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u32::leading_zeros intrinic requires weird extension to work #210

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Firestar99 opened this issue Jan 27, 2025 · 2 comments · May be fixed by #213
Open

u32::leading_zeros intrinic requires weird extension to work #210

Firestar99 opened this issue Jan 27, 2025 · 2 comments · May be fixed by #213
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@Firestar99
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Expected Behaviour

Using u32::leading_zeros() in any shader errors with:

error: extension SPV_INTEL_shader_integer_functions2 is required
[...]
note: used from within `<u32>::leading_zeros`

This should not even need some intel-specific extension, that function probably exists already in the GL450 extension. Especially since some hardware (AMD) has special instructions for bit counting.

System Info

rust-gpu master forked from 1932353
@Firestar99 Firestar99 added the bug Something isn't working label Jan 27, 2025
@LegNeato
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Here is the code:

rust-gpu/crates/rustc_codegen_spirv/src/builder/intrinsics.rs

Line 218 in 6e2c84d

.u_count_leading_zeros_intel(

Yeah, doesn't look right 🤔

@Firestar99
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Firestar99 commented Jan 29, 2025 •
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I'm having a look at it, it seems like GLSL.std.450 do have FindILsb, FindSMsb and FindUMsb instructions for this. Though they do slightly mismatch what rust expects when 0 is passed and are limited to 32 bits.

https://registry.khronos.org/SPIR-V/specs/unified1/GLSL.std.450.html

@Firestar99 Firestar99 linked a pull request Jan 29, 2025 that will close this issue
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add support for leading_zeros and trailing_zeros Rust-GPU/rust-gpu
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@LegNeato @Firestar99