[llvm] Rematerialize load RISCV backend (PR #73910)

[Niwin Anto via llvm-commits]

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@@ -1567,6 +1573,48 @@ bool RISCVInstrInfo::isAsCheapAsAMove(const MachineInstr &MI) const {
   return MI.isAsCheapAsAMove();
 }
 
+bool RISCVInstrInfo::isReallyTriviallyReMaterializable(
+    const MachineInstr &MI) const {
+  if (TargetInstrInfo::isReallyTriviallyReMaterializable(MI))
+    return true;
+
+  const MachineFunction &MF = *MI.getMF();
+  const MachineRegisterInfo &MRI = MF.getRegInfo();
+
+  const MachineOperand &Dest = MI.getOperand(0);
+  if (!MRI.hasOneUse(Dest.getReg()))
+    return false;
+
+  MachineInstr *UseMI = &*MRI.use_instr_begin(Dest.getReg());
+  MachineBasicBlock::const_iterator DefItr(MI);
+  MachineBasicBlock::const_iterator UseItr(UseMI);
+
+  const MachineBasicBlock *MBB = nullptr;
+  if ((MBB = MI.getParent()) != UseMI->getParent())
+    return false;
+
+  // When loading from stack and the stack slot is not modifed before its use,
+  // then materialize this load.
+  int FrameIdx = 0;
+  if (isLoadFromStackSlot(MI, FrameIdx) && AggressiveLoadRemat) {
+    for (; DefItr != UseItr && DefItr != MBB->end(); DefItr++) {
+      int StoreFrameIdx = 0;
+      if ((*DefItr).isCall() || (isStoreToStackSlot(*DefItr, StoreFrameIdx) &&
----------------
niwinanto wrote:

> What if there is store, but we don't know exactly where it stores to? Don't we need to conservatively assume it could write to the slot we're loading?

@topperc I did some mistakes with the PR workflow and it is failing, plan to drop this PR and open new. Will add you to new PR and lets address it there.

https://github.com/llvm/llvm-project/pull/73910