cc-wrapper: fix -mtune= validation, add ARM, add fallbacks #205091
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Before this commit, cc-wrapper/default.nix was using `isGccArchSupported` to validate `-mtune=` values. This has two problems: - On x86, `-mtune=` can take the same values as `-march`, plus two additional values `generic` and `intel` which are not valid for `-march`. - On ARM, `-mtune=` does not take the same values as `-march=`; instead it takes the same values as `-mcpu`. This commit fixes these two problems by adding a new `isGccTuneSupported` function. For `isx86` this returns `true` for the two special values and otherwise defers to `isGccArchSupported`. This commit also adds support for `-mtune=` on Aarch64. Unfortunately on Aarch64, Clang does not accept as wide a variety of `-mtune=` values as Gcc does. In particular, Clang does not tune for big.LITTLE mixed-model chips like the very popular RK3399, which is targeted using `-march=cortex-a72.cortex-a53` in gcc. To address this problem, this commit also adds a function `findBestTuneApproximation` which can be used to map clang-unsupported tunings like `cortex-a72.cortex-a53` to less-precise tunings like `cortex-a53`. The work which led to this commit arose because we now have packages, like `crosvm`, which use *both* `clang` *and* `gcc`. Previously I had been using `overrideAttrs` to set `NIX_CFLAGS_COMPILE` on a package-by-package basis based on which compiler that package used. Since we now have packages which use *both* compilers, this strategy no longer works. I briefly considered splitting `NIX_CFLAGS_COMPILE` into `NIX_CFLAGS_COMPILE_GCC` and `NIX_CFLAGS_COMPILE_CLANG`, but since `NIX_CFLAGS_COMPILE` is sort of a hack to begin with I figured that adding the logic to `cc-wrapper` would be preferable.
Atemu
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Description of changes
Before this commit, cc-wrapper/default.nix was using
isGccArchSupportedto validate-mtune=values. This has three problems:On x86,
-mtune=can take the same values as-march, plus two additional valuesgenericandintelwhich are not valid for-march.On Arm64,
-mtune=does not take the same values as-march=; instead it takes the same values as-mcpu.On PowerPC, all
-march=flags are rejected, soisGccArchSupportedought to unconditionally return false (see cc-wrapper: -march= is not allowed on powerpc #205242), yet-mtuneis allowed.This commit fixes these three problems by adding a new
isGccTuneSupportedfunction. Forisx86this returnstruefor the two special values and otherwise defers toisGccArchSupported. This commit also adds support for-mtune=on Arm64, PowerPC, and MIPS.On Arm64, Clang does not accept as wide a variety of
-mtune=values as Gcc does. In particular, Clang does not tune for big.LITTLE mixed-model chips like the very popular RK3399 (found in 6 of the 33 ARM boards on the NixOS wiki), which is-mtune=cortex-a72.cortex-a53. To address this problem, this commit also adds a functionfindBestTuneApproximationwhich can be used to map clang-unsupported tunings likecortex-a72.cortex-a53to less-precise tunings likecortex-a53.The work which led to this commit arose because we now have packages, like
crosvm, which use bothclangandgcc;NIX_CFLAGS_COMPILEcan no longer be used for those packages since it doesn't distinguish between clang and gcc.Things done
gcc.arch="bdver1"; gcc.tune="native")gcc.tune="cortex-a72.cortex-a53")gcc.tune="power9")gcc.tune="octeon3")