diff mbox series

[v6,3/3] Documentation/llvm: update CROSS_COMPILE inferencing

Message ID 20210802183910.1802120-4-ndesaulniers@google.com (mailing list archive)
State New, archived
Headers show
Series infer --target from SRCARCH for CC=clang | expand

Commit Message

Nick Desaulniers Aug. 2, 2021, 6:39 p.m. UTC
As noted by Masahiro, document how we can generally infer CROSS_COMPILE
(and the more specific details about --target and --prefix) based on
ARCH.

Change use of env vars to command line parameters.

Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Fangrui Song <maskray@google.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
Changes v5 -> v6:
* Pick up Fangrui's RB tag.
* Change use of env vars to command line parameters for consistency.

 Documentation/kbuild/llvm.rst | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

Comments

Nathan Chancellor Aug. 2, 2021, 7:16 p.m. UTC | #1
On 8/2/2021 11:39 AM, 'Nick Desaulniers' via Clang Built Linux wrote:
> As noted by Masahiro, document how we can generally infer CROSS_COMPILE
> (and the more specific details about --target and --prefix) based on
> ARCH.
> 
> Change use of env vars to command line parameters.
> 
> Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
> Reviewed-by: Fangrui Song <maskray@google.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

Thanks for the update, I think this is much easier for non-CBL folks to 
understand.

Reviewed-by: Nathan Chancellor <nathan@kernel.org>

> ---
> Changes v5 -> v6:
> * Pick up Fangrui's RB tag.
> * Change use of env vars to command line parameters for consistency.
> 
>   Documentation/kbuild/llvm.rst | 19 ++++++++++++++++++-
>   1 file changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
> index b18401d2ba82..f8a360958f4c 100644
> --- a/Documentation/kbuild/llvm.rst
> +++ b/Documentation/kbuild/llvm.rst
> @@ -38,7 +38,7 @@ Cross Compiling
>   A single Clang compiler binary will typically contain all supported backends,
>   which can help simplify cross compiling. ::
>   
> -	ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang
> +	make ARCH=arm64 CC=clang CROSS_COMPILE=aarch64-linux-gnu-
>   
>   ``CROSS_COMPILE`` is not used to prefix the Clang compiler binary, instead
>   ``CROSS_COMPILE`` is used to set a command line flag: ``--target=<triple>``. For
> @@ -63,6 +63,23 @@ They can be enabled individually. The full list of the parameters: ::
>   Currently, the integrated assembler is disabled by default. You can pass
>   ``LLVM_IAS=1`` to enable it.
>   
> +Omitting CROSS_COMPILE
> +----------------------
> +
> +As explained above, ``CROSS_COMPILE`` is used to set ``--target=<triple>``.
> +
> +Unless ``LLVM_IAS=1`` is specified, ``CROSS_COMPILE`` is also used to derive
> +``--prefix=<path>`` to search for the GNU assembler and linker.
> +
> +If ``CROSS_COMPILE`` is not specified, the ``--target=<triple>`` is inferred
> +from ``ARCH``.
> +
> +That means if you use only LLVM tools, ``CROSS_COMPILE`` becomes unnecessary.
> +
> +For example, to cross-compile the arm64 kernel::
> +
> +	make ARCH=arm64 LLVM=1 LLVM_IAS=1
> +
>   Supported Architectures
>   -----------------------
>   
>
Masahiro Yamada Aug. 5, 2021, 1:57 p.m. UTC | #2
On Tue, Aug 3, 2021 at 3:39 AM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@googlegroups.com> wrote:
>
> As noted by Masahiro, document how we can generally infer CROSS_COMPILE
> (and the more specific details about --target and --prefix) based on
> ARCH.
>
> Change use of env vars to command line parameters.
>
> Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
> Reviewed-by: Fangrui Song <maskray@google.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
> Changes v5 -> v6:
> * Pick up Fangrui's RB tag.
> * Change use of env vars to command line parameters for consistency.
>
>  Documentation/kbuild/llvm.rst | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
> index b18401d2ba82..f8a360958f4c 100644
> --- a/Documentation/kbuild/llvm.rst
> +++ b/Documentation/kbuild/llvm.rst
> @@ -38,7 +38,7 @@ Cross Compiling
>  A single Clang compiler binary will typically contain all supported backends,
>  which can help simplify cross compiling. ::
>
> -       ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang
> +       make ARCH=arm64 CC=clang CROSS_COMPILE=aarch64-linux-gnu-
>
>  ``CROSS_COMPILE`` is not used to prefix the Clang compiler binary, instead
>  ``CROSS_COMPILE`` is used to set a command line flag: ``--target=<triple>``. For
> @@ -63,6 +63,23 @@ They can be enabled individually. The full list of the parameters: ::
>  Currently, the integrated assembler is disabled by default. You can pass
>  ``LLVM_IAS=1`` to enable it.
>
> +Omitting CROSS_COMPILE
> +----------------------
> +
> +As explained above, ``CROSS_COMPILE`` is used to set ``--target=<triple>``.
> +
> +Unless ``LLVM_IAS=1`` is specified, ``CROSS_COMPILE`` is also used to derive
> +``--prefix=<path>`` to search for the GNU assembler and linker.


Is there any place where we rely on --prefix
to search for the linker?

In general, the compiler stops after generating an object
since it is passed with the -c option.
The linking stage is separated.

In the old days, VDSO was an exceptional case
where $(CC) was used as the linker driver, but
commit fe00e50b2db8c60e4ec90befad1f5bab8ca2c800 fixed it.







> +
> +If ``CROSS_COMPILE`` is not specified, the ``--target=<triple>`` is inferred
> +from ``ARCH``.
> +
> +That means if you use only LLVM tools, ``CROSS_COMPILE`` becomes unnecessary.
> +
> +For example, to cross-compile the arm64 kernel::
> +
> +       make ARCH=arm64 LLVM=1 LLVM_IAS=1
> +
>  Supported Architectures
>  -----------------------
>
> --
> 2.32.0.554.ge1b32706d8-goog
>
> --
> You received this message because you are subscribed to the Google Groups "Clang Built Linux" group.
> To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe@googlegroups.com.
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Nick Desaulniers Aug. 5, 2021, 6:27 p.m. UTC | #3
On Thu, Aug 5, 2021 at 6:58 AM Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> On Tue, Aug 3, 2021 at 3:39 AM 'Nick Desaulniers' via Clang Built
> Linux <clang-built-linux@googlegroups.com> wrote:
> > diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
> > index b18401d2ba82..f8a360958f4c 100644
> > --- a/Documentation/kbuild/llvm.rst
> > +++ b/Documentation/kbuild/llvm.rst
> > @@ -63,6 +63,23 @@ They can be enabled individually. The full list of the parameters: ::
> >  Currently, the integrated assembler is disabled by default. You can pass
> >  ``LLVM_IAS=1`` to enable it.
> >
> > +Omitting CROSS_COMPILE
> > +----------------------
> > +
> > +As explained above, ``CROSS_COMPILE`` is used to set ``--target=<triple>``.
> > +
> > +Unless ``LLVM_IAS=1`` is specified, ``CROSS_COMPILE`` is also used to derive
> > +``--prefix=<path>`` to search for the GNU assembler and linker.
>
>
> Is there any place where we rely on --prefix
> to search for the linker?
>
> In general, the compiler stops after generating an object
> since it is passed with the -c option.
> The linking stage is separated.
>
> In the old days, VDSO was an exceptional case
> where $(CC) was used as the linker driver, but
> commit fe00e50b2db8c60e4ec90befad1f5bab8ca2c800 fixed it.

See my previous reply to Fangrui.
https://lore.kernel.org/lkml/CAKwvOdnK=SUm1_--tcLRO3LVeXd_2Srfv2tsZCUW0uXXa1W_pg@mail.gmail.com/

To be more specific, I believe this is still a problem for ppc vdso.
https://github.com/ClangBuiltLinux/linux/issues/774

I had sent patches for that, but binutils 2.26 would crash (IIUC,
newer GNU binutils are ok).  See this thread:
https://lore.kernel.org/lkml/b2066ccd-2b81-6032-08e3-41105b400f75@csgroup.eu/

So "we'd prefer the linker was used as the driver, but there's at
least one place I know of in the tree where that's not currently the
case."
Nick Desaulniers Aug. 5, 2021, 6:29 p.m. UTC | #4
On Thu, Aug 5, 2021 at 11:27 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Thu, Aug 5, 2021 at 6:58 AM Masahiro Yamada <masahiroy@kernel.org> wrote:
> >
> > On Tue, Aug 3, 2021 at 3:39 AM 'Nick Desaulniers' via Clang Built
> > Linux <clang-built-linux@googlegroups.com> wrote:
> > > diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
> > > index b18401d2ba82..f8a360958f4c 100644
> > > --- a/Documentation/kbuild/llvm.rst
> > > +++ b/Documentation/kbuild/llvm.rst
> > > @@ -63,6 +63,23 @@ They can be enabled individually. The full list of the parameters: ::
> > >  Currently, the integrated assembler is disabled by default. You can pass
> > >  ``LLVM_IAS=1`` to enable it.
> > >
> > > +Omitting CROSS_COMPILE
> > > +----------------------
> > > +
> > > +As explained above, ``CROSS_COMPILE`` is used to set ``--target=<triple>``.
> > > +
> > > +Unless ``LLVM_IAS=1`` is specified, ``CROSS_COMPILE`` is also used to derive
> > > +``--prefix=<path>`` to search for the GNU assembler and linker.
> >
> >
> > Is there any place where we rely on --prefix
> > to search for the linker?
> >
> > In general, the compiler stops after generating an object
> > since it is passed with the -c option.
> > The linking stage is separated.
> >
> > In the old days, VDSO was an exceptional case
> > where $(CC) was used as the linker driver, but
> > commit fe00e50b2db8c60e4ec90befad1f5bab8ca2c800 fixed it.
>
> See my previous reply to Fangrui.
> https://lore.kernel.org/lkml/CAKwvOdnK=SUm1_--tcLRO3LVeXd_2Srfv2tsZCUW0uXXa1W_pg@mail.gmail.com/
>
> To be more specific, I believe this is still a problem for ppc vdso.
> https://github.com/ClangBuiltLinux/linux/issues/774
>
> I had sent patches for that, but binutils 2.26 would crash (IIUC,
> newer GNU binutils are ok).  See this thread:
> https://lore.kernel.org/lkml/b2066ccd-2b81-6032-08e3-41105b400f75@csgroup.eu/
>
> So "we'd prefer the linker was used as the driver, but there's at
> least one place I know of in the tree where that's not currently the
> case."

Also, I think the CC_CAN_LINK functionality also fits the bill.
https://github.com/ClangBuiltLinux/linux/issues/1290
Masahiro Yamada Aug. 5, 2021, 11:26 p.m. UTC | #5
On Fri, Aug 6, 2021 at 3:29 AM Nick Desaulniers <ndesaulniers@google.com> wrote:
>
> On Thu, Aug 5, 2021 at 11:27 AM Nick Desaulniers
> <ndesaulniers@google.com> wrote:
> >
> > On Thu, Aug 5, 2021 at 6:58 AM Masahiro Yamada <masahiroy@kernel.org> wrote:
> > >
> > > On Tue, Aug 3, 2021 at 3:39 AM 'Nick Desaulniers' via Clang Built
> > > Linux <clang-built-linux@googlegroups.com> wrote:
> > > > diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
> > > > index b18401d2ba82..f8a360958f4c 100644
> > > > --- a/Documentation/kbuild/llvm.rst
> > > > +++ b/Documentation/kbuild/llvm.rst
> > > > @@ -63,6 +63,23 @@ They can be enabled individually. The full list of the parameters: ::
> > > >  Currently, the integrated assembler is disabled by default. You can pass
> > > >  ``LLVM_IAS=1`` to enable it.
> > > >
> > > > +Omitting CROSS_COMPILE
> > > > +----------------------
> > > > +
> > > > +As explained above, ``CROSS_COMPILE`` is used to set ``--target=<triple>``.
> > > > +
> > > > +Unless ``LLVM_IAS=1`` is specified, ``CROSS_COMPILE`` is also used to derive
> > > > +``--prefix=<path>`` to search for the GNU assembler and linker.
> > >
> > >
> > > Is there any place where we rely on --prefix
> > > to search for the linker?
> > >
> > > In general, the compiler stops after generating an object
> > > since it is passed with the -c option.
> > > The linking stage is separated.
> > >
> > > In the old days, VDSO was an exceptional case
> > > where $(CC) was used as the linker driver, but
> > > commit fe00e50b2db8c60e4ec90befad1f5bab8ca2c800 fixed it.
> >
> > See my previous reply to Fangrui.
> > https://lore.kernel.org/lkml/CAKwvOdnK=SUm1_--tcLRO3LVeXd_2Srfv2tsZCUW0uXXa1W_pg@mail.gmail.com/
> >
> > To be more specific, I believe this is still a problem for ppc vdso.
> > https://github.com/ClangBuiltLinux/linux/issues/774
> >
> > I had sent patches for that, but binutils 2.26 would crash (IIUC,
> > newer GNU binutils are ok).  See this thread:
> > https://lore.kernel.org/lkml/b2066ccd-2b81-6032-08e3-41105b400f75@csgroup.eu/
> >
> > So "we'd prefer the linker was used as the driver, but there's at
> > least one place I know of in the tree where that's not currently the
> > case."
>
> Also, I think the CC_CAN_LINK functionality also fits the bill.
> https://github.com/ClangBuiltLinux/linux/issues/1290
> --
> Thanks,
> ~Nick Desaulniers


Ah, this is it.

Thanks.
diff mbox series

Patch

diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
index b18401d2ba82..f8a360958f4c 100644
--- a/Documentation/kbuild/llvm.rst
+++ b/Documentation/kbuild/llvm.rst
@@ -38,7 +38,7 @@  Cross Compiling
 A single Clang compiler binary will typically contain all supported backends,
 which can help simplify cross compiling. ::
 
-	ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang
+	make ARCH=arm64 CC=clang CROSS_COMPILE=aarch64-linux-gnu-
 
 ``CROSS_COMPILE`` is not used to prefix the Clang compiler binary, instead
 ``CROSS_COMPILE`` is used to set a command line flag: ``--target=<triple>``. For
@@ -63,6 +63,23 @@  They can be enabled individually. The full list of the parameters: ::
 Currently, the integrated assembler is disabled by default. You can pass
 ``LLVM_IAS=1`` to enable it.
 
+Omitting CROSS_COMPILE
+----------------------
+
+As explained above, ``CROSS_COMPILE`` is used to set ``--target=<triple>``.
+
+Unless ``LLVM_IAS=1`` is specified, ``CROSS_COMPILE`` is also used to derive
+``--prefix=<path>`` to search for the GNU assembler and linker.
+
+If ``CROSS_COMPILE`` is not specified, the ``--target=<triple>`` is inferred
+from ``ARCH``.
+
+That means if you use only LLVM tools, ``CROSS_COMPILE`` becomes unnecessary.
+
+For example, to cross-compile the arm64 kernel::
+
+	make ARCH=arm64 LLVM=1 LLVM_IAS=1
+
 Supported Architectures
 -----------------------