Re: [MLton-user] MLton on ARM Mac?

[Matthew F. <mat...@gm...>]

On Mon, Dec 28, 2020 at 11:11 AM Chris Cannam
<ca...@al...> wrote:
>
> On Mon, 28 Dec 2020, at 14:52, Matthew Fluet wrote:
> > > With some script tweaks [...]
> >
> > It would help to see these patches as well.
>
> The first of these is just a hack to try to force matters, but the platform bit seemed sensible to me:
>
> diff --git a/bin/mlton-script b/bin/mlton-script
> index 2e9f0d78d..e239d5456 100644
> --- a/bin/mlton-script
> +++ b/bin/mlton-script
> @@ -82,6 +82,8 @@ doit "$lib" \
>          -cc-opt '-O1 -fno-strict-aliasing'                       \
>          -cc-opt '-foptimize-sibling-calls'                       \
>          -cc-opt '-w'                                             \
> +        -cc-opt '-arch arm64'                                             \
> +        -link-opt '-arch arm64'                                             \
>          -cc-opt-quote "-I$lib/include"                           \
>          $gmpCCOpts $gmpLinkOpts                                  \
>          -llvm-llc-opt '-O2'                                      \
> diff --git a/bin/platform b/bin/platform
> index fed549259..999be668b 100755
> --- a/bin/platform
> +++ b/bin/platform
> @@ -88,12 +88,15 @@ i?86_64)
>  amd64)
>          HOST_ARCH=amd64
>  ;;
> -arm*)
> -        HOST_ARCH=arm
> +arm64)
> +        HOST_ARCH=arm64
>  ;;
>  aarch64)
>          HOST_ARCH=arm64
>  ;;
> +arm*)
> +        HOST_ARCH=arm
> +;;
>  parisc*)
>          HOST_ARCH=hppa
>  ;;
>
> > How is the macOS C compiler distinguishing between targeting amd64 and arm64?
>
> Various ways. One is the -arch option. "cc -arch x86_64" targets amd64, "cc -arch arm64" targets arm64, and "cc -arch x86_64 -arch arm64" creates a fat binary containing both architectures.
>
> If you don't give an -arch option, then it targets the same architecture as the process that invoked cc - which is not necessarily the native machine architecture. That means, I believe, that a compiler exec'd directly by an amd64 MLton will target amd64 even on an ARM Mac unless you specify otherwise. This seems logical but can be a bit confusing, especially since many programs that exec compilers (e.g. the system-supplied make) are fat binaries, so the compiler will default to whichever slice was in use for the parent process, something that is not necessarily externally obvious.
>
> There's also a command called "arch" that selects a specific slice from a fat binary. So if you run e.g. "arch -x86_64 make" you will run make in amd64 mode, and any cc it forks will default to amd64. Or you can "arch -arm64 cc" to get a cc that, I believe, both runs in and targets by default arm64, although it can still target the other architecture if you ask it to (e.g. "arch -arm64 cc -arch x86_64" produces an amd64-only binary).
>
> If you run cc directly from the shell without any of the above going on then it targets the native architecture, though I assume this is just a special case of following the parent process, since login shells are native.

Given what you say here, I'm hopeful that you won't need any changes
to `bin/mlton-script`, because at the end of the bootstrap, you will
have an arm64 MLton, which, when invoking cc, will automatically
generate arm64 executables.

-Matthew

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