Module level, inline assembly for wasm segfaults.

[gcoakes]

Zig Version

0.10.0-dev.4254+7f508480f

Steps to Reproduce

main.zig:

comptime {
    asm (
        \\.globl wasm_nop
        \\wasm_nop:
        \\  .functype wasm_nop () -> ()
        \\  nop
        \\  end_function
    );
}

Compile:

$ zig build-lib -dynamic -target wasm32-freestanding main.zig
LLVM Emit Object... Segmentation fault at address 0x0
???:?:?: 0x7f2d2771d249 in ??? (???)
???:?:?: 0x72732f73656b616e in ??? (???)
fish: Job 1, 'zig build-lib -dynamic -target…' terminated by signal SIGABRT (Abort)

Expected Behavior

A wasm file at least containing:

(module (func $wasm_nop nop))

Actual Behavior

$ zig build-lib -dynamic -target wasm32-freestanding main.zig
LLVM Emit Object... Segmentation fault at address 0x0
???:?:?: 0x7f2d2771d249 in ??? (???)
???:?:?: 0x72732f73656b616e in ??? (???)
fish: Job 1, 'zig build-lib -dynamic -target…' terminated by signal SIGABRT (Abort)

[nektro]

export fn wasm_nop() void {}

this is the way to do that. using asm inside a comptime block is illegal. however, this issue should stay open because that should be a compiler error not a compiler panic

[gcoakes]

using asm inside a comptime block is illegal

I don't think this is generally correct. There are behavior tests for this, and #256 added support for it. Are you saying it's just not legal for WASM?

Also, my use case is not strictly the minimal example I gave. I want to be able to define wrapper functions (a la wasm-bindgen) with parameters which are not expressible in zig (#10491).

[Luukdegram]

The code in the OP is perfectly valid. The segfault occurs in LLVM when we try to generate a binary. This smells like an issue upstream, but that requires some extra research. Here's a stack trace of the segfault which I'll update once I have the time to compile a debug version of LLVM:

LLVM Emit Object... Segmentation fault at address 0x6c2f73726573552f
???:?:?: 0x19baef104 in ??? (???)
???:?:?: 0x10334fa5f in _f128M_roundToInt (???)
???:?:?: 0x10334e1cf in _f128M_roundToInt (???)
???:?:?: 0x102f3f11f in __ZN4llvm16cast_convert_valIN5clang16EnumConstantDeclEPNS1_4DeclES4_E4doitEPKS3_ (???)
???:?:?: 0x1032c3833 in _f128M_roundToInt (???)
???:?:?: 0x102c9d523 in __ZN4llvm16cast_convert_valIN5clang16EnumConstantDeclEPNS1_4DeclES4_E4doitEPKS3_ (???)
???:?:?: 0x102c9801f in __ZN4llvm16cast_convert_valIN5clang16EnumConstantDeclEPNS1_4DeclES4_E4doitEPKS3_ (???)
???:?:?: 0x1023b6faf in _ZigLLVMTargetMachineEmitToFile (???)
/Users/luuk/dev/zig-dev/src/codegen/llvm.zig:795:43: 0x100da4803 in flushModule (zig)
        if (self.target_machine.emitToFile(
                                          ^
/Users/luuk/dev/zig-dev/src/link/Wasm.zig:2116:47: 0x100db4737 in flushModule (zig)
            return try llvm_object.flushModule(comp, prog_node);
                                              ^
/Users/luuk/dev/zig-dev/src/link/Wasm.zig:2872:29: 0x100daea97 in linkWithLLD (zig)
        try wasm.flushModule(comp, prog_node);
                            ^
/Users/luuk/dev/zig-dev/src/link/Wasm.zig:2104:32: 0x100da0e23 in flush (zig)
        return wasm.linkWithLLD(comp, prog_node);
                               ^
/Users/luuk/dev/zig-dev/src/link.zig:658:70: 0x100d9dde3 in flush (zig)
            .wasm => return @fieldParentPtr(Wasm, "base", base).flush(comp, prog_node),
                                                                     ^
/Users/luuk/dev/zig-dev/src/Compilation.zig:2440:24: 0x100f1a833 in flush (zig)
    comp.bin_file.flush(comp, prog_node) catch |err| switch (err) {
                       ^
/Users/luuk/dev/zig-dev/src/Compilation.zig:2421:23: 0x100f1c86f in update (zig)
        try comp.flush(main_progress_node);
                      ^
/Users/luuk/dev/zig-dev/src/main.zig:3383:20: 0x100f42dbb in updateModule (zig)
    try comp.update();
                   ^
/Users/luuk/dev/zig-dev/src/main.zig:3068:17: 0x100cd5e0b in buildOutputType (zig)
    updateModule(gpa, comp, hook) catch |err| switch (err) {
                ^
/Users/luuk/dev/zig-dev/src/main.zig:234:31: 0x100cb319b in mainArgs (zig)
        return buildOutputType(gpa, arena, args, .{ .build = .Obj });
                              ^
/Users/luuk/dev/zig-dev/src/stage1.zig:48:24: 0x100f8f217 in main (zig)
        stage2.mainArgs(gpa, arena, args) catch unreachable;

We can verify the correctness of the assembly using the following steps:

• Generate assembly from the code using:

zig build-obj test.zig -target wasm32-freestanding -fno-emit-bin -femit-asm=test.s

• And then using LLVM's playground:

llvm-mc -filetype=obj -triple=wasm32-unknown-unknown test.s -o test.o

• Generates a valid Wasm file:

> wasm-objdump test.o -x

test.wasm:	file format wasm 0x1

Section Details:

Type[1]:
 - type[0] () -> nil
Import[1]:
 - memory[0] pages: initial=0 <- env.__linear_memory
Function[1]:
 - func[0] sig=0 <wasm_nop>
Code[1]:
 - func[0] size=3 <wasm_nop>
Custom:
 - name: "linking"
  - symbol table [count=1]
   - 0: F <wasm_nop> func=0 [ binding=global vis=default ]
Custom:
 - name: "producers"

[gcoakes]

Slightly off topic, but I tried building the failing case using a debug build of LLVM as described in the wiki (which took ages). It produced identical output to my original report. @Luukdegram, was there something different you used to produce your stack trace?

[Luukdegram]

Slightly off topic, but I tried building the failing case using a debug build of LLVM as described in the wiki (which took ages). It produced identical output to my original report. @Luukdegram, was there something different you used to produce your stack trace?

Did you also build a debug version of the Zig compiler and not only of LLVM?

[gcoakes]

It's not all module-level asm. Maybe something specific to defining a function there? The following compiles:

comptime {
    asm (
        \\.globl __refs
        \\.tabletype __refs, externref, 32
        \\__refs:
    );
}

export fn refsSize() usize {
    return asm volatile (
        \\table.size __refs
        \\local.set %[ret]
        : [ret] "=r" (-> usize),
    );
}