[fpc-devel] Discussion on a particular optimisation development (WARNING: Technical!)

[Florian Klämpfl]

Am 03.02.21 um 22:36 schrieb J. Gareth Moreton via fpc-devel:
> On 03/02/2021 21:18, Florian Klämpfl via fpc-devel wrote:
>> Am 03.02.21 um 22:14 schrieb J. Gareth Moreton via fpc-devel:
>>> Rats, I might have messed up with some of the arithmetic, as well as 
>>> the dangers of crossing bitwise with logical Boolean operations, 
>>> although some combinations still work - if the conditions are "x = 0" 
>>> rather than "x <> 0" in the example:
>>>
>>>       testq    %rbx,%rbx
>>>       seteb    %al
>>>       testq    %rsi,%rsi
>>>       seteb    %dl
>>>       andb     %dl,%al
>>>
>>> Then this would equate to:
>>>
>>>       orq      %rbx,%rsi
>>>       seteb    %al
>>
>> Indeed:
>>
>> # [6] b:=(x=0) and (y=0);
>>     orl    %edx,%eax
>> # Var b located in register al
>>     seteb    %al
>>
>> :)
>>
> Suddenly I'm feeling embarrassed!  I'll have to look up to see how that 
> one is optimised!
>>>
>>> Mistake aside, I'm looking for acceptable solutions for a 
>>> platform-dependent node transformation, like a flag of some kind ($if 
>>> defined etc. might get untidy, although is an option).
>>>
>>
>> This depends on the transformation. Most transformations are 
>> beneficial for all platforms.
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>>
> I'll look around to see if there are other possible optimisations that 
> revolve around DeMorgan's Laws first.  For "b:=(x<>0) and (y<>0);", 
> there might not be a simple transformation available after all because, 
> generally, bitwise not <> logical not.  

I am pretty sure we miss some of the bit fiddeling stuff listed here: 
https://en.wikipedia.org/wiki/Bitwise_operation#Boolean_algebra

But there are more, like (typical pattern in the sha algorithm, I think 
I have even an unfinished patch for this one):

(a and b) or (c and not(b)) => c xor ((c xor a) and b)

One operation less and no additional temp. needed. Some googling might 
reveal more.

> The main question regarding 
> transformations is that there might be a place for "nor" and "nand" 
> operations (more traditional operations for electronics since you can 
> make circuits that do all the basic Boolean operations using just NOR 
> and NAND gates... e.g. a NAND gate where both inputs are wired to the 
> same signal becomes a NOT gate), and Intel does have "and not" (ANDN) as 
> a single instruction on later processors (anything that supports BMI1).

Those can be done normally one the fly during code generation,