[llvm] [AArch64] Disable variadic tail-calls for ARM64EC (PR #78914)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-aarch64

Author: Billy Laws (bylaws)

<details>
<summary>Changes</summary>

Arm64EC varargs calls expect that x4 = sp at entry, this complicates tail call handling so disable for now.

CC: @<!-- -->cjacek 

---
Full diff: https://github.com/llvm/llvm-project/pull/78914.diff


2 Files Affected:

- (modified) llvm/lib/Target/AArch64/AArch64ISelLowering.cpp (+5) 
- (modified) llvm/lib/Target/AArch64/GISel/AArch64CallLowering.cpp (+6) 


``````````diff
diff --git a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
index 96ea692d03f563..264c301d81194f 100644
--- a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
+++ b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
@@ -7235,6 +7235,11 @@ bool AArch64TargetLowering::isEligibleForTailCallOptimization(
   const Function &CallerF = MF.getFunction();
   CallingConv::ID CallerCC = CallerF.getCallingConv();
 
+  // Arm64EC varargs calls expect that x4 = sp at entry, this complicates tail
+  // call handling so disable for now.
+  if (IsVarArg && Subtarget->isWindowsArm64EC())
+    return false;
+
   // SME Streaming functions are not eligible for TCO as they may require
   // the streaming mode or ZA to be restored after returning from the call.
   SMEAttrs CallerAttrs(MF.getFunction());
diff --git a/llvm/lib/Target/AArch64/GISel/AArch64CallLowering.cpp b/llvm/lib/Target/AArch64/GISel/AArch64CallLowering.cpp
index 84057ea8d2214a..43a279de035ac7 100644
--- a/llvm/lib/Target/AArch64/GISel/AArch64CallLowering.cpp
+++ b/llvm/lib/Target/AArch64/GISel/AArch64CallLowering.cpp
@@ -909,6 +909,7 @@ bool AArch64CallLowering::isEligibleForTailCallOptimization(
   CallingConv::ID CalleeCC = Info.CallConv;
   MachineFunction &MF = MIRBuilder.getMF();
   const Function &CallerF = MF.getFunction();
+  const AArch64Subtarget &Subtarget = MF.getSubtarget<AArch64Subtarget>();
 
   LLVM_DEBUG(dbgs() << "Attempting to lower call as tail call\n");
 
@@ -926,6 +927,11 @@ bool AArch64CallLowering::isEligibleForTailCallOptimization(
     return false;
   }
 
+  // Arm64EC varargs calls expect that x4 = sp at entry, this complicates tail
+  // call handling so disable for now.
+  if (Info.IsVarArg && Subtarget.isWindowsArm64EC())
+    return false;
+
   // Byval parameters hand the function a pointer directly into the stack area
   // we want to reuse during a tail call. Working around this *is* possible (see
   // X86).

``````````

</details>


https://github.com/llvm/llvm-project/pull/78914