callconv(.Fastcall) broken on i386-windows

[drew-gpf]

Zig Version

0.9.0-dev.1945+efdb94486

Steps to Reproduce

test.zig:

const std = @import("std");
extern fn call_zig(function: usize) callconv(.Fastcall) void;

fn fastcall(ecx: u32, edx: u32) callconv(.Fastcall) void {
    std.debug.print("{}, {}\n", .{ecx, edx});
}

pub fn main() void {
    @call(.{ .modifier = .never_inline }, call_zig, .{@ptrToInt(fastcall)});
}

test.c:

#include <stdint.h>

typedef void(__fastcall*zig_exported_fn)(uint32_t ecx, uint32_t edx);

void __fastcall call_zig(zig_exported_fn fn)
{
    fn(56, 1337);
}

Build with: zig build-exe --strip -OReleaseSmall -target i386-windows test.zig test.c (stripped and made small because IDA Pro likes to crash when loading a PDB generated by zig)

Expected Behavior

This should produce test.exe which prints "56, 1337" when ran.

Actual Behavior

This will not link:

lld-link: error: undefined symbol: @call_zig@4
>>> referenced by test.exe.lto.obj:(_wWinMainCRTStartup@0)
error: LLDReportedFailure

zig will correctly mangle call_zig's name with a leading @ to mean fastcall, but it miscalculates the number of (stack) bytes needed for arguments, as the first argument should be present in the ecx register and not in a stack location. It seems to be handling arguments as if it were stdcall but still doing name mangling for fastcall.

Removing __fastcall and callconv(.Fastcall) from call_zig will link but yield an incorrect (undefined) output (ex. 0, 0) because fastcall() will read the arguments ecx and edx from stack locations--call_zig() in test.c will instead correctly pass them in their corresponding registers, ecx and edx.

When not used in a situation where link errors are involved - I ran into this while using MS Detours - it either causes something to crash because it pops too many arg bytes off the stack or it just behaves incorrectly. Conveniently for me it turned out the C code being replaced only used __fastcall to emulate __thiscall, which works correctly with callconv(.Thiscall).

[LemonBoy]

Good catch. Zig must specify inreg for the function parameters when applicable.