Re: [PATCH] RISCV: Add -m(no)-omit-leaf-frame-pointer support.

["Stefan O'Rear" <sorear@fastmail.com>]

On Sat, Jun 24, 2023, at 11:01 AM, Jeff Law via Gcc-patches wrote:
> On 6/21/23 02:14, Wang, Yanzhang wrote:
>> Hi Jeff, sorry for the late reply.
>> 
>>> The long branch handling is done at the assembler level.  So the clobbering
>>> of $ra isn't visible to the compiler.  Thus the compiler has to be
>>> extremely careful to not hold values in $ra because the assembler may
>>> clobber $ra.
>> 
>> If assembler will modify the $ra behavior, it seems the rules we defined in
>> the riscv.cc will be ignored. For example, the $ra saving generated by this
>> patch may be modified by the assmebler and all others depends on it will be
>> wrong. So implementing the long jump in the compiler is better.
> Basically correct.  The assembler potentially clobbers $ra.  That's why 
> in the long jump patches $ra becomes a fixed register -- the compiler 
> doesn't know when it's clobbered by the assembler.
>
> Even if this were done in the compiler, we'd still have to do something 
> special with $ra.  The point at which decisions about register 
> allocation and such are made is before the point where we know the final 
> positions of jumps/labels.  It's a classic problem in GCC's design.

Do you have a reference for more information on the long jump patches?

I'm particularly curious about why $ra was selected as the temporary instead
of $t1 like the tail pseudoinstruction uses.

-s