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

["Feng Wang" <wangfeng@eswincomputing.com>]

On 2023-03-26 02:18  Jeff Law<jeffreyalaw@gmail.com> wrote:
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>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？
Thanks.

Feng Wang