[llvm] [AIX][TLS] Optimize the small local-exec access sequence for non-zero offsets (PR #71485)

[Amy Kwan via llvm-commits]

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@@ -7567,8 +7567,63 @@ static void reduceVSXSwap(SDNode *N, SelectionDAG *DAG) {
   DAG->ReplaceAllUsesOfValueWith(SDValue(N, 0), N->getOperand(0));
 }
 
+// For non-TOC-based local-exec access where an addi is feeding into another
+// addi, fold this sequence into a single addi if possible.
+static void foldADDIForLocalExecAccesses(SDNode *N, SelectionDAG *DAG) {
+  const PPCSubtarget &Subtarget =
+      DAG->getMachineFunction().getSubtarget<PPCSubtarget>();
+  // This optimization is only performed for non-TOC-based local-exec accesses.
+  if (!Subtarget.hasAIXSmallLocalExecTLS())
+    return;
+
+  if (N->getMachineOpcode() != PPC::ADDI8)
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amy-kwan wrote:

I think `PPC::LA8` can only be produced at assembly printing time and not during instruction selection. Thus, I think checking for `ADDI8` here is sufficient. This is because I see places in assembly printing where we set the opcode of the `ADDI8` to be `LA8` specifically.

Furthermore, there is a pattern (`(set i64:$RST, (add i64:$RA, (PPClo tglobaladdr:$D, 0)))`) for the `LA8` case but I don't think this pattern is really being used as removing this pattern has no effect.

https://github.com/llvm/llvm-project/pull/71485