On 2020-04-06, Masahiro Yamada wrote:
On Mon, Apr 6, 2020 at 8:55 AM 'Fangrui Song' via Clang Built Linux
<clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
On 2020-04-06, Masahiro Yamada wrote:
>On Sat, Apr 4, 2020 at 3:24 AM Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
>>
>> On Thu, Apr 2, 2020 at 10:17 PM Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote:
>> >
>> > As Documentation/kbuild/llvm.rst implies, building the kernel with a
>> > full set of LLVM tools gets very verbose and unwieldy.
>> >
>> > Provide a single switch 'LLVM' to use Clang and LLVM tools instead of
>> > GCC and Binutils. You can pass LLVM=1 from the command line or as an
>> > environment variable. Then, Kbuild will use LLVM toolchains in your
>> > PATH environment.
>> >
>> > Please note LLVM=1 does not turn on the LLVM integrated assembler.
>> > You need to explicitly pass AS=clang to use it. When the upstream
>> > kernel is ready for the integrated assembler, I think we can make
>> > it default.
>>
>> Having this behavior change over time may be surprising. I'd rather
>> that if you want to not use the integrated assembler, you explicitly
>> negate it, or just don't use the LLVM=1 syntax, ie. `make CC=clang
>> LD=ld.lld ...`.
>>
>> We could modify how `-no-integrated-as` is chosen when LLVM=1.
>>
>> make LLVM=1 LLVMIA=0 ... # add `-no-integrated-as`
>> # what the flag is doesn't really matter to me, something shorter might be nice.
>> make LLVM=1 # use all LLVM tools
>>
>> Since we got rid of $(AS), it would be appropriate to remove/change it
>> there, since no one really relies on AS=clang right now. (We do have 1
>> of our 60+ CI targets using it, but we can also change that trivially.
>> So I think we have a lot of freedom to change how `-no-integrated-as`
>> is set.
>>
>> This could even be independent of this patch.
>
>
>I also thought a boolean flag is preferred.
>
>AS=clang will not live long anyway, and
>I hesitated to break the compatibility
>for the short-term workaround.
>
>But, if this is not a big deal, I can
>replace AS=clang with LLVMIA=1.
My mere complaint is that it may be difficult to infer the intention (integrated
assembler) from the abbreviation "IA" in "LLVMIA" :/
Something with "AS" in the name may be easier for a user to understand,
e.g. CLANG_AS or LLVM_AS.
I see 'llvm-as' in my PATH,
but it is a different kind of tool, right?
(converter from LLVM assembler *.ll to LLVM bit code *.bc)
So, I thought "LLVM_AS" might be confusing.
You are right. llvm-as converts a textual form of LLVM IR (.ll) to
a binary form bitcode (.bc). LLVM_AS is confusing. CLANG_AS/CLANGAS might be
suitable.
clang a.c '-###' => clang -cc1 # like gcc invokes cc1
clang a.s '-###' => clang -cc1as # this invokes the integrated assembler
