[GlobalIsel] Modernize truncate of ext. (#100338)

Credits:
https://github.com/llvm/llvm-project/pull/90964
https://reviews.llvm.org/D87050

combine-trunc.mir

Functional changes intended.

2 files changed, 48 insertions, 34 deletions

diff --git a/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
index 208f554..8c05931 100644
--- a/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
@@ -2582,40 +2582,6 @@ void CombinerHelper::applyCombineExtOfExt(
   }
 }
 
-bool CombinerHelper::matchCombineTruncOfExt(
-    MachineInstr &MI, std::pair<Register, unsigned> &MatchInfo) {
-  assert(MI.getOpcode() == TargetOpcode::G_TRUNC && "Expected a G_TRUNC");
-  Register SrcReg = MI.getOperand(1).getReg();
-  MachineInstr *SrcMI = MRI.getVRegDef(SrcReg);
-  unsigned SrcOpc = SrcMI->getOpcode();
-  if (SrcOpc == TargetOpcode::G_ANYEXT || SrcOpc == TargetOpcode::G_SEXT ||
-      SrcOpc == TargetOpcode::G_ZEXT) {
-    MatchInfo = std::make_pair(SrcMI->getOperand(1).getReg(), SrcOpc);
-    return true;
-  }
-  return false;
-}
-
-void CombinerHelper::applyCombineTruncOfExt(
-    MachineInstr &MI, std::pair<Register, unsigned> &MatchInfo) {
-  assert(MI.getOpcode() == TargetOpcode::G_TRUNC && "Expected a G_TRUNC");
-  Register SrcReg = MatchInfo.first;
-  unsigned SrcExtOp = MatchInfo.second;
-  Register DstReg = MI.getOperand(0).getReg();
-  LLT SrcTy = MRI.getType(SrcReg);
-  LLT DstTy = MRI.getType(DstReg);
-  if (SrcTy == DstTy) {
-    MI.eraseFromParent();
-    replaceRegWith(MRI, DstReg, SrcReg);
-    return;
-  }
-  if (SrcTy.getSizeInBits() < DstTy.getSizeInBits())
-    Builder.buildInstr(SrcExtOp, {DstReg}, {SrcReg});
-  else
-    Builder.buildTrunc(DstReg, SrcReg);
-  MI.eraseFromParent();
-}
-
 static LLT getMidVTForTruncRightShiftCombine(LLT ShiftTy, LLT TruncTy) {
   const unsigned ShiftSize = ShiftTy.getScalarSizeInBits();
   const unsigned TruncSize = TruncTy.getScalarSizeInBits();
diff --git a/llvm/lib/CodeGen/GlobalISel/CombinerHelperCasts.cpp b/llvm/lib/CodeGen/GlobalISel/CombinerHelperCasts.cpp
index 8fe69f2..d36685b 100644
--- a/llvm/lib/CodeGen/GlobalISel/CombinerHelperCasts.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/CombinerHelperCasts.cpp
@@ -113,3 +113,51 @@ bool CombinerHelper::matchNonNegZext(const MachineOperand &MO,
 
   return false;
 }
+
+bool CombinerHelper::matchTruncateOfExt(const MachineInstr &Root,
+                                        const MachineInstr &ExtMI,
+                                        BuildFnTy &MatchInfo) {
+  const GTrunc *Trunc = cast<GTrunc>(&Root);
+  const GExtOp *Ext = cast<GExtOp>(&ExtMI);
+
+  if (!MRI.hasOneNonDBGUse(Ext->getReg(0)))
+    return false;
+
+  Register Dst = Trunc->getReg(0);
+  Register Src = Ext->getSrcReg();
+  LLT DstTy = MRI.getType(Dst);
+  LLT SrcTy = MRI.getType(Src);
+
+  if (SrcTy == DstTy) {
+    // The source and the destination are equally sized. We need to copy.
+    MatchInfo = [=](MachineIRBuilder &B) { B.buildCopy(Dst, Src); };
+
+    return true;
+  }
+
+  if (SrcTy.getScalarSizeInBits() < DstTy.getScalarSizeInBits()) {
+    // If the source is smaller than the destination, we need to extend.
+
+    if (!isLegalOrBeforeLegalizer({Ext->getOpcode(), {DstTy, SrcTy}}))
+      return false;
+
+    MatchInfo = [=](MachineIRBuilder &B) {
+      B.buildInstr(Ext->getOpcode(), {Dst}, {Src});
+    };
+
+    return true;
+  }
+
+  if (SrcTy.getScalarSizeInBits() > DstTy.getScalarSizeInBits()) {
+    // If the source is larger than the destination, then we need to truncate.
+
+    if (!isLegalOrBeforeLegalizer({TargetOpcode::G_TRUNC, {DstTy, SrcTy}}))
+      return false;
+
+    MatchInfo = [=](MachineIRBuilder &B) { B.buildTrunc(Dst, Src); };
+
+    return true;
+  }
+
+  return false;
+}