[llvm] [RISCV] Rematerialize load (PR #73924)

[Niwin Anto via llvm-commits]

================
@@ -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 modified 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:

@topperc Copied from stale PR, `_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?_`

https://github.com/llvm/llvm-project/pull/73924