Re: [PATCH] RISC-V: Produce better code with complex constants [PR95632] [PR106602]

[Palmer Dabbelt <palmer@dabbelt.com>]

On Thu, 08 Dec 2022 10:15:47 PST (-0800), gcc-patches@gcc.gnu.org wrote:
>
>
> On 12/8/22 10:53, Palmer Dabbelt wrote:
>> On Wed, 07 Dec 2022 12:55:17 PST (-0800), rzinsly@ventanamicro.com wrote:
>>> Due to RISC-V limitations on operations with big constants combine
>>> is failing to match such operations and is not being able to
>>> produce optimal code as it keeps splitting them. By pretending we
>>> can do those operations we can get more opportunities for
>>> simplification of surrounding instructions.
>>
>> I saw Jeff's comments.  This is always the kind of thing that worries
>> me: we're essentially lying to the optimizer in order to trick it into
>> generating better code, which might just make it generate worse code.
>> It's always easy to see a small example that improves, but those could
>> be wiped out by secondary effects in real code.  So I'd usually want to
>> have some benchmarking for a patch like this.
>>
>> That said, if this is just the standard way of doing things then maybe
>> it's just fine?
> Bridge combiner patterns are pretty standard.  The insn's condition of
> cse_not_expected is also in there to minimize the potential for
> surprises by not exposing this too early.

OK, I'm fine with this, then -- aside from the fairly minor issues 
pointed out.