Re: [PATCH] invoke: RISC-V's -march doesn't take ISA strings

[Palmer Dabbelt <palmer@rivosinc.com>]

On Tue, 08 Nov 2022 05:40:10 PST (-0800), christoph.muellner@vrull.eu wrote:
> On Mon, Nov 7, 2022 at 8:01 PM Palmer Dabbelt <palmer@rivosinc.com> wrote:
>
>> The docs say we take ISA strings, but that's never really been the case:
>> at a bare minimum we've required lower case strings, but there's
>> generally been some subtle differences as well in things like version
>> handling and such.  We talked about removing the lower case requirement
>> in the last GNU toolchain meeting and we've always called other
>> differences just bugs.  We don't have profile support yet, but based on
>> the discussions on the RISC-V lists it looks like we're going to have
>> some differences there as well.
>
>
>> So let's just stop pretending these are ISA strings.  That's been a
>> headache for years now, if we're meant to just be ISA-string-like here
>> then we don't have to worry about all these long-tail ISA string parsing
>> issues.
>>
>
> You are right, we should first properly specify the -march string,
> before we talk about the implementation details of the parser.
>
> I tried to collect all the recent change requests and undocumented
> properties of the -march string and worked on a first draft specification.
> As the -march flag should share a common behavior across different
> compilers and tools, I've made a PR to the RISC-V toolchain-conventions
> repo:
>   https://github.com/riscv-non-isa/riscv-toolchain-conventions/pull/26
>
> Do you mind if we continue the discussion there?

IMO trying to handle this with another RISC-V spec is a waste of time: 
we've spent many years trying to follow the specs here, it's pretty 
clear they're just not meant to be read in that level of detail.  This 
sort of problem is all over the place in RISC-V land, moving to a 
different spec doesn't fix the problem.

>> Link: https://lists.riscv.org/g/sig-toolchains/message/486
>>
>> gcc/ChangeLog
>>
>>         doc/invoke.texi (RISC-V): -march doesn't take ISA strings.
>>
>> ---
>>
>> This is now woefully under-documented, as we can't even fall back on the
>> "it's just an ISA string" excuse any more.  I'm happy to go document
>> that, but figured I'd just send this along now so we can have the
>> discussion.
>> ---
>>  gcc/doc/invoke.texi | 8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
>> index 94a2e20cfc1..780b0364c52 100644
>> --- a/gcc/doc/invoke.texi
>> +++ b/gcc/doc/invoke.texi
>> @@ -28617,11 +28617,11 @@ Produce code conforming to version 20191213.
>>  The default is @option{-misa-spec=20191213} unless GCC has been configured
>>  with @option{--with-isa-spec=} specifying a different default version.
>>
>> -@item -march=@var{ISA-string}
>> +@item -march=@var{target-string}
>>  @opindex march
>> -Generate code for given RISC-V ISA (e.g.@: @samp{rv64im}).  ISA strings
>> must be
>> -lower-case.  Examples include @samp{rv64i}, @samp{rv32g}, @samp{rv32e},
>> and
>> -@samp{rv32imaf}.
>> +Generate code for given target (e.g.@: @samp{rv64im}).  Target strings
>> are
>> +similar to ISA strings, but must be lower-case.  Examples include
>> @samp{rv64i},
>> +@samp{rv32g}, @samp{rv32e}, and @samp{rv32imaf}.
>>
>>  When @option{-march=} is not specified, use the setting from
>> @option{-mcpu}.
>>
>> --
>> 2.38.1
>>
>>