[llvm] [AArch64] Skip over shadow space for ARM64EC entry thunk variadic calls (PR #80994)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-aarch64

Author: Billy Laws (bylaws)

<details>
<summary>Changes</summary>

When in an entry thunk the x64 SP is passed in x4 but this cannot be directly passed through since x64 varargs calls have a 32 byte shadow store at SP followed by the in-stack parameters. ARM64EC varargs calls on the other hand expect x4 to point to the first in-stack parameter.

CC: @<!-- -->efriedma-quic @<!-- -->cjacek @<!-- -->mstorsjo 

---
Full diff: https://github.com/llvm/llvm-project/pull/80994.diff


2 Files Affected:

- (modified) llvm/lib/Target/AArch64/AArch64ISelLowering.cpp (+11-1) 
- (modified) llvm/test/CodeGen/AArch64/arm64ec-entry-thunks.ll (+1-1) 


``````````diff
diff --git a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
index 8573939b04389..b7e1c96c23728 100644
--- a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
+++ b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
@@ -8010,7 +8010,17 @@ AArch64TargetLowering::LowerCall(CallLoweringInfo &CLI,
 
   if (IsVarArg && Subtarget->isWindowsArm64EC()) {
     SDValue ParamPtr = StackPtr;
-    if (IsTailCall) {
+    if (MF.getFunction().getCallingConv() == CallingConv::ARM64EC_Thunk_X64) {
+      // When in an entry thunk the x64 SP is passed via x4. This cannot
+      // be directly passed through since x64 varargs calls have a 32 byte
+      // shadow store at SP followed by the in-stack parameters,
+      // Arm64EC varargs calls on the other hand expect x4
+      // to point to the first in-stack parameter.
+      Register VReg = MF.addLiveIn(AArch64::X4, &AArch64::GPR64RegClass);
+      SDValue Val = DAG.getCopyFromReg(Chain, DL, VReg, MVT::i64);
+      SDValue PtrOff = DAG.getIntPtrConstant(32, DL);
+      ParamPtr = DAG.getNode(ISD::ADD, DL, PtrVT, Val, PtrOff);
+    } else if (IsTailCall) {
       // Create a dummy object at the top of the stack that can be used to get
       // the SP after the epilogue
       int FI = MF.getFrameInfo().CreateFixedObject(1, FPDiff, true);
diff --git a/llvm/test/CodeGen/AArch64/arm64ec-entry-thunks.ll b/llvm/test/CodeGen/AArch64/arm64ec-entry-thunks.ll
index 0083818def151..bb9ba05f7a272 100644
--- a/llvm/test/CodeGen/AArch64/arm64ec-entry-thunks.ll
+++ b/llvm/test/CodeGen/AArch64/arm64ec-entry-thunks.ll
@@ -147,7 +147,7 @@ define void @has_varargs(...) nounwind {
 ; CHECK-NEXT:     add     x29, sp, #160
 ; CHECK-NEXT:     .seh_add_fp     160
 ; CHECK-NEXT:     .seh_endprologue
-; CHECK-NEXT:     mov     x4, sp
+; CHECK-NEXT:     add     x4, x4, #32
 ; CHECK-NEXT:     mov     x5, xzr
 ; CHECK-NEXT:     blr     x9
 ; CHECK-NEXT:     adrp    x8, __os_arm64x_dispatch_ret

``````````

</details>


https://github.com/llvm/llvm-project/pull/80994