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,
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