[fpc-pascal] round(2.5)=2

[Wolf]

Why not ask the CPU to do the rounding? This is what Intel offers in 
volume 1 of the Intel ® 64 and IA-32 Architectures Software Developer's 
Manual:


4.8.4.1
  Rounding Control (RC) Fields
In the Intel 64 and IA-32 architectures, the rounding mode is controlled 
by a 2-bit rounding-control (RC) field
(Table 4-8 shows the encoding of this field). The RC field is 
implemented in two different locations:
•
  x87 FPU control register (bits 10 and 11)
•
  The MXCSR register (bits 13 and 14)
Although these two RC fields perform the same function, they control 
rounding for different execution environ-
ments within the processor. The RC field in the x87 FPU control register 
controls rounding for computations
performed with the x87 FPU instructions; the RC field in the MXCSR 
register controls rounding for SIMD floating-
point computations performed with the SSE/SSE2 instructions./
/
Do you want more?

Wolf


On 13/06/2018 09:58, Klaus Hartnegg wrote:
> Am 12.06.2018 um 23:24 schrieb Jonas Maebe:
>> On 12/06/18 23:12, Klaus Hartnegg wrote:
>>> This behaviour does have advantages in the special case of adding up 
>>> many rounded numbers, but it ruins the graphics output of my 
>>> software. It should at least be configurable. And in {$mode tp} it 
>>> should behave like Turbo Pascal, because that's what this mode is 
>>> made for.
>>
>> The basic issue is that FPC doesn't support the 6 byte real type used 
>> by TP in {$n-}, nor the associated behaviours such as rounding, 
>> range/precision cut-offs etc. FPC indeed can only support TP {$n+} 
>> mode compatibility at this time (and even that is not always exact, 
>> as it depends on the FPU precision support by the hardware and OS).
>
> Nevertheless it could offer an option to use the other rounding method.
>
> Yes, there would still be deviations in some math results, because of 
> the different precision. But these deviations are much smaller than 
> those from the other rounding method.
>
> Klaus
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