Aarch64: Emit a minimal SEH prologue when needed (#158173)

In some cases, with very simple thunks, it is possible that the
`.seh_endprologue` is not emitted. This causes issues in the assembler
because the epilogue ends up starting before the prologue has ended.

Bug: swiftlang#11377

Author: Steelskin

llvm/lib/Target/AArch64/AArch64PrologueEpilogue.cpp

@@ -541,6 +541,13 @@ void AArch64PrologueEmitter::emitPrologue() {
   // to determine the end of the prologue.
   DebugLoc DL;
 
+  // In some cases, particularly with CallingConv::SwiftTail, it is possible to
+  // have a tail-call where the caller only needs to adjust the stack pointer in
+  // the epilogue. In this case, we still need to emit a SEH prologue sequence.
+  // See `seh-minimal-prologue-epilogue.ll` test cases.
+  if (AFI->getArgumentStackToRestore())
+    HasWinCFI = true;
+
   if (AFI->shouldSignReturnAddress(MF)) {
     // If pac-ret+leaf is in effect, PAUTH_PROLOGUE pseudo instructions
     // are inserted by emitPacRetPlusLeafHardening().

llvm/test/CodeGen/AArch64/seh-minimal-prologue-epilogue.ll

@@ -0,0 +1,53 @@
+; RUN: llc -mtriple=aarch64-windows %s -o - | FileCheck %s
+
+; This test verifies that functions requiring Windows CFI that have minimal
+; or no prologue instructions still emit proper SEH directives, specifically
+; ensuring .seh_endprologue is emitted before .seh_startepilogue.
+;
+; This reproduces the issue where Swift async functions with swifttailcc
+; calling convention would fail with:
+; "error: starting epilogue (.seh_startepilogue) before prologue has ended (.seh_endprologue)"
+
+; Test 1: Swift-style tail call function with minimal prologue
+define swifttailcc void @test_swifttailcc_minimal(ptr %async_ctx, ptr %arg1, ptr %arg2) {
+; CHECK-LABEL: test_swifttailcc_minimal:
+; CHECK-NOT:   .seh_proc test_swifttailcc_minimal
+; CHECK-NOT:   .seh_endprologue
+; CHECK-NOT:   .seh_startepilogue
+; CHECK-NOT:   .seh_endepilogue
+; CHECK-NOT:   .seh_endproc
+entry:
+  %ptr1 = getelementptr inbounds i8, ptr %async_ctx, i64 16
+  %ptr2 = getelementptr inbounds i8, ptr %async_ctx, i64 24
+  store ptr %arg1, ptr %ptr1, align 8
+  store ptr %arg2, ptr %ptr2, align 8
+  musttail call swifttailcc void @external_swift_function(ptr %async_ctx, ptr %arg1)
+  ret void
+}
+
+; Test 2: Function similar to the original failing case
+define linkonce_odr hidden swifttailcc void @test_linkonce_swifttailcc(ptr swiftasync %async_ctx, ptr %arg1, ptr noalias dereferenceable(40) %arg2, ptr %arg3, i64 %value, ptr %arg4, ptr %arg5, ptr %arg6, i1 %flag, ptr %arg7, ptr noalias dereferenceable(40) %arg8) {
+; CHECK-LABEL: test_linkonce_swifttailcc:
+; CHECK-NEXT:  .seh_proc
+; CHECK:       .seh_endprologue
+; CHECK:       .seh_startepilogue
+; CHECK:       .seh_endepilogue
+; CHECK:       .seh_endproc
+entry:
+  %frame_ptr = getelementptr inbounds nuw i8, ptr %async_ctx, i64 16
+  %ctx1 = getelementptr inbounds nuw i8, ptr %async_ctx, i64 400
+  %ctx2 = getelementptr inbounds nuw i8, ptr %async_ctx, i64 1168
+  %spill1 = getelementptr inbounds nuw i8, ptr %async_ctx, i64 2392
+  store ptr %arg8, ptr %spill1, align 8
+  %spill2 = getelementptr inbounds nuw i8, ptr %async_ctx, i64 2384
+  store ptr %arg7, ptr %spill2, align 8
+  %spill3 = getelementptr inbounds nuw i8, ptr %async_ctx, i64 2225
+  store i1 %flag, ptr %spill3, align 1
+  %spill4 = getelementptr inbounds nuw i8, ptr %async_ctx, i64 2376
+  store ptr %arg6, ptr %spill4, align 8
+  musttail call swifttailcc void @external_swift_continuation(ptr swiftasync %async_ctx, i64 0, i64 0)
+  ret void
+}
+
+declare swifttailcc void @external_swift_function(ptr, ptr)
+declare swifttailcc void @external_swift_continuation(ptr swiftasync, i64, i64)

llvm/test/CodeGen/AArch64/wincfi-seh-only-in-epilogue.ll renamed to llvm/test/CodeGen/AArch64/wincfi-minimal-seh-prologue.ll

@@ -5,8 +5,9 @@ entry:
   ret void
 }
 
-; Check that there is no .seh_endprologue but there is seh_startepilogue/seh_endepilogue.
-; CHECK-NOT: .seh_endprologue
+; Check that there is a minimal SEH prologue with seh_startepilogue/seh_endepilogue.
+; CHECK:     .seh_proc test
+; CHECK:     .seh_endprologue
 ; CHECK:     .seh_startepilogue
 ; CHECK:     add sp, sp, #48
 ; CHECK:     .seh_stackalloc 48