[AArch64] Preserve X8 for thunks ending in variadic musttail calls

Summary: On Windows, X8 may be used to pass in the address of an aggregate that is returned indirectly. Therefore, it should be forwarded to variadic musttail calls and preserved in thunks. Fixes PR41997 Reviewers: mgrang, efriedma Subscribers: javed.absar, kristof.beyls, hiraditya, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D62344 llvm-svn: 361585
llvm · May 24, 2019 · b7a78c7 · b7a78c7
1 parent ed595e8
commit b7a78c7
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Showing 2 changed files with 9 additions and 3 deletions.
diff --git a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
@@ -3207,6 +3207,12 @@ SDValue AArch64TargetLowering::LowerFormalArguments(
                                        FuncInfo->getForwardedMustTailRegParms();
       CCInfo.analyzeMustTailForwardedRegisters(Forwards, RegParmTypes,
                                                CC_AArch64_AAPCS);
+
+      // Conservatively forward X8, since it might be used for aggregate return.
+      if (!CCInfo.isAllocated(AArch64::X8)) {
+        unsigned X8VReg = MF.addLiveIn(AArch64::X8, &AArch64::GPR64RegClass);
+        Forwards.push_back(ForwardedRegister(X8VReg, AArch64::X8, MVT::i64));
+      }
     }
   }
 

diff --git a/llvm/test/CodeGen/AArch64/vararg-tallcall.ll b/llvm/test/CodeGen/AArch64/vararg-tallcall.ll
@@ -28,7 +28,7 @@ entry:
 attributes #1 = { noinline optnone "thunk" }
 
 ; CHECK: mov     v16.16b, v0.16b
-; CHECK: ldr     x8, [x0]
-; CHECK: ldr     x8, [x8]
+; CHECK: ldr     x9, [x0]
+; CHECK: ldr     x9, [x9]
 ; CHECK: mov     v0.16b, v16.16b
-; CHECK: br      x8
+; CHECK: br      x9