[taylnos]

Problem

reflectionprobe_gl_test failed once on the Linux x64 test job (#554's first attempt) and passed on the re-run, on the same commit, image and Mesa. It failed only in the screen-space reflection and hero probe case, with the classic sky off, through sampleReflectionProbes (2,136 pixels) and sampleReflectionProbesWater (2,840). The one-pass build's gloss had a NaN green where red and blue matched the list build exactly, e.g. at 23,0: 0.670816, -nan, 0.530254 against 0.670816, 0.700022, 0.530254.

Cause

llvmpipe compiles the shaders for the CPU it runs on, and the runners are not all the same CPU. On an AMD Zen 4 host (-mcpu=znver4), Ubuntu 26.04's Mesa 26.0.8 with LLVM 21.1.8 compiles the one-pass probe shader to AVX-512 code that leaves a NaN in the green of the screen-space reflection. It hits about a tenth of the opaque marches. The failing attempt ran in Azure eastus and the passing re-run in centralus.

The failure reproduces exactly: the Linux test binary at the failing commit (996d100), on Ubuntu 26.04's own Mesa in WSL, under Intel SDE with a Zen 4 CPUID. It gives the same 2,136 and 2,840 pixels and the same values as CI. These all pass:

• Zen 4 with AVX-512 cleared from Mesa's CPU caps (AVX2 code, still tuned for znver4).
• Zen 4 with LLVM's IR passes off (GALLIVM_PERF=nopt), or with 128-bit vectors.
• Zen 3 (no AVX-512), and Intel's AVX-512 CPUs under SDE (Sapphire Rapids, Ice Lake server, Skylake-X, Granite Rapids).
• An i7-14700K natively.
• Every Windows build of Mesa tried (26.0.8 on LLVM 22, 26.0.6 on LLVM 21.1.8), under every one of those profiles, Zen 4 included.

The shader is not at fault. Its NIR has no undefined float values, and red, green and blue go through the same code. Giving traceScreenRay's out hitDepth a variable of its own, rather than depth, changes nothing.

Change

al_add_test_binary runs every GL binary with GALLIUM_OVERRIDE_CPU_CAPS=avx, Mesa's documented llvmpipe override. It clears AVX-512 from what Mesa sees of the CPU, so every runner's llvmpipe compiles the same AVX2 code. The variable reaches every entry, combined and per-case runs included, in both the build and sanitizer workflows and in a local ctest. A GPU's driver draws nothing differently for it. doc/TESTING.md's GL bullet says so, for a binary run by hand.

Develop does not trip it today. With #553's change to the probe shader, develop's whole alrender_gl_tests suite passes under the Zen 4 profile without the override. The shader's new code happens to dodge the bad code. This holds it off for the next edit to the probe or reflection shaders.

Testing

Under SDE with a Zen 4 CPUID, on Ubuntu 26.04's Mesa:

• At 996d100, the whole alrender_gl_tests suite: without the override, only this case fails (52 cases); with it, all 52 pass.
• At this branch, ctest's entry for reflectionprobe_gl_test carries GALLIUM_OVERRIDE_CPU_CAPS=avx and passes.

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