[PATCH] D113779: [Clang] Add mfp16, mfp16fml and mdotprod flags for ARM target features.

Mon Nov 15 01:55:18 PST 2021

fhahn added a comment.

In D113779#3130744 <https://reviews.llvm.org/D113779#3130744>, @DavidSpickett wrote:

> There was a similar proposal for crypto https://reviews.llvm.org/D60472.
>
> Quoting @manojgupta for the pitch for that:
>
>> The motivation for this change is to make "crypto" setting an additive option e.g. like "-mavx" used in many media packages. Some packages in Chrome want to enable crypto conditionally for a few files to allow crypto feature to be used based on runtime cpu detection. They set "-march=armv8+crypto" flag but it gets overridden by the global "-march=armv8a" flag set by the build system in Chrome OS because the target cpu does not support crypto causing compile-time errors.
>> Ability to specify "-mcrypto" standalone makes it an additive option and ensures that it it is not lost. i.e. this will help in decoupling "-mcrypto" from "-march" so that they could be set independently. The current additive alternate is '-Xclang -target-feature -Xclang "+crypto" ' which is ugly.
>
> Concerns were raised there.
>
> Seems like we're balancing adding a bunch of (clang only for at least the short term) options versus making the build system(s) cope with having to track what the last `-march` was.

Thanks for sharing the related patch/discussion.

> versus making the build system(s) cope with having to track what the last `-march` was.

Please see my earlier response. There can be use cases where the build system may not know what the last `-march` was, because the user may not have specified an explicit `-march` (e.g. they want to use the default picked by the compiler). Is there a way for the build system to conveniently determine what the default architecture version of the compiler is?

Also pushing this to the build system vendors means we need to convince X build-system vendors to adjust their implementations.

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https://reviews.llvm.org/D113779