[llvm] [RegAllocFast] fold foldable inline asm (PR #74344)

[Nick Desaulniers via llvm-commits]

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@@ -1626,6 +1637,69 @@ void RegAllocFast::allocateBasicBlock(MachineBasicBlock &MBB) {
   LLVM_DEBUG(MBB.dump());
 }
 
+void RegAllocFast::preemptivelySpillInlineAsmOperands(MachineInstr *MI) {
+  assert(MI->isInlineAsm() && "should only be used by inline asm");
+  // BOTH MI and its number of operands may change in this loop; cache neither.
+  for (unsigned I = InlineAsm::MIOp_FirstOperand; I < MI->getNumOperands();
+       ++I) {
+    MachineOperand &MO = MI->getOperand(I);
+    if (!(MO.isReg() && MI->mayFoldInlineAsmRegOp(I)))
+      continue;
+
+    const bool IsDef = MO.isDef();
+    const bool IsUse = MO.isUse();
+    const bool IsKill = MO.isKill();
+    const MachineOperand *TiedOp = nullptr;
+
+    if (MO.isTied())
+      if (MachineOperand *T = &MI->getOperand(MI->findTiedOperandIdx(I)))
+        if (T->isUse())
+          TiedOp = T;
+
+    Register Reg = MO.getReg();
+    const bool IsVirt = Reg.isVirtual();
+    const TargetRegisterClass *RC =
+        IsVirt ? MRI->getRegClass(Reg) : TRI->getMinimalPhysRegClass(Reg);
+    int FrameIndex = IsVirt
+                         ? getStackSpaceFor(Reg)
+                         : MFI->CreateSpillStackObject(TRI->getSpillSize(*RC),
+                                                       TRI->getSpillAlign(*RC));
+
+    MachineInstr *NewMI = TII->foldMemoryOperand(*MI, {I}, FrameIndex);
+    // foldMemoryOperand always puts the new instruction before its first param.
+    // Here, we want the newly created replacement to go after the old one, so
+    // that we don't attempt to regalloc it again.
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nickdesaulniers wrote:

This comment is stale, but I'll remove it once I get additional feedback from reviewers.

https://github.com/llvm/llvm-project/pull/74344