Sema: improve handling of always_tail call modifier

Closes #4301
Closes #5692
Closes #6281
Closes #10786
Closes #11149
Closes #11776

3 files changed, 84 insertions, 0 deletions

diff --git a/test/behavior/call.zig b/test/behavior/call.zig
index eafd2ef4e9..4c697ed542 100644
--- a/test/behavior/call.zig
+++ b/test/behavior/call.zig
@@ -261,3 +261,57 @@ test "arguments to comptime parameters generated in comptime blocks" {
     };
     S.foo(S.fortyTwo());
 }
+
+test "forced tail call" {
+    if (builtin.zig_backend == .stage1) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_c) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
+
+    const S = struct {
+        fn fibonacciTailInternal(n: u16, a: u16, b: u16) u16 {
+            if (n == 0) return a;
+            if (n == 1) return b;
+            return @call(
+                .{ .modifier = .always_tail },
+                fibonacciTailInternal,
+                .{ n - 1, b, a + b },
+            );
+        }
+
+        fn fibonacciTail(n: u16) u16 {
+            return fibonacciTailInternal(n, 0, 1);
+        }
+    };
+    try expect(S.fibonacciTail(10) == 55);
+}
+
+test "inline call preserves tail call" {
+    if (builtin.zig_backend == .stage1) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_c) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
+
+    const max = std.math.maxInt(u16);
+    const S = struct {
+        var a: u16 = 0;
+        fn foo() void {
+            return bar();
+        }
+
+        inline fn bar() void {
+            if (a == max) return;
+            // Stack overflow if not tail called
+            var buf: [max]u16 = undefined;
+            buf[a] = a;
+            a += 1;
+            return @call(.{ .modifier = .always_tail }, foo, .{});
+        }
+    };
+    S.foo();
+    try expect(S.a == std.math.maxInt(u16));
+}
diff --git a/test/cases/compile_errors/invalid_tail_call.zig b/test/cases/compile_errors/invalid_tail_call.zig
new file mode 100644
index 0000000000..cdeb9df930
--- /dev/null
+++ b/test/cases/compile_errors/invalid_tail_call.zig
@@ -0,0 +1,12 @@
+fn myFn(_: usize) void {
+    return;
+}
+pub export fn entry() void {
+    @call(.{ .modifier = .always_tail }, myFn, .{0});
+}
+
+// error
+// backend=llvm
+// target=native
+//
+// :5:5: error: unable to perform tail call: type of function being called 'fn(usize) void' does not match type of calling function 'fn() callconv(.C) void'
diff --git a/test/cases/taill_call_noreturn.zig b/test/cases/taill_call_noreturn.zig
new file mode 100644
index 0000000000..0c2497d6ce
--- /dev/null
+++ b/test/cases/taill_call_noreturn.zig
@@ -0,0 +1,18 @@
+const std = @import("std");
+const builtin = std.builtin;
+pub fn foo(message: []const u8, stack_trace: ?*builtin.StackTrace) noreturn {
+    @call(.{ .modifier = .always_tail }, bar, .{ message, stack_trace });
+}
+pub fn bar(message: []const u8, stack_trace: ?*builtin.StackTrace) noreturn {
+    _ = message;
+    _ = stack_trace;
+    std.process.exit(0);
+}
+
+pub fn main() void {
+    foo("foo", null);
+}
+
+// run
+// backend=llvm
+// target=native