Re: [PATCH] tree-optimization/110963 - more PRE when optimizing for size

[Richard Biener via Gcc-patches]

> Am 10.08.2023 um 17:01 schrieb Jeff Law via Gcc-patches 
> <gcc-patches@gcc.gnu.org>:
> 
> 
> 
>> On 8/10/23 06:41, Richard Biener via Gcc-patches wrote:
>> The following adjusts the heuristic when we perform PHI insertion
>> during GIMPLE PRE from requiring at least one edge that is supposed
>> to be optimized for speed to also doing insertion when the expression
>> is available on all edges (but possibly with different value) and
>> we'd at most have one copy from a constant.  The first ensures
>> we optimize two computations on all paths to one plus a possible
>> copy due to the PHI, the second makes sure we do not need to insert
>> many possibly large copies from constants, disregarding the
>> cummulative size cost of the register copies when they are not
>> coalesced.
>> The case in the testcase is
>>   <bb 5>
>>   _14 = h;
>>   if (_14 == 0B)
>>     goto <bb 7>;
>>   else
>>     goto <bb 6>;
>>   <bb 6>
>>   h = 0B;
>>   <bb 7>
>>   h.6_12 = h;
>> and we want to optimize that to
>>   <bb 7>
>>   # h.6_12 = PHI <_14(5), 0B(6)>
>> If we want to consider the cost of the register copies I think the
>> only simplistic enough way would be to restrict the special-case to
>> two incoming edges - we'd assume one register copy is coalesced
>> leaving one copy from a register or from a constant.
>> As with every optimization the downstream effects are probably
>> bigger than what we can locally estimate.
>> Bootstrapped and tested on x86_64-unknown-linux-gnu.
>> Any comments?
>> Thanks,
>> Richard.
>>    PR tree-optimization/110963
>>    * tree-ssa-pre.cc (do_pre_regular_insertion): Also insert
>>    a PHI node when the expression is available on all edges
>>    and we insert at most one copy from a constant.
>>    * gcc.dg/tree-ssa/ssa-pre-34.c: New testcase.
> The other thing in this space is to extend it to the case where multiple phi 
> args have the same constant.  My recollection is we had some bits in the 
> out-of-ssa code to factor those into a single path -- if that still works in 
> the more modern expansion approach, the it'd likely be a win to support as 
> well.

Yes, though it comes at the cost of another branch, no?  The other thing to 
consider is that undoing the transform is much more difficult for constants, we 
usually have no idea what expression to re-materialize for it.

Richard.

> jeff