Re: [middle-end PATCH] Prefer PLUS over IOR in RTL expansion of multi-word shifts/rotates.

[Jeff Law <jeffreyalaw@gmail.com>]

On 1/22/24 00:45, Richard Biener wrote:
> On Fri, Jan 19, 2024 at 5:06 PM Georg-Johann Lay <avr@gjlay.de> wrote:
>>
>>
>>
>> Am 18.01.24 um 20:54 schrieb Roger Sayle:
>>>
>>> This patch tweaks RTL expansion of multi-word shifts and rotates to use
>>> PLUS rather than IOR for disjunctive operations.  During expansion of
>>> these operations, the middle-end creates RTL like (X<<C1) | (Y>>C2)
>>> where the constants C1 and C2 guarantee that bits don't overlap.
>>> Hence the IOR can be performed by any any_or_plus operation, such as
>>> IOR, XOR or PLUS; for word-size operations where carry chains aren't
>>> an issue these should all be equally fast (single-cycle) instructions.
>>> The benefit of this change is that targets with shift-and-add insns,
>>> like x86's lea, can benefit from the LSHIFT-ADD form.
>>>
>>> An example of a backend that benefits is ARC, which is demonstrated
>>> by these two simple functions:
>>
>> But there are also back-ends where this is bad.
>>
>> The reason is that with ORI, the back-end needs only to operate no
>> these sub-words where the sub-mask is non-zero.  But for PLUS this
>> is not the case because the back-end does not know that intermediate
>> carry will be zero.  Hence, with PLUS, more instructions are needed.
>> An example is AVR, but maybe much more target with multi-word operations
>> are affected in a bad way.
>>
>> Take for example the case with 2 words and a value of 1.
>>
>> LO |= 1
>> HI |= 0
>>
>> can be optimized to
>>
>> LO |= 1
>>
>> but for addition this is not the case:
>>
>> LO += 1
>> HI +=c 0 ;; Does not know that always carry = 0.
> 
> I wonder if the PLUS can be done on the lowpart only to make this
> detail obvious?
In theory, yes.   This class of problems has often been punted to the 
target expanders (far from ideal).

I still suspect the way forward here is to have the exp* code query one 
or more target properties to guide IOR vs PLUS selection.

Jeff