On 3/23/23 20:45, juzhe.zh...@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
