Re: [PATCH] riscv: implement TARGET_MODE_REP_EXTENDED

[Alexander Monakov <amonakov@ispras.ru>]

[-- Attachment #1: Type: text/plain, Size: 1305 bytes --]


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).

> -- 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).

Alexander