[llvm] [X86] When expanding LCMPXCHG16B_SAVE_RBX, substitute RBX in base (PR #134109)

[Aaron Puchert via llvm-commits]

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@@ -439,8 +439,18 @@ bool X86ExpandPseudo::expandMI(MachineBasicBlock &MBB,
     TII->copyPhysReg(MBB, MBBI, DL, X86::RBX, InArg.getReg(), false);
     // Create the actual instruction.
     MachineInstr *NewInstr = BuildMI(MBB, MBBI, DL, TII->get(X86::LCMPXCHG16B));
-    // Copy the operands related to the address.
-    for (unsigned Idx = 1; Idx < 6; ++Idx)
+    // Copy the operands related to the address. If we access a frame variable,
+    // we need to replace the RBX base with SaveRbx, as RBX has another value.
+    const MachineOperand &Base = MBBI->getOperand(1);
+    if (Base.getReg() == X86::RBX || Base.getReg() == X86::EBX)
+      NewInstr->addOperand(MachineOperand::CreateReg(
+          Base.getReg() == X86::RBX
+              ? SaveRbx
+              : Register(TRI->getSubReg(SaveRbx, X86::sub_32bit)),
+          /*IsDef=*/false));
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aaronpuchert wrote:

Thought about that. I ended up writing it like this to factor out the common `NewInstr->addOperand(MachineOperand::CreateReg(...), /*IsDef=*/false))`, which would otherwise be duplicated. (Just the register is different.)

But if you think I should spell it out that's fine for me as well.

https://github.com/llvm/llvm-project/pull/134109