I just repeated the test on i386-win32 and it's the same thing... with
one notable exception:
procedure DoDivMod(N, D: Int64; out Q, R: LongInt); noinline;
begin
Q := LongInt(N) div LongInt(D);
R := LongInt(N) mod LongInt(D);
end;
This reduces the operation to 32 bits, but does NOT generate a
conditional check for a divisor of -1, so LongInt($0000000080000000) div
LongInt(-1) will raise an exception, but won't raise an exception on
x86_64-win64, thus behaviour between platforms is different.
Can others confirm this?
Kit
On 21/05/2023 00:00, J. Gareth Moreton via fpc-devel wrote:
Hi Florian,
Can I have a specific code example where this is absolutely necessary,
and, if applicable, a target where it's known to cause a problem
otherwise? I've tried to create the example listed in the e-mail with
the following:
procedure DoDivMod(N, D: Int64; out Q, R: LongInt); noinline;
begin
Q := N div D;
R := N mod D;
end;
I'm testing on x86_64-win64. In this case, the "expanding to 64-bit"
is necessary and so "doremoveinttypeconvs" is never called, thus the
condition is not inserted. The "idiv" operation is 64-bit and the
result downsized to 32-bit - this also occurs if I do an explicit
typecast on the result:
procedure DoDivMod(N, D: Int64; out Q, R: LongInt); noinline;
begin
Q := LongInt(N div D);
R := LongInt(N mod D);
end;
However, if I do an explicit typecast on the operands...
procedure DoDivMod(N, D: Int64; out Q, R: LongInt); noinline;
begin
Q := LongInt(N) div LongInt(D);
R := LongInt(N) mod LongInt(D);
end;
... then "doremoveinttypeconvs" is called and the condition inserted.
I would argue here though that typecasting a value to a LongInt, where
an output of $80000000 is a real possibility, should raise an
exception if you try to divide it by -1 since the programmer is asking
to downsize values that could potentially be out of range.
Kit
On 19/05/2023 21:55, Florian Klämpfl via fpc-devel wrote:
Am 19.05.23 um 21:14 schrieb J. Gareth Moreton via fpc-devel:
So I need to ask... should the check for a divisor of -1 still be
performed?
Yes. This is the result of "down sizing" a division. In case of
longint(int64 div int64) can be converted only into longint(int64)
div longint(int64) if this check is carried out. longint($80000000
div $ffffffff) must silently result in $8000000 in this case.
The case of doing "min_int div -1", even with unsigned-to-signed
typecasting, seems very contrived and the programmer should expect
problems if "min_int" and "-1" appear as the operands. Is there a
specific example where this implicit check is absolutely necessary?
As others have pointed out, silently returning "min_int" as the
answer seems more unexpected (granted this is just the behaviour of
an optimisation that converts the nodes equating to "x div -1" to
"-x", and Intel's NEG instruction doesn't return an error if min_int
is its input operand, but I can't be sure if the same applies to
non-Intel processors and their equivalent instructions).
Kit
On 17/05/2023 09:51, J. Gareth Moreton via fpc-devel wrote:
Logically yes, but using 16-bit as an example, min_int is -32,768,
and signed 16-bit integers range from -32,768 to 32,767. So -32,768
÷ -1 = 32,768, which is out of range. This is where the problem lies.
Internally, negation involves inverting all of the bits and then
adding 1 (essentially how you subtract a number using two's
complement), so min_int, which is 1000000000000000, becomes
0111111111111111 and then, after incrementing, 1000000000000000,
which is min_int again.
Kit
On 16/05/2023 13:13, Jean SUZINEAU via fpc-devel wrote:
Le 16/05/2023 à 01:47, Stefan Glienke via fpc-devel a écrit :
min_int div -1
"min_int div -1" should give "- min_int" ?
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