[llvm] [ARM] support -mlong-calls -fPIC on arm32 #39970 (PR #147313)

[via llvm-commits]

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@@ -2839,7 +2846,8 @@ ARMTargetLowering::LowerCall(TargetLowering::CallLoweringInfo &CLI,
             PtrVt, dl, DAG.getEntryNode(), Addr,
             MachinePointerInfo::getConstantPool(DAG.getMachineFunction()));
       }
-    } else if (ExternalSymbolSDNode *S=dyn_cast<ExternalSymbolSDNode>(Callee)) {
+    } else if (ExternalSymbolSDNode *S =
+                   dyn_cast<ExternalSymbolSDNode>(Callee)) {
       const char *Sym = S->getSymbol();
 
       if (Subtarget->genExecuteOnly()) {
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jiangxuezhi wrote:

Thank you for your suggestion. I have attempted to make the external-symbol code path work with PIC.

During this process, I found that using ARMISD::WrapperPIC caused issues in generating correct SDValues for certain symbols. For example, the following error was triggered:

```
LLVM ERROR: Cannot select: t20: i32 = ARMISD::WrapperPIC TargetExternalSymbol:i32 'memset' [TF=8]
```
This suggests that using the GOT-based approach for PIC is not suitable in this case.

To address this, I instead used the ARMISD::PIC_ADD node to compute PC-relative addresses, which allows proper handling of external symbols under PIC.

In addition, I have added test cases for llvm.memset in: 

llvm-project/llvm/test/CodeGen/ARM/subtarget-features-long-calls.ll

Please help review this change — I'd appreciate your feedback on whether this implementation is appropriate.

https://github.com/llvm/llvm-project/pull/147313