Re: [PATCH] RISC-V: Optimize zbb ins sext.b and sext.h in rv64

[Jeff Law <jeffreyalaw@gmail.com>]

On 3/26/23 19:32, Feng Wang wrote:
> On 2023-03-26 02:18  Jeff Law<jeffreyalaw@gmail.com> wrote:
>>
>>
>>
>> On 3/23/23 20:45, juzhe.zhong@rivai.ai wrote:
>>> Sounds like you are looking at redundant extension problem in RISC-V port.
>>> This is the issue I want to fix but I don't find the time to do that.
>>> My first impression is that we need to fix redundant extension in "ree"
>>> PASS.
>>> I am not sure.
>> It's actually quite a bit more complicated.
>>
>> Some extension elimination can and probably should be happening in
>> gimple. In gimple you have access to type information as well as range
>> information.  So you have the opportunity to do things like rewrite the
>> IL to use different types when it's safe to do so, or to use range
>> information to identify when an object is already properly extended and
>> thus eliminate the extension before we expand gimple into RTL.
>>
>> Once in RTL, you can use forward propagation to eliminate extensions, or
>> at least fold them into existing operations.  combine can eliminate
>> extensions and it has the ability to track (for example) if the upper
>> bits are copies of the sign bit, if they're known zero, etc.  combine is
>> also capable of recognizing that a load implicitly extends and using
>> that knowledge to eliminate extensions or to discover that a pair of
>> shifts are just zero or sign extending a value, etc etc.  combine also
>> interacts with simplify-rtx which is used by other passes, so there's a
>> chance that work in simplify-rtx can eliminate extensions not just in
>> combine, but in other passes as well.
>>
>> REE is a post-register allocation pass and kind of the last chance to
>> eliminate extensions.
>>
>> So for any given redundant extension, the way to go (IMHO) is to walk
>> through the optimizer pipeline to see where it can potentially be
>> eliminated.  In general, the earlier in the optimizer pipeline the
>> extension can be eliminated, the better.
>>
>> Jeff
> Hi Jeff，Do you think my patch modification is suitable?What else needs to be improved？
I haven't looked at it in any detail.  We're in stage4 right now, so 
it's regression bugfixes only going into the tree.  Once gcc-13 branches 
I'll be focused on helping folks move RVV forward, submitting/refining 
various RISC-V patches from Ventana and reviewing other RISC-V related 
patches.

Jeff