LLVM: fix missing alignment on wrapping instructions

Previously, when lowering AIR instructions `wrap_errunion_payload`,
`wrap_errunion_err`, and `wrap_optional`, the LLVM backend would create
an alloca instruction to store the result, but did not set the alignment
on it. This caused UB which went undetected for a long time until we
started enabling the stack protector.

Closes #12594
Unblocks #12508
Inspires #12634

Tests passed locally:
 * test-behavior
 * test-cases

3 files changed, 39 insertions, 3 deletions

diff --git a/src/codegen/llvm.zig b/src/codegen/llvm.zig
index d872057beb..d1c68b430c 100644
--- a/src/codegen/llvm.zig
+++ b/src/codegen/llvm.zig
@@ -6188,7 +6188,9 @@ pub const FuncGen = struct {
         }
         const llvm_optional_ty = try self.dg.lowerType(optional_ty);
         if (isByRef(optional_ty)) {
+            const target = self.dg.module.getTarget();
             const optional_ptr = self.buildAlloca(llvm_optional_ty);
+            optional_ptr.setAlignment(optional_ty.abiAlignment(target));
             const payload_ptr = self.builder.buildStructGEP(optional_ptr, 0, "");
             var ptr_ty_payload: Type.Payload.ElemType = .{
                 .base = .{ .tag = .single_mut_pointer },
@@ -6208,20 +6210,21 @@ pub const FuncGen = struct {
         if (self.liveness.isUnused(inst)) return null;
 
         const ty_op = self.air.instructions.items(.data)[inst].ty_op;
-        const inst_ty = self.air.typeOfIndex(inst);
+        const err_un_ty = self.air.typeOfIndex(inst);
         const operand = try self.resolveInst(ty_op.operand);
         const payload_ty = self.air.typeOf(ty_op.operand);
         if (!payload_ty.hasRuntimeBitsIgnoreComptime()) {
             return operand;
         }
         const ok_err_code = (try self.dg.lowerType(Type.anyerror)).constNull();
-        const err_un_llvm_ty = try self.dg.lowerType(inst_ty);
+        const err_un_llvm_ty = try self.dg.lowerType(err_un_ty);
 
         const target = self.dg.module.getTarget();
         const payload_offset = errUnionPayloadOffset(payload_ty, target);
         const error_offset = errUnionErrorOffset(payload_ty, target);
-        if (isByRef(inst_ty)) {
+        if (isByRef(err_un_ty)) {
             const result_ptr = self.buildAlloca(err_un_llvm_ty);
+            result_ptr.setAlignment(err_un_ty.abiAlignment(target));
             const err_ptr = self.builder.buildStructGEP(result_ptr, error_offset, "");
             const store_inst = self.builder.buildStore(ok_err_code, err_ptr);
             store_inst.setAlignment(Type.anyerror.abiAlignment(target));
@@ -6256,6 +6259,7 @@ pub const FuncGen = struct {
         const error_offset = errUnionErrorOffset(payload_ty, target);
         if (isByRef(err_un_ty)) {
             const result_ptr = self.buildAlloca(err_un_llvm_ty);
+            result_ptr.setAlignment(err_un_ty.abiAlignment(target));
             const err_ptr = self.builder.buildStructGEP(result_ptr, error_offset, "");
             const store_inst = self.builder.buildStore(operand, err_ptr);
             store_inst.setAlignment(Type.anyerror.abiAlignment(target));
diff --git a/test/behavior/error.zig b/test/behavior/error.zig
index b355c85819..684b01a797 100644
--- a/test/behavior/error.zig
+++ b/test/behavior/error.zig
@@ -796,3 +796,19 @@ test "error union of noreturn used with catch" {
     const err = NoReturn.testCatch();
     try expect(err == error.OtherFailure);
 }
+
+test "alignment of wrapping an error union payload" {
+    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest;
+
+    const S = struct {
+        const I = extern struct { x: i128 };
+
+        fn foo() anyerror!I {
+            var i: I = .{ .x = 1234 };
+            return i;
+        }
+    };
+    try expect((S.foo() catch unreachable).x == 1234);
+}
diff --git a/test/behavior/optional.zig b/test/behavior/optional.zig
index 28261daf1f..eb693147e6 100644
--- a/test/behavior/optional.zig
+++ b/test/behavior/optional.zig
@@ -412,3 +412,19 @@ test "orelse on C pointer" {
     const d = foo orelse @compileError("bad");
     try expectEqual([*c]const u8, @TypeOf(d));
 }
+
+test "alignment of wrapping an optional payload" {
+    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest;
+
+    const S = struct {
+        const I = extern struct { x: i128 };
+
+        fn foo() ?I {
+            var i: I = .{ .x = 1234 };
+            return i;
+        }
+    };
+    try expect(S.foo().?.x == 1234);
+}