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SubjectRe: [PATCH v7 21/25] Kbuild: add Rust support
> +choice
> + prompt "Optimization level"
> + default RUST_OPT_LEVEL_SIMILAR_AS_CHOSEN_FOR_C
> + depends on RUST
> + help
> + Controls rustc's `-Copt-level` codegen option.
> +
> + This flag controls the optimization level.
> +
> + If unsure, say "Similar as chosen for C".
> +
> +config RUST_OPT_LEVEL_SIMILAR_AS_CHOSEN_FOR_C
> + bool "Similar as chosen for C"
> + help
> + This choice will pick a similar optimization level as chosen in
> + the "Compiler optimization level" for C:
> +
> + -O2 is currently mapped to -Copt-level=2
> + -O3 is currently mapped to -Copt-level=3
> + -Os is currently mapped to -Copt-level=s
> +
> + The mapping may change over time to follow the intended semantics
> + of the choice for C as sensibly as possible.
> +
> + This is the default.
> +
> +config RUST_OPT_LEVEL_0
> + bool "No optimizations (-Copt-level=0)"
> + help
> + Not recommended for most purposes. It may come in handy for debugging
> + suspected optimizer bugs, unexpected undefined behavior, etc.
> +
> + Note that this level will *not* enable debug assertions nor overflow
> + checks on its own (like it happens when interacting with rustc
> + directly). Use the corresponding configuration options to control
> + that instead, orthogonally.
> +
> + Note this level may cause excessive stack usage, which can lead to stack
> + overflow and subsequent crashes.
> +
> +config RUST_OPT_LEVEL_1
> + bool "Basic optimizations (-Copt-level=1)"
> + help
> + Useful for debugging without getting too lost, but without
> + the overhead and boilerplate of no optimizations at all.
> +
> + Note this level may cause excessive stack usage, which can lead to stack
> + overflow and subsequent crashes.
> +
> +config RUST_OPT_LEVEL_2
> + bool "Some optimizations (-Copt-level=2)"
> + help
> + The sensible choice in most cases.
> +
> +config RUST_OPT_LEVEL_3
> + bool "All optimizations (-Copt-level=3)"
> + help
> + Yet more performance (hopefully).
> +
> +config RUST_OPT_LEVEL_S
> + bool "Optimize for size (-Copt-level=s)"
> + help
> + Smaller kernel, ideally without too much performance loss.
> +
> +config RUST_OPT_LEVEL_Z
> + bool "Optimize for size, no loop vectorization (-Copt-level=z)"
> + help
> + Like the previous level, but also turn off loop vectorization.

I'm super not into having the rust optimization level differ from the
C optimization level. This is just someone having too much fun
wrapping every compiler flag in a kbuild option. Either folks wan't
smaller size or more optimizations. Allowing for RUST_OPT_LEVEL_S and
CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE or RUST_OPT_LEVEL_3 and
CONFIG_CC_OPTIMIZE_FOR_SIZE is just wacky nonsense that's going to
make randconfig bug reports more confusing to tease out.
--
Thanks,
~Nick Desaulniers

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