Re: [GCC RFC]A new and simple pass merging paired load store instructions

[Jeff Law <law at redhat dot com>]

On 05/19/14 00:38, Bin.Cheng wrote:
> 1) Should we do it in a separated pass, or just along with scheduler?
ISTM that when we're able to combine insns that can impact the schedule 
we'd like to generate, possibly in significant ways.  That argues for a 
separate pass that runs before the scheduler.

So I'd probably look at splitting out the code which builds the 
dependency information, running that first, then the pass to merge these 
memory ops (incrementally updating the dependency info), then the scheduler.


> 2) When should we run the new pass, before or after RA?  There are
> both advantages and disadvantages and very depends on the target for
> which we are compiling.
> I have no simple answer to this.  Maybe we can run the pass twice or
> follow Oleg's suggestion.  I think it's a new strategy for GCC to let
> backend decide when to run a pass.
If we only run the dependency stuff once and share the data between the 
new pass and the sceduler, then I think running the new pass twice just 
before scheduling would be fine.

> 3) Do we need a new target hook interface?
> I answered this in other messages and I still think it's target dependent.
Still pondering that one :-)

> 4) The optimization should be able to handle cases with more than 2
> consecutive load/store instructions.
> The current implementation can't handle such cases and need further extension.
Yea, probably.  I think if we build the infrastructure right extension 
for > 2 load/stores shouldn't be terribly bad to implement.

Jeff