Re: [PATCH] RISC-V: Add the option "-mdisable-multilib-check" to avoid multilib checks breaking the compilation.

[Jeff Law <jeffreyalaw@gmail.com>]

On 5/28/23 21:46, Jin Ma wrote:
>>>>> When testing a extension, it is often necessary for a certain program not to
>>>>> need some kind of extension, such as the bitmanip extension, to evaluate the
>>>>> performance or codesize of the extension. However, the current multilib rules
>>>>> will report an error when it is not a superset of the MULTILIB_REQUIRED list,
>>>>> which will cause the program to be unable to link normally, thus failing to
>>>>> achieve the expected purpose.
>>>>
>>>>   Hmm, I have troubles understanding what is going on here.  What do you
>>>> refer to by saying: "it is not a superset of the MULTILIB_REQUIRED list"?
>>>
>>> This is a new matching rule added by kito for the multilib of riscv:
>>> https://github.com/gcc-mirror/gcc/commit/d72ca12b846a9f5c01674b280b1817876c77888f
>>>
>>>>   There should be no problem with linking compiled modules together that
>>>> make use of different extensions, with the static linker figuring out the
>>>> combined set of extensions actually required at run time for the program
>>>> loader to consider, as long as the modules do not have contradicting
>>>> requirements, e.g. big vs little endianness or RV32 vs RV64.
>>>>
>>>>   Can you give me a specific example (compilation options and multilibs
>>>> available) of a failure you refer to?
>>>
>>> A simple example:
>>> 1. Use "--disable-multilib --with-abi =lp64d --with-arch =rv64imafdc_zba_zbb_zbc_zbs"
>>> to build the toolchain".
>>> 2. Use the toolchain to test the impact of zba_zbb_zbc_zbs extensions on the
>>> performance and codesize of some functions or files in the program.
>>>
>>> In this case, I may need to use the command "-mabi=lp64d -march=rv64imafdc" for
>>> the compilation of a specific .c file in the program, which will cause the link to
>>> fail and throw the following error: "FATAL ERROR: Can't find suitable multilib set for
>>> '-march=rv64imafdc'/'-mabi=lp64d'". This does not satisfy the purpose of the test.
>>
>> I feel this case should be build with --with-arch =rv64imafdc and test
>> with -march=rv64imafdc and  -march=rv64imafdc_zba_zbb_zbc_zbs,
>> but anyway I am OK with option :P
> 
> Yes, but with "--with-arch=rv64imafdc" building toolchains, the library will not contain
> zba_zbb_zbc_zbs extensions, so how can we quickly and easily eliminate the impact of not
> using zba_zbb_zbc_zbs extensions in a certain program on program performance and codesize?
> 
> Although-mno-multilib-check is unsafe, it is useful during the development and testing phases.
But I'm not sure that's a good reason to include an unsafe option like 
this in the official GCC sources.

This is the kind of thing that I'd tend to think belongs as a local change.

jeff