Re: [PATCH] riscv: implement TARGET_MODE_REP_EXTENDED

[Jeff Law <jeffreyalaw@gmail.com>]

On 11/21/22 06:49, Alexander Monakov wrote:
> On Sun, 20 Nov 2022, Jeff Law wrote:
>
>>> The concern, as far as I understand would be the case where the
>>> assembly-sequence leaves an incompatible extension in the register.
>> Right.  The question in my mind is whether or not the responsibility should be
>> on the compiler or on the developer to ensure the ASM output is properly
>> extended.  If someone's writing ASMs, then to a large degree, I consider it
>> their responsibility to make sure things are properly extended
> Right. They should also find out the hard way, with zero documentation telling
> them they had to (let alone *why* they had to), and without a hypothetical
> -fsanitize=abi that would catch exactly the point of the missing extension
> instead of letting the program crash mysteriously at a much later point.
> Uphill both ways, in a world where LLVM does not place such burden on the
> programmer, and even among GCC targets only mips64 made a precedent (also
> without documenting the new requirement).

They're writing assembly code -- in my book that means they'd better 
have a pretty good understanding of the architecture, its limitations 
and quirks.

  I'm happy to give them some diagnostics, guardrails, etc etc, but 
slowing down standard C code to placate someone who doesn't really know 
the architecture, but is trying to write assembly code is a step too far 
for me.


>
>> -- even more so
>> if having the compiler do it results in slower code independent of ASMs.
> I think LLVM demonstrates well enough that a compiler can do a better job
> than GCC at eliminating redundant extensions without upgrading requirements
> for inline asm (in the usual C code, not for sub-word outputs of an asm).

Perhaps.  But it's also the case the LLVM and GCC have some significant 
differences in implementation.  Sometimes those differences in 
impementations have notable impacts on how code is generated.


jeff

>
> Alexander