cmov: add generic masksel function to portable backend
#1342
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Generically implements selecting between two values based upon a provided mask value, rather than duplicating this logic
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tarcieri
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Uses the `masksel` function added in #1342 to impl `CmovEq`. This should be less circuitious than the previous implementation. Mask generation still goes through `masknz32`/`masknz64` which either use `testnz32`/`testnz64` and `black_box` or use an optimized assembly implementation of `masknz32` (vicariously for `masknz64`), which means these functions still have a strategy to prevent branch insertion.
tarcieri
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Uses the `masksel` function added in #1342 to impl `CmovEq`. This should be less circuitious than the previous implementation. This approach means we can get rid of all of the `test*` functions like `testeq*`, `testne*`, and `testnz*` and implement everything in terms of mask generation. The pure Rust versions of `masknz32` and `masknz64` still internally use `black_box` to prevent optimizations around the non-zero bit extracted before computing the mask.
tarcieri
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Jan 15, 2026
Uses the `masksel` function added in #1342 to impl `CmovEq`. This should be less circuitious than the previous implementation. This approach means we can get rid of all of the `test*` functions like `testeq*`, `testne*`, and `testnz*` and implement everything in terms of mask generation. The pure Rust versions of `masknz32` and `masknz64` still internally use `black_box` to prevent optimizations around the non-zero bit extracted before computing the mask.
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Generically implements selecting between two values based upon a provided mask value, rather than duplicating this logic