Re: [PATCH 1/2] Add new target hook: constant_ok_for_cprop_p

[Jeff Law <jeffreyalaw@gmail.com>]

On 9/12/22 01:35, Richard Biener via Gcc-patches wrote:
> On Sun, Sep 11, 2022 at 10:51 PM Takayuki 'January June' Suwa via
> Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
>> Hi,
>>
>> Many RISC machines, as we know, have some restrictions on placing register-width constants in the source of load-immediate machine instructions, so the target must provide a solution for that in the machine description.
>>
>> A naive way would be to solve it early, ie. to replace with read constants pooled in memory when expanding to RTL.
>>
>> Alternatively, a more fancy approach would be to forgo placement in the constant pool until somewhere before the reload/LRA eg. the "split1" pass to give the optimization passes that involve immediates a chance to work.
>>
>> If we choose the latter, we can expect better results with RTL if-conversion, constant folding, etc., but it often propagates constants that are too large in size to resolve to a simple load-immediate instruction.
>>
>> This is because constant propagation has no way of telling about it, so this patch provides it.
> What does prevent other passes like fwprop, CSE and PRE from doing the
> same propagation?  Can that be the solution for
> constant propagation as well?

I would think this should be driven by costing rather than a new hook.  
I'm pretty sure we already use costing to determine some of this stuff 
for CSE/PRE.


jeff