[llvm] [AArch64] Define constructive EXT_ZZZI pseudo instruction (PR #152552)

[Paul Walker via llvm-commits]

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@@ -639,10 +644,20 @@ bool AArch64ExpandPseudo::expand_DestructiveOp(
           .addImm(0);
     }
   } else if (DstReg != MI.getOperand(DOPIdx).getReg()) {
-    assert(DOPRegIsUnique && "The destructive operand should be unique");
-    PRFX = BuildMI(MBB, MBBI, MI.getDebugLoc(), TII->get(MovPrfx))
-               .addReg(DstReg, RegState::Define)
-               .addReg(MI.getOperand(DOPIdx).getReg(), DOPRegState);
+    if (DOPRegIsUnique) {
+      PRFX = BuildMI(MBB, MBBI, MI.getDebugLoc(), TII->get(MovPrfx))
+                 .addReg(DstReg, RegState::Define)
+                 .addReg(MI.getOperand(DOPIdx).getReg(), DOPRegState);
+    } else {
+      // MOVPRFX requires unique operands: Just build a COPY (Using ORR directly
+      // as we are past PostRAPseudo expansion).
+      assert(DType == AArch64::DestructiveRegRegImmUnpred &&
+             "Unexpected destructive operation type");
+      PRFX = BuildMI(MBB, MBBI, MI.getDebugLoc(), TII->get(AArch64::ORR_ZZZ))
----------------
paulwalker-arm wrote:

What if `DstReg` is not unique? I think this will corrupt the `SrcIdx` operand for the case `A = EXT_ZZZI, B, A, Imm`.

There's an assumption with the pseudo expansion that if `DstReg` must be written to, then they'll always be a way to do this with `MOVPFRX` and thus is can be done without corrupting any of the inputs.

This is the reason we never completed the rollout to cover all instructions.  Ideally we want a way to restrict the isel. I have an old patch where I tried to introduce the concept of a late read but never managed to finish it (it was quite risky).

In this instance I don't think you care about a fully general `EXT_ZZZI`? but rather just want a pseudo to represent `AArch64ext A, A, Imm` (i.e. a pseudo that takes a single register operand) that can be expanded to either  `MOVPFRX B, A; EXT_ZZI B, B, Imm` or `EXT_ZZI A, A, Imm` depending on whether the destination is unique or not.

https://github.com/llvm/llvm-project/pull/152552