On Sat, Aug 7, 2021 at 4:53 AM Nathan Chancellor [off-list ref] wrote:

On Fri, Aug 06, 2021 at 10:27:01AM -0700, Nick Desaulniers wrote:

quoted

LLVM_IAS=1 controls enabling clang's integrated assembler via
-integrated-as. This was an explicit opt in until we could enable
assembler support in Clang for more architecures. Now we have support
and CI coverage of LLVM_IAS=1 for all architecures except a few more
bugs affecting s390 and powerpc.

The powerpc and s390 folks have been testing with clang, I think they
should have been on CC for this change (done now).

quoted

This commit flips the default from opt in via LLVM_IAS=1 to opt out via
LLVM_IAS=0.  CI systems or developers that were previously doing builds
with CC=clang or LLVM=1 without explicitly setting LLVM_IAS must now
explicitly opt out via LLVM_IAS=0, otherwise they will be implicitly
opted-in.

This finally shortens the command line invocation when cross compiling
with LLVM to simply:

$ make ARCH=arm64 LLVM=1

Signed-off-by: Nick Desaulniers <redacted>

I am still not really sure how I feel about this. I would prefer not to
break people's builds but I suppose this is inevitabile eventually.

A little support matrix that I drafted up where based on ARCH and clang
version for LLVM_IAS=1 support:

             | 10.x | 11.x | 12.x | 13.x | 14.x |
ARCH=arm     |  NO  |  NO  |  NO  |  YES |  YES |
ARCH=arm64   |  NO  |  YES |  YES |  YES |  YES |
ARCH=i386    |  YES |  YES |  YES |  YES |  YES |
ARCH=mips*   |  YES |  YES |  YES |  YES |  YES |
ARCH=powerpc |  NO  |  NO  |  NO  |  NO  |  NO  |
ARCH=s390    |  NO  |  NO  |  NO  |  NO  |  NO  |
ARCH=x86_64  |  NO  |  YES |  YES |  YES |  YES |

The main issue that I have with this change is that all of these
architectures work fine with CC=clang and their build commands that used
to work fine will not with this change, as they will have to specify
LLVM_IAS=0. I think that making this change for LLVM=1 makes sense but
changing the default for just CC=clang feels like a bit much at this
point in time. I would love to hear from others on this though, I am not
going to object much further than this.

Regardless of that concern, this patch does what it says so:

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

Applied to linux-kbuild.
Thanks.


-- 
Best Regards
Masahiro Yamada