt &Val = (ExtOpc == TargetOpcode::G_SEXT)
+                           ? SrcVal.sext(WideTy.getSizeInBits())
+                           : SrcVal.zext(WideTy.getSizeInBits());
     Observer.changingInstr(MI);
     SrcMO.setCImm(ConstantInt::get(Ctx, Val));
 
diff --git a/llvm/lib/CodeGen/GlobalISel/LegalizerInfo.cpp b/llvm/lib/CodeGen/GlobalISel/LegalizerInfo.cpp
index 70045512fae..f897f9c7e20 100644
--- a/llvm/lib/CodeGen/GlobalISel/LegalizerInfo.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/LegalizerInfo.cpp
@@ -685,6 +685,10 @@ bool LegalizerInfo::legalizeIntrinsic(MachineInstr &MI,
   return true;
 }
 
+unsigned LegalizerInfo::getExtOpcodeForWideningConstant(LLT SmallTy) const {
+  return SmallTy.isByteSized() ? TargetOpcode::G_SEXT : TargetOpcode::G_ZEXT;
+}
+
 /// \pre Type indices of every opcode form a dense set starting from 0.
 void LegalizerInfo::verify(const MCInstrInfo &MII) const {
 #ifndef NDEBUG