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

[via llvm-commits]

================
@@ -1504,12 +1513,72 @@ void PPCAsmPrinter::emitInstruction(const MachineInstr *MI) {
     // Verify alignment is legal, so we don't create relocations
     // that can't be supported.
     unsigned OpNum = (MI->getOpcode() == PPC::STD) ? 2 : 1;
+    // For non-TOC-based local-exec TLS accesses with non-zero offsets, the
+    // machine operand (which is a TargetGlobalTLSAddress) is expected to be
+    // the same operand for both loads and stores.
+    for (const MachineOperand &TempMO : MI->operands()) {
+      if (((TempMO.getTargetFlags() == PPCII::MO_TPREL_FLAG)) &&
+          TempMO.getOperandNo() == 1)
+        OpNum = 1;
+    }
     const MachineOperand &MO = MI->getOperand(OpNum);
     if (MO.isGlobal()) {
       const DataLayout &DL = MO.getGlobal()->getParent()->getDataLayout();
       if (MO.getGlobal()->getPointerAlignment(DL) < 4)
         llvm_unreachable("Global must be word-aligned for LD, STD, LWA!");
     }
+    // As these load/stores share common code with the following load/stores,
+    // fall through to the subsequent cases in order to either process the
+    // non-TOC-based local-exec sequence or to process the instruction normally.
+    [[fallthrough]];
+  }
+  case PPC::LBZ:
+  case PPC::LBZ8:
+  case PPC::LHA:
+  case PPC::LHA8:
+  case PPC::LHZ:
+  case PPC::LHZ8:
+  case PPC::LWZ:
+  case PPC::LWZ8:
+  case PPC::STB:
+  case PPC::STB8:
+  case PPC::STH:
+  case PPC::STH8:
+  case PPC::STW:
+  case PPC::STW8:
+  case PPC::LFS:
+  case PPC::STFS:
+  case PPC::LFD:
+  case PPC::STFD:
+  case PPC::ADDI8: {
+    // A faster non-TOC-based local-exec sequence is represented by `addi`
----------------
diggerlin wrote:

Nit: for performance, I  will suggest that put following code here.

```
if(!HasAIXSmallLocalExecTLS) 
   break;
```

instead of assert later

```
  assert(HasAIXSmallLocalExecTLS &&
             "addi, or load/stores with thread-pointer only expected with "
             "local-exec small TLS");
```

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