If the code breaks down to "x div 1000000000" (with 1000000000 as an
explicit constant and x being a 64-bit integer), then yes it will. If
the divisor is a variable though, then it won't. However, the "magic
div" algorithm only works for numerators up to a given bit size; if the
number exceeds it, you'll lose precision and risk getting an incorrect
answer (a warning for if you want to try to implement the algorithm
yourself).
Gareth aka. Kit
On 16/11/2021 17:52, Benito van der Zander via fpc-devel wrote:
Hi,
in my big decimal unit, I need to divide by 1000000000 all the time,
e.g.
https://github.com/benibela/bigdecimalmath/blob/master/bigdecimalmath.pas#L1324-L1325
would the magic div help there much?
Bye,
Benito
On 09.11.21 22:12, J. Gareth Moreton via fpc-devel wrote:
This one for Marģers specifically,
You'll be pleased to know that your insight has been partially
implemented!
https://gitlab.com/freepascal.org/fpc/source/-/merge_requests/51
This only expands 32-bit divisions to 64-bit, since the smaller sizes
requires more work at the node level, but is certainly possible and
will happily continue to experiment with seeing if it can be
implemented. This was fun to do!
The test "bench/bdiv.pp" (also "tests/test/cg/tmoddiv6.pp" which just
includes the given file) is my benchmark test for division and
modulus operations if anyone wants to see if they can make the
fundamental routine faster on any platform.
Gareth aka. Kit
On 24/08/2021 20:14, Marģers . via fpc-devel wrote:
I came up with even shorter variant of div
example
function teDWordDivBy7_v4( divided : dword):dword; assembler;
nostackframe;
asm
mov ecx,divided
mov rax,2635249153693862181
mul rcx
mov eax,edx
end;
current version for comparison
function teDWordDivBy7_v0( divided : dword):dword; assembler;
nostackframe;
asm
mov ecx,divided
mov eax,613566757
mul ecx
add edx,ecx
rcr edx,1
shr edx,2
mov eax,edx
end;
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