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GPU target: if with two or more else-if branches and no else loses the first branch's assignment #21
Description
On the GPU target, an if followed by two or more else if branches and no final else loses the assignment made in the first branch: the variable reads back as 0 instead of the assigned value (and not as its initial value either). The same function compiled for WebAssembly is correct.
Reproduction
A fragment shader that should draw four grey bands, left to right 1.0, 0.25, 0.75, 0.5:
import lib/shader_art.dl
to get the level for {floating-point number:kind}:
set amount to 0.5
if kind = 1.0:
set amount to 1.0
else if kind = 2.0:
set amount to 0.25
else if kind = 3.0:
set amount to 0.75
return amount
set band to (the floor of (((the fragment x coordinate) / (the shader width)) * 4.0)) + 1.0
set shade to the level for band
set the fragment color with a red channel of shade, a green channel of shade, a blue channel of shade and an alpha channel of 1.0
Open https://dynlex.com/ide/index.html?mode=shader, paste it, and look at the live preview. It compiles without diagnostics.
Expected and observed
Red channel read back from the preview canvas in the middle of each band (measured):
| Variant of the branch chain | Band 1 | Band 2 | Band 3 | Band 4 |
|---|---|---|---|---|
| Expected | 255 | 64 | 191 | 128 |
if + 2 × else if (the code above) |
0 | 64 | 191 | 128 |
if + 1 × else if (third branch removed; band 3 then expects 128) |
255 | 64 | 128 | 128 |
if + else if + else (last branch as else; band 4 then expects 191) |
255 | 64 | 191 | 191 |
three plain ifs |
255 | 64 | 191 | 128 |
Only the first row of code is wrong, and only in its first branch.
The same function as a program (import lib/std.dl, print line the level for 1.0 … 4.0, run in the IDE, WebAssembly target) prints 1, 0.25, 0.75, 0.5, which is correct. An integer version prints 100, 25, 75, 50, also correct.
It is not specific to scalars: it was first seen in a function returning a color, where a chain of six branches returned black for the first one. There, later branches (second, fourth) were correct, and putting a never-taken branch in front of the chain made the former first branch correct.
Environment
- dynlex.com IDE shader preview, release
fb835f53298c82b304993c1a63c7ff5cde2fd563, on 2026-11-10 - Chromium-based browser on Linux;
navigator.gpuis present. Which preview backend was active (WebGPU or WebGL) was not checked.
Not determined
Whether the fault is in SPIR-V emission or in the translation of the emitted SPIR-V to WGSL/GLSL. The native compiler was not tried.
Workaround
End the chain with else, keep it to one else if, or use separate if statements when the conditions are mutually exclusive.
Activity