[llvm] [GlobalIsel] combine ext of trunc with flags (PR #87115)

[Matt Arsenault via llvm-commits]

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@@ -7138,3 +7138,69 @@ bool CombinerHelper::matchAddOverflow(MachineInstr &MI, BuildFnTy &MatchInfo) {
 
   return false;
 }
+
+void CombinerHelper::applyBuildFnMO(const MachineOperand &MO,
+                                    BuildFnTy &MatchInfo) {
+  MachineInstr *Root = getDefIgnoringCopies(MO.getReg(), MRI);
+  Builder.setInstrAndDebugLoc(*Root);
+  MatchInfo(Builder);
+  Root->eraseFromParent();
+}
+
+bool CombinerHelper::matchSextOfTrunc(const MachineOperand &MO,
+                                      BuildFnTy &MatchInfo) {
+  GSext *Sext = getOpcodeDef<GSext>(MO.getReg(), MRI);
+  if (!Sext)
+    return false;
+
+  GTrunc *Trunc = getOpcodeDef<GTrunc>(Sext->getSrcReg(), MRI);
+  if (!Trunc)
+    return false;
+
+  // The trunc must have the nsw flag.
+  if (!Trunc->getFlag(MachineInstr::MIFlag::NoSWrap))
+    return false;
+
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arsenm wrote:

All of this is already in the pattern?  Don't need to double check it? 

https://github.com/llvm/llvm-project/pull/87115