[STATEPOINT] Mark LR is early-clobber implicit def.

[skatkov]

Summary

LR is modified at the moment of the call and before any use is read.

[reames]

LGTM

This must have been fun to debug...

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

#endif
     llvm_unreachable("Unexpected instruction for custom inserter!");
 
   case AArch64::F128CSEL:
     return EmitF128CSEL(MI, BB);
 
   case TargetOpcode::STATEPOINT:
     // STATEPOINT is a pseudo instruction which has no implicit defs/uses
     // while bl call instruction (where statepoint will be lowered at the end)
-    // has implicit def. Add this implicit dead def here as a workaround.
-    MI.addOperand(*MI.getMF(), MachineOperand::CreateReg(AArch64::LR, true,
-                                                         true, false, true));
+    // has implicit def. This def is early-clobber as it will be set at
+    // the moment of the call and earlier than any use is read.
+    // Add this implicit dead def here as a workaround.
+    MI.addOperand(*MI.getMF(),
+                  MachineOperand::CreateReg(
+                      AArch64::LR, /*isDef*/ true,
+                      /*isImp*/ true, /*isKill*/ false, /*isDead*/ true,
+                      /*isUndef*/ false, /*isEarlyClobber*/ true));
     LLVM_FALLTHROUGH;
   case TargetOpcode::STACKMAP:
   case TargetOpcode::PATCHPOINT:
     return emitPatchPoint(MI, BB);
 
   case AArch64::CATCHRET:
     return EmitLoweredCatchRet(MI, BB);
   }

llvm/test/CodeGen/AArch64/statepoint-call-lowering-lr.ll

+; RUN: llc -mtriple aarch64-none-linux-gnu -verify-machineinstrs -stop-after=prologepilog < %s | FileCheck %s
+
+; Check that STATEPOINT instruction has an early clobber implicit def for LR.
+target datalayout = "e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "aarch64-unknown-linux-gnu"
+
+define void @test() "frame-pointer"="all" gc "statepoint-example" {
+entry:
+  %safepoint_token = tail call token (i64, i32, void ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_i1f(i64 0, i32 0, void ()* elementtype(void ()) @return_i1, i32 0, i32 0, i32 0, i32 0) ["gc-live" ()]
+; CHECK: STATEPOINT 0, 0, 0, @return_i1, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, 2, 0, csr_aarch64_aapcs, implicit-def $sp, implicit-def dead early-clobber $lr
+  ret void
+}
+
+
+declare void @return_i1()
+declare token @llvm.experimental.gc.statepoint.p0f_i1f(i64, i32, void ()*, i32, i32, ...)