[AArch64] Fix variadic tail-calls on ARM64EC (#79774)

ARM64EC varargs calls expect that x4 = sp at entry, special handling is
needed to ensure this with tail calls since they occur after the
epilogue and the x4 write happens before.

I tried going through AArch64MachineFrameLowering for this, hoping to
avoid creating the dummy object but this was the best I could do since
the stack info that uses isn't populated at this stage,
CreateFixedObject also explicitly forbids 0 sized objects.

Author: bylaws

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -7991,11 +7991,19 @@ AArch64TargetLowering::LowerCall(CallLoweringInfo &CLI,
   }
 
   if (IsVarArg && Subtarget->isWindowsArm64EC()) {
+    SDValue ParamPtr = StackPtr;
+    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);
+      ParamPtr = DAG.getFrameIndex(FI, PtrVT);
+    }
+
     // For vararg calls, the Arm64EC ABI requires values in x4 and x5
     // describing the argument list.  x4 contains the address of the
     // first stack parameter. x5 contains the size in bytes of all parameters
     // passed on the stack.
-    RegsToPass.emplace_back(AArch64::X4, StackPtr);
+    RegsToPass.emplace_back(AArch64::X4, ParamPtr);
     RegsToPass.emplace_back(AArch64::X5,
                             DAG.getConstant(NumBytes, DL, MVT::i64));
   }

llvm/test/CodeGen/AArch64/arm64ec-varargs.ll

@@ -100,5 +100,42 @@ define void @varargs_many_argscalleer() nounwind {
   ret void
 }
 
+define void @varargs_caller_tail() nounwind {
+; CHECK-LABEL: varargs_caller_tail:
+; CHECK:        // %bb.0:
+; CHECK-NEXT:        sub     sp, sp, #48
+; CHECK-NEXT:        mov     x4, sp
+; CHECK-NEXT:        add     x8, sp, #16
+; CHECK-NEXT:        mov     x9, #4617315517961601024        // =0x4014000000000000
+; CHECK-NEXT:        mov     x0, #4607182418800017408        // =0x3ff0000000000000
+; CHECK-NEXT:        mov     w1, #2                          // =0x2
+; CHECK-NEXT:        mov     x2, #4613937818241073152        // =0x4008000000000000
+; CHECK-NEXT:        mov     w3, #4                          // =0x4
+; CHECK-NEXT:        mov     w5, #16                         // =0x10
+; CHECK-NEXT:        stp     xzr, x30, [sp, #24]             // 8-byte Folded Spill
+; CHECK-NEXT:        stp     x9, x8, [sp]
+; CHECK-NEXT:        str     xzr, [sp, #16]
+; CHECK-NEXT:        .weak_anti_dep  varargs_callee
+; CHECK-NEXT:.set varargs_callee, "#varargs_callee"@WEAKREF
+; CHECK-NEXT:        .weak_anti_dep  "#varargs_callee"
+; CHECK-NEXT:.set "#varargs_callee", varargs_callee@WEAKREF
+; CHECK-NEXT:        bl      "#varargs_callee"
+; CHECK-NEXT:        ldr     x30, [sp, #32]                  // 8-byte Folded Reload
+; CHECK-NEXT:        add     x4, sp, #48
+; CHECK-NEXT:        mov     x0, #4607182418800017408        // =0x3ff0000000000000
+; CHECK-NEXT:        mov     w1, #4                          // =0x4
+; CHECK-NEXT:        mov     w2, #3                          // =0x3
+; CHECK-NEXT:        mov     w3, #2                          // =0x2
+; CHECK-NEXT:        mov     x5, xzr
+; CHECK-NEXT:        add     sp, sp, #48
+; CHECK-NEXT:        .weak_anti_dep  varargs_callee
+; CHECK-NEXT:.set varargs_callee, "#varargs_callee"@WEAKREF
+; CHECK-NEXT:        .weak_anti_dep  "#varargs_callee"
+; CHECK-NEXT:.set "#varargs_callee", varargs_callee@WEAKREF
+; CHECK-NEXT:        b       "#varargs_callee"
+  call void (double, ...) @varargs_callee(double 1.0, i32 2, double 3.0, i32 4, double 5.0, <2 x double> <double 0.0, double 0.0>)
+  tail call void (double, ...) @varargs_callee(double 1.0, i32 4, i32 3, i32 2)
+  ret void
+}
 
 declare void @llvm.va_start(ptr)

llvm/test/CodeGen/AArch64/vararg-tallcall.ll

@@ -1,5 +1,6 @@
 ; RUN: llc -mtriple=aarch64-windows-msvc %s -o - | FileCheck %s
 ; RUN: llc -mtriple=aarch64-linux-gnu %s -o - | FileCheck %s
+; RUN: llc -mtriple=arm64ec-windows-msvc %s -o - | FileCheck %s --check-prefixes=CHECK-EC
 ; RUN: llc -global-isel -global-isel-abort=2 -verify-machineinstrs -mtriple=aarch64-windows-msvc %s -o - | FileCheck %s
 ; RUN: llc -global-isel -global-isel-abort=2 -verify-machineinstrs -mtriple=aarch64-linux-gnu %s -o - | FileCheck %s
 
@@ -32,3 +33,10 @@ attributes #1 = { noinline optnone "thunk" }
 ; CHECK: ldr     x9, [x9]
 ; CHECK: mov     v0.16b, v16.16b
 ; CHECK: br      x9
+; CHECK-EC: mov     v7.16b, v0.16b
+; CHECK-EC: ldr     x9, [x0]
+; CHECK-EC: ldr     x11, [x9]
+; CHECK-EC: mov     v0.16b, v7.16b
+; CHECK-EC: add     x4, sp, #64
+; CHECK-EC: add     sp, sp, #64
+; CHECK-EC: br      x11