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RE: [PATCH 01/20] Hexagon HVX (target/hexagon) README

From: Brian Cain
Subject: RE: [PATCH 01/20] Hexagon HVX (target/hexagon) README
Date: Mon, 19 Jul 2021 13:39:35 +0000

> -----Original Message-----
> From: Rob Landley <rob@landley.net>
> On 7/12/21 8:42 AM, Brian Cain wrote:
> > and there's also a binary hexagon-linux cross toolchain that
> > we shared for use by kernel developers.  The hexagon linux
> > toolchain is built on Ubuntu 16.04.
> Where's that one?

        - Built on Ubuntu 16.04, similar dynamic dependencies as 
releases.llvm.org binaries
        - Manifest:
                - llvm+clang 12.0.0 tag
                - Linux 5.6.18
                - github.com/qemu/qemu 15106f7dc3290ff3254611f265849a314a93eb0e
                - github.com/quic/musl aff74b395fbf59cd7e93b3691905aa1af6c0778c

> > But when building your toolchain, omitting LLVM_ENABLE_LLD should work
> just fine.
> It did, thanks.
> Now I'm trying to figure out what all the extra CFLAGS are for.

+Sid for some perspective on the rationale of these flags.  Some of these flags 
may be workarounds for toolchain issues.

> The clang_rt build has CMAKE_ASM_FLAGS="-G0 -mlong-calls -fno-pic
> --target=hexagon-unknown-linux-musl" which
> https://clang.llvm.org/docs/ClangCommandLineReference.html defines as:
> -G<size>
>   Put objects of at most <size> bytes into small data section (MIPS / Hexagon)
> -mlong-calls
>   Generate branches with extended addressability, usually via indirect jumps.
> I don't understand why your libcc build needs no-pic? (Are we only building
> a static libgcc.a instead of a dynamic one? I'm fine with that if so, but
> this needs to be specified in the MAKE_ASM_FLAGS why?)
> Why is it saying --target=hexagon-random-nonsense-musl to a toolchain
> we built with exactly one target type? How does it NOT default to hexagon?
> (Is this related to the build writing a hexagon-potato-banana-musl.cfg file
> in the bin directory, meaning the config file is in the $PATH? Does clang only
> look for it in the same directory the clang executable lives in?)
> And while we're at it, the CONTENTS of hexagon-gratuitous-gnu-format.cfg is:
> cat <<EOF > hexagon-unknown-linux-musl.cfg
> -G0 --sysroot=${HEX_SYSROOT}
> Which is ALREADY saying -G0? (Why does it want to do that globally? Some
> sort of
> bug workaround?) So why do we specify it again here?
> Next up build_musl_headers does CROSS_CFLAGS="-G0 -O0 -mv65 -fno-builtin
> -fno-rounding-math --target=hexagon-unknown-linux-musl" which:
> -O0
>   disable most of the optimizer
> -mv65
>   -mtune for a specific hexagon generation.
>   (Why? Does qemu only support that but not newer?)
> -fno-builtin
>   musl's ./configure already probes for this and will add it if
>   the compiler supports it.
> -fno-rounding-math
>   the docs MENTION this, but do not explain it.
> And again with the -G0.
> These flags probably aren't needed _here_ because this is just the headers
> install (which is basically a cp -a isn't it?). This looks like it's copied
> verbatim from the musl library build. But that library build happens in a bit,
> so relevant-ish I guess...
> (Also, why does building librt-but-not-really need the libc headers?
> The libgcc build EXPLICITLY does not require that, because otherwise you
> have this kind of BS circular dependency. Also, how do you EVER build a
> bare metal ELF toolchain with that dependency in there?)
> Next up build_kernel_headers has KBUILD_CFLAGS_KERNEL="-mlong-calls"
> which
> again, A) why does the compiler not do by default, B) can't be needed here
> because you don't even have to specify a cross compiler when doing
> headers_install. (I just confirmed this by diffing installs with an without a
> cross compiler specified: they were identical. I remembered this from
> https://github.com/torvalds/linux/commit/e0e2fa4b515c but checked again to
> be
> sure.) Presumably this is more "shared with full kernel build".
> And then build_musl, covered above under the headers build: lotsa flags, not
> sure why.
> > -Brian
> >
> Rob
> P.S. It took me a while to figure out that clang_rt is NOT librt.a, I think 
> it's
> their libgcc? Especially confusing since librt.a has existed for decades and
> was on solaris before it was on linux, and the OBVIOUS name is libcc
> the same way "cc" is the generic compiler name instead of "gcc".
> (In fact that was the posix compiler name until they decided to replace
> it with "c99" and everybody ignored them the way tar->pax was ignored,
> largely because make's $CC defaults to "cc" so it Just Works, and yes
> the cross compiler should have that name but the prepackaged clang tarball
> above does not. *shrug* I fix that up when making my prefix symlinks. The
> android NDK guys at least have the excuse of shipping NINE different
> x86_64-linux-android*-clang with API version numbers and thus not wanting to
> pick a default to single out, so leave making the -cc link as an exercise for
> the reader. I give instructions for doing so on the toybox cross compiling 
> page
> I linked above. :)

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