Selfhosted x86_64 backend hits unreachable code when passing sliced array of structs with field that has type "type" #30602

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opened 2025-12-27 15:32:21 +01:00 by Vidar · 0 comments

Vidar commented

2025-12-27 15:32:21 +01:00

Zig Version

0.16.0-dev.1854+e55e6b552

Steps to Reproduce and Observed Behavior

repro.zig:

const Foo = struct {
    type: type,
};

fn bar(a: []const Foo) void {
    _ = a;
}

pub fn main() void {
    const foos = [1]Foo{.{
        .type = u8,
    }};
    bar(foos[1..]);
}

❯ ../../zig/build/stage3/bin/zig run repro.zig
Compiler crash context:
(no context)

thread 1052961 panic: reached unreachable code
/home/vidar/dev/zig/src/link/Dwarf.zig:4210:38: 0x3d928b0 in updateLazyValue (main.zig)
                        .arr_elem => unreachable,
                                     ^
/home/vidar/dev/zig/src/link/Dwarf.zig:2342:46: 0x36174f2 in updateLazy (main.zig)
            try wip_nav.dwarf.updateLazyValue(wip_nav.pt, src_loc, pending_val, &wip_nav.pending_lazy)
                                             ^
/home/vidar/dev/zig/src/link/Dwarf.zig:3049:27: 0x3c4c526 in finishWipNavWriterError (main.zig)
    try wip_nav.updateLazy(zcu.navSrcLoc(nav_index));
                          ^
/home/vidar/dev/zig/src/link/Dwarf.zig:3023:41: 0x3428370 in finishWipNav (main.zig)
    return dwarf.finishWipNavWriterError(pt, nav_index, wip_nav) catch |err| switch (err) {
                                        ^
/home/vidar/dev/zig/src/link/Dwarf.zig:3014:27: 0x471098e in finishWipNavFuncWriterError (main.zig)
    try dwarf.finishWipNav(pt, nav_index, wip_nav);
                          ^
/home/vidar/dev/zig/src/link/Dwarf.zig:2907:45: 0x4075c8f in finishWipNavFunc (main.zig)
    return dwarf.finishWipNavFuncWriterError(pt, nav_index, code_size, wip_nav) catch |err| switch (err) {
                                            ^
/home/vidar/dev/zig/src/link/Elf/ZigObject.zig:1581:64: 0x392fd70 in updateFunc (main.zig)
    if (debug_wip_nav) |*wip_nav| self.dwarf.?.finishWipNavFunc(pt, func.owner_nav, code.len, wip_nav) catch |err|
                                                               ^
/home/vidar/dev/zig/src/link/Elf.zig:1696:44: 0x32c8d11 in updateFunc (main.zig)
    return self.zigObjectPtr().?.updateFunc(self, pt, func_index, mir);
                                           ^
/home/vidar/dev/zig/src/link.zig:820:82: 0x2c7c0c8 in updateFunc (main.zig)
                return @as(*tag.Type(), @fieldParentPtr("base", base)).updateFunc(pt, func_index, mir);
                                                                                 ^
/home/vidar/dev/zig/src/link.zig:1541:30: 0x279b542 in doZcuTask (main.zig)
                lf.updateFunc(pt, func, &mir) catch |err| switch (err) {
                             ^
/home/vidar/dev/zig/src/link/Queue.zig:196:27: 0x2c21dfe in runLinkTasks (main.zig)
            link.doZcuTask(comp, tid, task);
                          ^
/home/vidar/dev/zig/lib/std/Io.zig:2126:13: 0x277a4e2 in start (std.zig)
            result_casted.* = @call(.auto, function, args_casted.*);
            ^
/home/vidar/dev/zig/lib/std/Io/Threaded.zig:1087:16: 0x1b6168e in start (std.zig)
        ac.func(ac.contextPointer(), ac.resultPointer());
               ^
/home/vidar/dev/zig/lib/std/Io/Threaded.zig:759:26: 0x1beeb45 in worker (std.zig)
            closure.start(closure, t);
                         ^
/home/vidar/dev/zig/lib/std/Thread.zig:558:13: 0x1baae20 in callFn__anon_37240 (std.zig)
            @call(.auto, f, args);
            ^
/home/vidar/dev/zig/lib/std/Thread.zig:829:30: 0x1b62209 in entryFn (std.zig)
                return callFn(f, args_ptr.*);
                             ^
???:?:?: 0x7f4adb49698a in ??? (/usr/lib/libc.so.6)
???:?:?: 0x7f4adb51a9cb in ??? (/usr/lib/libc.so.6)
zsh: IOT instruction (core dumped)  ../../zig/build/stage3/bin/zig run repro.zig

Compiling with LLVM/LLD (-fllvm) seems to work fine. Originally ran into this on 0.15.2 but seems to still be present on master. The context where this came up was passing struct fields (std.builtin.Type.StructField) to a function while excluding the first two entries. repro.zig compiles just fine if you get rid of the slicing of foos (also replacing the 1 with a 0 works). I was able to work around this for my use case by making the offending function (bar in this case) inline.

Expected Behavior

For the program run and compile like it does when using the LLVM backend

### Zig Version 0.16.0-dev.1854+e55e6b552 ### Steps to Reproduce and Observed Behavior repro.zig: ```zig const Foo = struct { type: type, }; fn bar(a: []const Foo) void { _ = a; } pub fn main() void { const foos = [1]Foo{.{ .type = u8, }}; bar(foos[1..]); } ``` ```shell ❯ ../../zig/build/stage3/bin/zig run repro.zig Compiler crash context: (no context) thread 1052961 panic: reached unreachable code /home/vidar/dev/zig/src/link/Dwarf.zig:4210:38: 0x3d928b0 in updateLazyValue (main.zig) .arr_elem => unreachable, ^ /home/vidar/dev/zig/src/link/Dwarf.zig:2342:46: 0x36174f2 in updateLazy (main.zig) try wip_nav.dwarf.updateLazyValue(wip_nav.pt, src_loc, pending_val, &wip_nav.pending_lazy) ^ /home/vidar/dev/zig/src/link/Dwarf.zig:3049:27: 0x3c4c526 in finishWipNavWriterError (main.zig) try wip_nav.updateLazy(zcu.navSrcLoc(nav_index)); ^ /home/vidar/dev/zig/src/link/Dwarf.zig:3023:41: 0x3428370 in finishWipNav (main.zig) return dwarf.finishWipNavWriterError(pt, nav_index, wip_nav) catch |err| switch (err) { ^ /home/vidar/dev/zig/src/link/Dwarf.zig:3014:27: 0x471098e in finishWipNavFuncWriterError (main.zig) try dwarf.finishWipNav(pt, nav_index, wip_nav); ^ /home/vidar/dev/zig/src/link/Dwarf.zig:2907:45: 0x4075c8f in finishWipNavFunc (main.zig) return dwarf.finishWipNavFuncWriterError(pt, nav_index, code_size, wip_nav) catch |err| switch (err) { ^ /home/vidar/dev/zig/src/link/Elf/ZigObject.zig:1581:64: 0x392fd70 in updateFunc (main.zig) if (debug_wip_nav) |*wip_nav| self.dwarf.?.finishWipNavFunc(pt, func.owner_nav, code.len, wip_nav) catch |err| ^ /home/vidar/dev/zig/src/link/Elf.zig:1696:44: 0x32c8d11 in updateFunc (main.zig) return self.zigObjectPtr().?.updateFunc(self, pt, func_index, mir); ^ /home/vidar/dev/zig/src/link.zig:820:82: 0x2c7c0c8 in updateFunc (main.zig) return @as(*tag.Type(), @fieldParentPtr("base", base)).updateFunc(pt, func_index, mir); ^ /home/vidar/dev/zig/src/link.zig:1541:30: 0x279b542 in doZcuTask (main.zig) lf.updateFunc(pt, func, &mir) catch |err| switch (err) { ^ /home/vidar/dev/zig/src/link/Queue.zig:196:27: 0x2c21dfe in runLinkTasks (main.zig) link.doZcuTask(comp, tid, task); ^ /home/vidar/dev/zig/lib/std/Io.zig:2126:13: 0x277a4e2 in start (std.zig) result_casted.* = @call(.auto, function, args_casted.*); ^ /home/vidar/dev/zig/lib/std/Io/Threaded.zig:1087:16: 0x1b6168e in start (std.zig) ac.func(ac.contextPointer(), ac.resultPointer()); ^ /home/vidar/dev/zig/lib/std/Io/Threaded.zig:759:26: 0x1beeb45 in worker (std.zig) closure.start(closure, t); ^ /home/vidar/dev/zig/lib/std/Thread.zig:558:13: 0x1baae20 in callFn__anon_37240 (std.zig) @call(.auto, f, args); ^ /home/vidar/dev/zig/lib/std/Thread.zig:829:30: 0x1b62209 in entryFn (std.zig) return callFn(f, args_ptr.*); ^ ???:?:?: 0x7f4adb49698a in ??? (/usr/lib/libc.so.6) ???:?:?: 0x7f4adb51a9cb in ??? (/usr/lib/libc.so.6) zsh: IOT instruction (core dumped) ../../zig/build/stage3/bin/zig run repro.zig ``` Compiling with LLVM/LLD (-fllvm) seems to work fine. Originally ran into this on 0.15.2 but seems to still be present on master. The context where this came up was passing struct fields (std.builtin.Type.StructField) to a function while excluding the first two entries. repro.zig compiles just fine if you get rid of the slicing of foos (also replacing the 1 with a 0 works). I was able to work around this for my use case by making the offending function (bar in this case) inline. ### Expected Behavior For the program run and compile like it does when using the LLVM backend

Vidar added the

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2025-12-27 15:32:21 +01:00

Vidar changed title from ~~Selfhosted x86_64 backend hits unreachable code when passing sliced array of structs with field with type "type"~~ to Selfhosted x86_64 backend hits unreachable code when passing sliced array of structs with field that has type "type"

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