The GL tests hold Mesa's llvmpipe to AVX2: LLVM 21's Zen 4 code puts NaN in the probe test

doc/TESTING.md

@@ -105,7 +105,7 @@ A binary is a link set. Its files share their libraries, their fakes and their d
 
 - **`UNIT`** compiles the library's own files into the binary in place of linking the library, for a test that stubs what they call. Such a test is a binary of its own.
 - **`SUPPORT`** names object libraries linked whole: fakes and checks. A case or listener in a static archive would be dropped by the linker.
-- **`GL`** labels the entries `gl` and registers them disabled with `-DAL_ENABLE_GL_TESTS=OFF`. **`PYTHON`** gives them the interpreter, and registers them disabled without one that has `llsd`.
+- **`GL`** labels the entries `gl`, registers them disabled with `-DAL_ENABLE_GL_TESTS=OFF`, and holds Mesa's llvmpipe to AVX2 (`GALLIUM_OVERRIDE_CPU_CAPS=avx`): llvmpipe compiles the shaders for the host's CPU, and LLVM 21's AVX-512 code for AMD's Zen 4 leaves a NaN in `reflectionprobe_gl`'s screen-space reflection. A GL binary run by hand on llvmpipe wants the same. **`PYTHON`** gives them the interpreter, and registers them disabled without one that has `llsd`.
 - **`ISA_TIER`** builds the binary at an x86-64 tier, and it skips on a host below it.
 - **`RERUN`** runs some files a second time with more environment. alscript's SLua tests run again under Luau's new solver, as `alscript_tests_newsolver/<file stem>`, with `AL_TEST_LUAU_SOLVER=new`.
 - **`<binary>/stray_cases`** fails when a case belongs to no file's entry, such as one written in a header.

indra/cmake/ALAddTestBinary.cmake

@@ -118,9 +118,10 @@ endif()
 # the environment and labels a file's entry would have.
 #
 # ENVIRONMENT sets variables for the runs, VAR=value each. GL, PYTHON and
-# ISA_TIER mean what they mean to al_add_test: a GL binary is labelled gl and
-# registered disabled without AL_ENABLE_GL_TESTS, a PYTHON binary is given the
-# interpreter and registered disabled without one that has llsd, and an
+# ISA_TIER mean what they mean to al_add_test: a GL binary is labelled gl,
+# registered disabled without AL_ENABLE_GL_TESTS and run with Mesa's llvmpipe
+# held to AVX2, a PYTHON binary is given the interpreter and registered
+# disabled without one that has llsd, and an
 # ISA_TIER binary is built at that x86-64 tier, named <name>_<tier>, labelled
 # isa, and skips on a host below it -- or below the tree's own tier, since it
 # links the tree's libraries, built at that one. RESOURCE_LOCK names what the binary's
@@ -283,6 +284,14 @@ function(al_add_test_binary name)
     if(NOT AL_ENABLE_GL_TESTS)
       set(disabled TRUE)
     endif()
+    # Mesa's llvmpipe, the Linux runners' GL, compiles the shaders for the
+    # host's CPU, and LLVM 21's AVX-512 code for AMD's Zen 4 (znver4) leaves
+    # a NaN in the green of reflectionprobe_gl's screen-space reflection. Its
+    # AVX2 code for the same CPU does not, nor its code for Zen 3 or for
+    # Intel's AVX-512 CPUs. Clearing AVX-512 from what Mesa sees of the CPU
+    # holds llvmpipe to AVX2 on every host; a driver that compiles for a GPU
+    # draws nothing differently.
+    list(APPEND environment "GALLIUM_OVERRIDE_CPU_CAPS=avx")
   endif()
   math(EXPR timeout "${timeout} * ${AL_TEST_TIMEOUT_SCALE}")