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

[Jeff Law <jeffreyalaw@gmail.com>]

On 6/24/23 19:40, Stefan O'Rear wrote:
> 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 can extract the patch Andrew wrote if that would be helpful.

> 
> I'm particularly curious about why $ra was selected as the temporary instead
> of $t1 like the tail pseudoinstruction uses.
$ra would be less disruptive from a code generation standpoint. 
Essentially whatever register is selected has to become a fixed 
register, meaning it's unavailable to the allocator.   Thus $t1 would be 
a horrible choice.  Ultimately this is defined by the assembler.

jeff