[llvm] [AArch64] Implement -fno-plt for SelectionDAG/GlobalISel (PR #78890)

[Fangrui Song via llvm-commits]

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@@ -1293,8 +1293,19 @@ bool AArch64CallLowering::lowerCall(MachineIRBuilder &MIRBuilder,
            !Subtarget.noBTIAtReturnTwice() &&
            MF.getInfo<AArch64FunctionInfo>()->branchTargetEnforcement())
     Opc = AArch64::BLR_BTI;
-  else
+  else {
+    // For an intrinsic call (e.g. memset), use GOT if "RtLibUseGOT" (-fno-plt)
+    // is set.
+    if (Info.Callee.isSymbol() && F.getParent()->getRtLibUseGOT()) {
+      auto Reg =
+          MRI.createGenericVirtualRegister(getLLTForType(*F.getType(), DL));
+      auto MIB = MIRBuilder.buildInstr(TargetOpcode::G_GLOBAL_VALUE);
+      DstOp(Reg).addDefToMIB(MRI, MIB);
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MaskRay wrote:

`buildInstr(TargetOpcode::G_GLOBAL_VALUE)` cannot be CSEed today, and I think this patch should not change it.

It seems that only the `buildInstr` overload with `DstOps/SrcOps` can perform CSE. `MachineIRBuilder::buildGlobalValue` does not call this overload.

https://github.com/llvm/llvm-project/pull/78890