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Shaders: zero-initialize the streams variable - #3447
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`streams = input[i]` reads the members the stage input does not carry back from streams, so writing only part of one left the rest undefined and fxc rejected the translated HLSL with X4000. Stride.BepuPhysics.Debug's SinglePassWireframeShader writes EdgeA.xyz and never EdgeA.w, so it stopped compiling. Bumps ShaderCompilerVersion.Effect: the change is mix-time, so the per-class SPIR-V cache stays valid. Test: GeometryStreamsAssignThenPartialWriteCompilesWithFxc, running fxc on the translated geometry shader. Full suite 840/840.
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PR details
SinglePassWireframeShader(the Bepu debug wireframe) stopped compiling on 4.4.0-beta7 witherror X4000: variable '_2168' used without having been completely initialized.Two commits caused it, both in beta7 and neither in beta6:
streams = input[i]read the members the stage input does not carry back fromstreamsinstead of zeroing them, so writing only part of one leaves the rest undefined. Thewireframe shader writes
EdgeA.xyzand neverEdgeA.w.ForceZeroInitializedVariables, which had been zero-initializing SPIRV-Crosshoisted temporaries and hiding that. Its replacement,
requires_phi_undef_zero_init, only coversloop variables.
The private
streamsvariable now gets anOpConstantNullinitializer, so reading a member beforeit is written is defined. Members the shader does write are dead stores that spirv-opt drops.
ForceZeroInitializedVariablesstays off on purpose: it hides real undefined reads.ShaderCompilerVersion.Effectgoes 0 -> 1. The change is mix-time, so the per-class SPIR-V cachestays valid, and the checked-in precompiled bytecodes are unaffected.
Test
GeometryStreamsAssignThenPartialWriteCompilesWithFxcruns fxc on the translated geometryshader and fails with the same X4000 without the fix. Nothing in the suite ran fxc before, which is
why this was not caught. Suite 840/840.