I do agree that populating the carry bit is not that straightforward,
especially on processors that don't have an explicit carry.
An alternative approach is to go for something similar to the SHRD
instruction, that pulls in bits from another register rather than the
carry bit - https://www.felixcloutier.com/x86/shrd - then again, this
might be overkill when you want to rotate with a single carry bit, which
is effectively akin to rotating a register that is 1 bit larger than the
normal word size.
Gareth aka. Kit
On 11/01/2021 18:53, Karoly Balogh via fpc-devel wrote:
Hi,
On Mon, 11 Jan 2021, Sven Barth via fpc-devel wrote:
So the idea is to add support for ROL/ROR with Carry support as the
current Rol*/Ror* functions don't support that. Florian then said that
he's open to suggestions under some restrictions. Considering that the
current Rol*/Ror* intrinsics aren't available for all platforms as
intrinsics either, but as functions then, this could be done to support
CPUs without suitable Carry support as well. And the code snippet that
jamie showed would be a nice base:
=== code begin ===
Function ROLByteWithCarry(Const AByte:Byte; Var ACarry:Boolean):Byte;Inline;
var
LocalIn:Boolean;
Begin
LocalIN := Acarry;
ACarry := ShortInt(Abyte) < 0;
Result := (AByte shl 1);
IF LocalIn then Result := Result or 1;
End;
We can name it rolx/rorx... I don't think such a long name is a good idea,
because such construct will be used a lot in longer formulas?
Btw, as the code, what's wrong with:
begin
result:=(abyte shl 1) or ord(acarry);
acarry:=shortint(abyte) < 0;
end;
No local var, no branches...
The problem is with this ROL-with-carry as library function approach, that
it tries to mimic internal working of some CPU, so it inherently leads to
less efficient code, because it's very difficult to actually map this into
a CPU instruction... Even when we provide an instrinsic for it, because
how on earth you populate the carry flag first based on that boolean
coming from that argument? So I feel people will try to use this for
"smart" algos and optimization, while it's totally counterproductive in a
high level language.
It's much better to write such code as <value * 2> + previous carry then
clamp it, then let the compiler optimize it, rather than trying to mess
with a rorx/rolx function.
I think whoever wants such a ching is just better off extending their byte
to a word, doing the shift/rotate, and masking the carry off as needed.
My 2 cents.
Charlie
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