[llvm] [PowerPC] Peephole address calculation in TOC memops (PR #76488)

[Sean Fertile via llvm-commits]

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@@ -7662,241 +7662,212 @@ static void foldADDIForLocalExecAccesses(SDNode *N, SelectionDAG *DAG) {
     DAG->RemoveDeadNode(InitialADDI.getNode());
 }
 
-void PPCDAGToDAGISel::PeepholePPC64() {
-  SelectionDAG::allnodes_iterator Position = CurDAG->allnodes_end();
-  bool HasAIXSmallLocalExecTLS = Subtarget->hasAIXSmallLocalExecTLS();
-
-  while (Position != CurDAG->allnodes_begin()) {
-    SDNode *N = &*--Position;
-    // Skip dead nodes and any non-machine opcodes.
-    if (N->use_empty() || !N->isMachineOpcode())
-      continue;
-
-    if (isVSXSwap(SDValue(N, 0)))
-      reduceVSXSwap(N, CurDAG);
-
-    // This optimization is performed for non-TOC-based local-exec accesses.
-    if (HasAIXSmallLocalExecTLS)
-      foldADDIForLocalExecAccesses(N, CurDAG);
+static bool isValidOffsetMemOp(SDNode *N, bool &IsLoad,
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mandlebug wrote:

We seems to be doing multiple things with this helper function:
1) Determine if the current machine node is a valid memory operation that we could forward a tac-operand to.
2) Determine if said memory operation is a load or a store
3) Determine if the memory operation has a stricter requirement on the offset that can be encoded - FWIW I belive changing the name from `RequiresMod4Offset` to `ExtraAlign` now obfuscates the fact it an encoding limitation and really an alignment limitation.
4) Determine if the toc operation that feeds the memory operation is a valid one to forward from.

While it will make the code bigger and likely duplicate the switch statement, I think if we split this into separate appropriately named helpers it will make this code easier to understand.

We could use 
- `isValidMemoryOperation(N->getOpcode())` that returns true or false, 
- `encodingModulous(N->getOpcode())` that returns either 4 or 1 as an unsigned value.
- can we reliably use `MachineInstr::mayLoad()` to determine if the node is a load or a store?
- `isValidForwardingTocoperation(Base->getOpcode())` to determine if the feeding operation is one that can be forwarded from.

https://github.com/llvm/llvm-project/pull/76488