stage2 LLVM: module fails to validate due to uninitialized global

[topolarity]

Zig Version

0.10.0-dev.2855+aab1284e1

Steps to Reproduce

test {
    comptime var st = .{
        .foo = &1,
        .bar = &2,
    };

    inline for (@typeInfo(@TypeOf(st)).Struct.fields) |field| { _ = field; }
}

Expected Behavior

Test should compile successfully and pass.

Actual Behavior

(The LLVM error message here is misleading. The real problem is that the global was never given a definition/initializer.)

LLVM Emit Object...
Global is external, but doesn't have external or weak linkage!
i8** @test.test_0__anon_981
Global is external, but doesn't have external or weak linkage!
i8** @test.test_0__anon_983

thread 23245 panic: LLVM module verification failed
/home/topolarity/repos/zig/src/Sema.zig:4515:5: 0x3473cae in Sema.zirBlock (zig)
    return sema.resolveBlockBody(parent_block, src, &child_block, body, inst, &label.merges);
    ^
/home/topolarity/repos/zig/src/Sema.zig:1206:52: 0x3285963 in Sema.analyzeBodyInner (zig)
                if (!block.is_comptime) break :blk try sema.zirBlock(block, inst);
                                                   ^
/home/topolarity/repos/zig/src/Sema.zig:13624:13: 0x3666c89 in Sema.analyzeRet (zig)
            return error.ComptimeReturn;
            ^
/home/topolarity/repos/zig/src/Sema.zig:13570:5: 0x34799c2 in Sema.zirRetNode (zig)
    return sema.analyzeRet(block, operand, src);
    ^
/home/topolarity/repos/zig/src/Sema.zig:1456:5: 0x3288080 in Sema.analyzeBodyInner (zig)
    return result;
    ^
/home/topolarity/repos/zig/src/Sema.zig:600:21: 0x326c86f in Sema.analyzeBody (zig)
        else => |e| return e,
                    ^
/home/topolarity/repos/zig/src/Sema.zig:1157:21: 0x32851d7 in Sema.analyzeBodyInner (zig)
                    return error.ComptimeBreak;
                    ^
/home/topolarity/repos/zig/src/Sema.zig:1157:21: 0x32851d7 in Sema.analyzeBodyInner (zig)
                    return error.ComptimeBreak;
                    ^
/home/topolarity/repos/zig/src/Sema.zig:1157:21: 0x32851d7 in Sema.analyzeBodyInner (zig)
                    return error.ComptimeBreak;
                    ^
/home/topolarity/repos/zig/src/Sema.zig:1157:21: 0x32851d7 in Sema.analyzeBodyInner (zig)
                    return error.ComptimeBreak;
                    ^
/home/topolarity/repos/zig/src/Sema.zig:4541:21: 0x364776b in Sema.resolveBlockBody (zig)
                    return error.ComptimeBreak;
                    ^
/home/topolarity/repos/zig/src/Sema.zig:8763:25: 0x3411f17 in Sema.zirSwitchBlock (zig)
                        return sema.resolveBlockBody(block, src, &child_block, body, inst, merges);
                        ^
/home/topolarity/repos/zig/src/Sema.zig:782:46: 0x327c0d2 in Sema.analyzeBodyInner (zig)
            .switch_block                 => try sema.zirSwitchBlock(block, inst),
                                             ^
/home/topolarity/repos/zig/src/Sema.zig:1157:21: 0x32851d7 in Sema.analyzeBodyInner (zig)
                    return error.ComptimeBreak;
                    ^
/home/topolarity/repos/zig/src/Sema.zig:1157:21: 0x32851d7 in Sema.analyzeBodyInner (zig)
                    return error.ComptimeBreak;
                    ^
/home/topolarity/repos/zig/src/Sema.zig:1157:21: 0x32851d7 in Sema.analyzeBodyInner (zig)
                    return error.ComptimeBreak;
                    ^
/home/topolarity/repos/zig/src/Sema.zig:1157:21: 0x32851d7 in Sema.analyzeBodyInner (zig)
                    return error.ComptimeBreak;
                    ^
/home/topolarity/repos/zig/src/Sema.zig:1157:21: 0x32851d7 in Sema.analyzeBodyInner (zig)
                    return error.ComptimeBreak;
                    ^
/home/topolarity/repos/zig/src/Sema.zig:13624:13: 0x3666c89 in Sema.analyzeRet (zig)
            return error.ComptimeReturn;
            ^
/home/topolarity/repos/zig/src/Sema.zig:13570:5: 0x34799c2 in Sema.zirRetNode (zig)
    return sema.analyzeRet(block, operand, src);
    ^
/home/topolarity/repos/zig/src/Sema.zig:1456:5: 0x3288080 in Sema.analyzeBodyInner (zig)
    return result;
    ^
/home/topolarity/repos/zig/src/Sema.zig:600:21: 0x326c86f in Sema.analyzeBody (zig)
        else => |e| return e,
                    ^
/home/topolarity/repos/zig/src/Sema.zig:1157:21: 0x32851d7 in Sema.analyzeBodyInner (zig)
                    return error.ComptimeBreak;
                    ^
/home/topolarity/repos/zig/src/Sema.zig:13302:9: 0x347b07c in Sema.zirCondbr (zig)
        return sema.analyzeBodyInner(parent_block, body);
        ^
/home/topolarity/repos/zig/src/Sema.zig:1456:5: 0x3288080 in Sema.analyzeBodyInner (zig)
    return result;
    ^
/home/topolarity/repos/zig/src/Sema.zig:1157:21: 0x32851d7 in Sema.analyzeBodyInner (zig)
                    return error.ComptimeBreak;
                    ^
/home/topolarity/repos/zig/src/Sema.zig:1157:21: 0x32851d7 in Sema.analyzeBodyInner (zig)
                    return error.ComptimeBreak;
                    ^
/home/topolarity/repos/zig/src/Sema.zig:1157:21: 0x32851d7 in Sema.analyzeBodyInner (zig)
                    return error.ComptimeBreak;
                    ^
/home/topolarity/repos/zig/src/Sema.zig:1157:21: 0x32851d7 in Sema.analyzeBodyInner (zig)
                    return error.ComptimeBreak;
                    ^
/home/topolarity/repos/zig/src/Sema.zig:13302:9: 0x347b07c in Sema.zirCondbr (zig)
        return sema.analyzeBodyInner(parent_block, body);
        ^
/home/topolarity/repos/zig/src/Sema.zig:1456:5: 0x3288080 in Sema.analyzeBodyInner (zig)
    return result;
    ^
/home/topolarity/repos/zig/src/Sema.zig:4541:21: 0x364776b in Sema.resolveBlockBody (zig)
                    return error.ComptimeBreak;
                    ^
/home/topolarity/repos/zig/src/codegen/llvm.zig:621:17: 0x2e2c821 in codegen.llvm.Object.flushModule (zig)
                @panic("LLVM module verification failed");
                ^
/home/topolarity/repos/zig/src/link/Elf.zig:960:47: 0x2e4098b in link.Elf.flushModule (zig)
            return try llvm_object.flushModule(comp, prog_node);
                                              ^
/home/topolarity/repos/zig/src/link/Elf.zig:1250:29: 0x2e3705e in link.Elf.linkWithLLD (zig)
        try self.flushModule(comp, prog_node);
                            ^
/home/topolarity/repos/zig/src/link/Elf.zig:946:32: 0x2e2ad6d in link.Elf.flush (zig)
        return self.linkWithLLD(comp, prog_node);
                               ^
/home/topolarity/repos/zig/src/link.zig:641:68: 0x2e27f95 in link.File.flush (zig)
            .elf => return @fieldParentPtr(Elf, "base", base).flush(comp, prog_node),
                                                                   ^
/home/topolarity/repos/zig/src/Compilation.zig:2325:24: 0x2def6f8 in Compilation.flush (zig)
    comp.bin_file.flush(comp, prog_node) catch |err| switch (err) {
                       ^
/home/topolarity/repos/zig/src/Compilation.zig:2306:23: 0x2de46fe in Compilation.update (zig)
        try comp.flush(main_progress_node);
                      ^
/home/topolarity/repos/zig/src/main.zig:3245:20: 0x2d4ef2f in updateModule (zig)
    try comp.update();
                   ^
/home/topolarity/repos/zig/src/main.zig:2934:17: 0x2d170aa in buildOutputType (zig)
    updateModule(gpa, comp, hook) catch |err| switch (err) {
                ^
/home/topolarity/repos/zig/src/main.zig:225:31: 0x2cfb9d6 in mainArgs (zig)
        return buildOutputType(gpa, arena, args, .zig_test);
                              ^
/home/topolarity/repos/zig/src/main.zig:174:20: 0x2cfaa24 in main (zig)
    return mainArgs(gpa, arena, args);
                   ^
/home/topolarity/repos/zig/lib/std/start.zig:581:37: 0x3220357 in std.start.callMain (zig)
            const result = root.main() catch |err| {
                                    ^
/home/topolarity/repos/zig/lib/std/start.zig:515:12: 0x2cfd477 in std.start.callMainWithArgs (zig)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/home/topolarity/repos/zig/lib/std/start.zig:480:12: 0x2cfd222 in std.start.main (zig)
    return @call(.{ .modifier = .always_inline }, callMainWithArgs, .{ @intCast(usize, c_argc), c_argv, envp });
           ^
???:?:?: 0x7fc998ab37fc in ??? (???)
[1]    23245 IOT instruction  ../zig/stage2/bin/zig test test.zig

[topolarity]

The problem seems to be related to DebugInfo: If running with --strip, the problem goes away.

It looks like we are generating an anonymous decl to a comptime-only type, which is never sent to the linker by Module.zig but ends up indirectly referenced by the DebugInfo given to LLVM:

; Globals declared here
@test.test_0__anon_980 = internal unnamed_addr constant [4 x i8] c"foo\00", align 1
@test.test_0__anon_981 = internal unnamed_addr global i8*   ; decl never sent to linker
@0 = private unnamed_addr constant %builtin.Type.StructField { { i8*, i64 } { i8* getelementptr inbounds ([4 x i8], [4 x i8]* @test.test_0__anon_980, i32 0, i32 0), i64 3 }, i8* bitcast (i8** @test.test_0__anon_981 to i8*), i1 true, [7 x i8] undef }, align 8

; ...

; Function Attrs: nounwind
define internal fastcc i16 @test.test_0(%builtin.StackTrace* nonnull %0) unnamed_addr #0 !dbg !4175 {
Entry:
  call void @llvm.dbg.declare(metadata %builtin.Type.StructField* @0, metadata !4189, metadata !DIExpression()), !dbg !4195
  call void @llvm.dbg.declare(metadata %builtin.Type.StructField* @1, metadata !4193, metadata !DIExpression()), !dbg !4196
  ret i16 0, !dbg !4197
}

[topolarity]

My knowledge of DebugInfo/the Module system is too limited for me to be sure which side the fix belongs in.

Nonetheless, this small patch to Module.zig forces the comptime-only decl to be generated, working around the bug:

diff --git a/src/Module.zig b/src/Module.zig
index 1f70a44df..e3d81dc32 100644
--- a/src/Module.zig
+++ b/src/Module.zig
@@ -5297,7 +5297,7 @@ pub fn initNewAnonDecl(
     // if the Decl is referenced by an instruction or another constant. Otherwise,
     // the Decl will be garbage collected by the `codegen_decl` task instead of sent
     // to the linker.
-    if (typed_value.ty.isFnOrHasRuntimeBits()) {
+    if (typed_value.ty.isFnOrHasRuntimeBitsIgnoreComptime()) {
         try mod.comp.bin_file.allocateDeclIndexes(new_decl_index);
         try mod.comp.anon_work_queue.writeItem(.{ .codegen_decl = new_decl_index });
     }