IssueTracker

In b/404353398, I landed quite a few workarounds targeted at code that LLVM happened to apply a specific transformation to:

• https://chromium-review.git.corp.google.com/c/chromiumos/overlays/chromiumos-overlay/+/6376233
• https://chromium-review.git.corp.google.com/c/chromiumos/overlays/chromiumos-overlay/+/6379063
• https://chromium-review.git.corp.google.com/c/chromiumos/overlays/chromiumos-overlay/+/6384184
• https://chromium-review.git.corp.google.com/c/chromiumos/overlays/chromiumos-overlay/+/6383321
• https://chromium-review.git.corp.google.com/c/chromiumos/overlays/chromiumos-overlay/+/6385588
• https://chromium-review.git.corp.google.com/c/chromiumos/overlays/chromiumos-overlay/+/6385587

The commit upstream that initially landed this transformation was https://github.com/llvm/llvm-project/commit/abd314938dda1b117f289be5e630e43e68533929. Critically, comments say that this implements the following transforms in terms of GFNI instructions:

• // Generate a GFNI gf2p8affine bitmask for vXi8 bitreverse/shift/rotate.
• // Attempt to use GFNI gf2p8affine to rotate vXi8 by an uniform constant.

tl;dr:

• The surface area for where this can pop up is "any dynamic library linked into any unittest" (I had to disable tensorflow tests & remove this instruction from libcxx builds)-
• This is likely to continue popping up through no fault of anyone.

So I'm filing this bug to look into whether there are better long-term solutions here. Preferably not landed near a branch point.