[spirv] Fix null pointer dereference

[sudonatalie]

Fix a null pointer dereference which can occur when an unimplemented
intrinsic is used. The repro shader from issue #2955 (added as a unit
test) now fails with a useful message rather than a silent crash.

[sudonatalie]

@jaebaek This should fix the crash, but of course will not generate valid SPIR-V. Is this new intrinsic one that has been discussed before or that warrants tracking separately for the SPIR-V backend?

[AppVeyorBot]

✅ Build DirectXShaderCompiler 1.0.337 completed (commit 26a68d1e8e by @sudonatalie)

[jaebaek]

We have not implemented GetAttributeAtVertex support for the SPIR-V backend. It is relatively new feature. I found some commits about it, but I cannot find the spec about it. The DXIL code gen uses @dx.op.attributeAtVertex.f32() intrinsic but I have no idea we have a SPIR-V instruction matches it.

@pow2clk could you please point out the document about GetAttributeAtVertex?

[ghost]

All CLA requirements met.

[AppVeyorBot]

✅ Build DirectXShaderCompiler 1.0.343 completed (commit a213072fb2 by @sudonatalie)

[AppVeyorBot]

✅ Build DirectXShaderCompiler 1.0.344 completed (commit e72b7d64b7 by @sudonatalie)

[pow2clk]

Hi @jaebaek!

GetAttributeAtPVertex is part of the barycentrics optional feature added in shader model 6.1: https://github.com/microsoft/DirectXShaderCompiler/wiki/SV_Barycentrics#per-vertex-attributes

Congrats @sudonatalie on your first PR to DXC! 😄

[jaebaek]

Thanks @pow2clk !