[llvm] [SystemZ] Enable MachineCombiner for FP reassociation (PR #83546)

[Ulrich Weigand via llvm-commits]

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@@ -610,6 +610,23 @@ void SystemZInstrInfo::insertSelect(MachineBasicBlock &MBB,
     .addImm(CCValid).addImm(CCMask);
 }
 
+MachineInstr *SystemZInstrInfo::optimizeLoadInstr(MachineInstr &MI,
+                                                  const MachineRegisterInfo *MRI,
+                                                  Register &FoldAsLoadDefReg,
+                                                  MachineInstr *&DefMI) const {
+  // Check whether we can move the DefMI load, and that it only has one use.
+  DefMI = MRI->getVRegDef(FoldAsLoadDefReg);
+  assert(DefMI);
+  bool SawStore = false;
+  if (!DefMI->isSafeToMove(nullptr, SawStore) ||
+      !MRI->hasOneNonDBGUse(FoldAsLoadDefReg))
+    return nullptr;
+
+  int UseOpIdx = MI.findRegisterUseOperandIdx(FoldAsLoadDefReg);
+  assert(UseOpIdx != -1 && "Expected FoldAsLoadDefReg to be used by MI.");
+  return foldMemoryOperand(MI, {((unsigned) UseOpIdx)}, *DefMI);
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uweigand wrote:

The X86 version clears FoldAsLoadDefReg if sucessful.  Not sure why since common code doesn't appear to rely on that.  But I think we should be consistent here.

https://github.com/llvm/llvm-project/pull/83546