Default member initialization crashes compiler

[exdal]

Default initialized RW/Textures/Resources seem to be crashing compiler despite a condition is set to false, this is started happening after I bumped slang version from v2024.17.4 to v2025.5

struct Foo {
    bool sample_bar = false;
    Texture2D<float4> bar = {};
};

float4 sample_foo(in Foo foo) {
    if (foo.sample_bar) {
        // Compiler crashes here despite `sample_bar` is set to false
        return bar.Sample(...);
    }

    return float4(0.0);
}

float4 main() {
    Foo foo;
    return sample_foo(foo);
}

Repro: https://gist.github.com/exdal/2d40533d7a8df813a6996771b1f9f95d
Command line:

./slangc -capability spirv_1_4 -target spirv test.slang -o ./test.bin
test.slang(59): warning 41016: use of uninitialized variable 'foo'
    const let result = process(foo);
                              ^
[1]    28517 segmentation fault (core dumped)  ./slangc -capability spirv_1_4 -target spirv test.slang -o ./test.bin

Version: v2025.5 (57b09a8986668626c37055e431fa0ac6449d7214)
Target profile: spirv_1_4

Link to original Discord post: https://discord.com/channels/1303735196696445038/1337851943942619298/1337851943942619298

[cheneym2]

I didn't get a repro using Slang @ 2/5/2025

commit d3e5f39cafbf1d65cf93cdd42c20c472c68197a2 (HEAD -> master, origin/master, origin/HEAD)
Author: Jay Kwak <[email protected]>
Date:   Wed Feb 5 00:43:02 2025 -0800
    Use 0.0.7 version of VK-GL-CTS-for-Slang on nightly CTS CI (#6278)

I compiled the shader in https://gist.github.com/exdal/2d40533d7a8df813a6996771b1f9f95d (named to repro.slang locally)

With this command:

$ build/Debug/bin/slangc -capability spirv_1_4 -target spirv-asm ~/repro.slang -o ./test.bin

[cheneym2]

I am able to reproduce with v2025.5 though

[cheneym2]

No repro on v2025.4, so regression is recent

[cheneym2]

#6058 Is the regression point. @kaizhangNV

[kaizhangNV]

test.slang(59): warning 41016: use of uninitialized variable 'foo'

this is as expected, because in the intializer list proposal SP004, we remove the implicit constructor call for a struct that dones't define an explicit constructor.

So for your current example since struct Foo doesn't define __init, statement like
Foo foo; won't be converted to Foo foo = Foo();

you have to explicit write it like
Foo foo = Foo();
or
Foo foo = {};

So your statement Foo foo; will be treated as uninitialized, check:
https://github.com/shader-slang/slang/blob/master/docs/proposals/004-initialization.md#variable-initialization
for more detail.

[csyonghe]

@kaizhangNV the warning is good, but the compiler shouldn't crash. Where are we crashing?

[kaizhangNV]

I'm currently looking into the crash.

It crashes at IR link stage, we eliminate the uninitialized variable at call site:

 Foo foo;
 const let result = process(foo);

becomes:

 const let result = process();

specifically, it's wiped out in this call legalizeResourceTypes.