Broken rendering on disc surface with Clang 14 and later with -march=native

[illwieckz]

See that vega place in viewpos 486 -57 96 147 24, the wrong black triangles on the disc surface on the right:

The bug disappears with r_deluxeMapping 0:

I'm reproducing it with current master but I was already noticing the problems on commit 9352952. I don't know when the bug was introduced, but the Unvanquished 0.55.2 release build doesn't reproduce the bug.

[VReaperV]

With what other settings? I can't reproduce this so far.

[illwieckz]

I use the ultra preset. I'm running it on an AMD Radeon W7600 PRO on Mesa radeonsi 24.2.8 (LLVM 19.1.1).

[VReaperV]

Still can't reproduce. Maybe a driver bug?

[VReaperV]

Also, it may be some other setting really, since disabling deluxe mapping disables a bunch of other features too.

[VReaperV]

@illwieckz Can you try on OGLP? Also, try with r_showDeluxeMaps 1, r_showNormalMaps 1, and r_showMaterialMaps 1 (separately) to see if any of those appear broken.

[illwieckz]

Maybe a driver bug?

Maybe, but the release build isn't affected. I may do a bisect at some point.

[VReaperV]

I don't think anything really changed there except making texture matrices 3x2.

[slipher]

r_depthShaders 0 is a good thing to try with these bugs

[VReaperV]

The texture matrix there is just an identity matrix, so it shouldn't be that. These triangles are also all part of the same surface. I wonder if this some different build of the map? Otherwise this bug would probably show up on other maps as well.

[illwieckz]

I first seen that on a custom built map with custom engine branch, but then I reverted to stock released map and master engine branch to reproduce the bug before reporting it.

[illwieckz]

I first noticed it on some Xonotic maps with similar geometry, Vega is just the first Unvanquished stock map I reproduced with, there is no reason other Unvanquished maps featuring the same kind of geometry would not be buggy as well.

[illwieckz]

I reproduce it with Clang 20 and -march=native on a Zen 2 CPU, the bug is still there with fast math disabled.

Since the release wasn't built that way, that may not be a regression.

A release-like build with Clang 20 (not using -march=native) is not affected.

[VReaperV]

Oh. Maybe it's some SIMD code breaking for triangle processing when loading the BSP, like the R_TBNtoQtangents()? Or a Clang bug.

[illwieckz]

I starts breaking with Clang 14:

compiler	status
Clang 20	❌️
Clang 19	❌️
Clang 18	❌️
Clang 17	❌️
Clang 16	❌️
Clang 15	❌️
Clang 14	❌️
Clang 13	✅️
Clang 12	✅️
Clang 11	✅️
Clang 10	✅️

[VReaperV]

#1451 (comment) seems to suggest that R_TBNtoQtangents() may, indeed, be the issue here.