[fpc-devel] I've asked this before, but perhaps I wasn't specific enough that time: what do I *personally*, specifically need to do to ensure that a native Windows 64-bit build winds up on the FPC website for the next release?

[Nikolay Nikolov]

On 1/13/22 05:59, Travis Siegel via fpc-devel wrote:
>
>
> On 1/12/2022 5:20 PM, Sven Barth via fpc-devel wrote:
>> When compiling from a target supporting Extended to one only 
>> supporting Double there isn't a loss of precision when calculating 
>> values at compile time. The other way around however, there *is* and 
>> that is the more crucial problem.
>>
>> Regards,
>> Sven
>>
> /I understand only part of this issue.  64-bit windows doesn't have 
> extended support, is there a reason for this? If it's simply 
> processors, and it works on linux, why does it not work on windows?/
>
> /Also, since it's 64-bit, wouldn't a double on a 64-bit system match 
> or exceed the numeric range on an extended range for a 32-bit system?/
>
> /I'm no expert on compiler numeric ranges, and 32/64 ranges aren't 
> something I've studied a whole lot of, other than to note that 64-bit 
> processors can handle *much* larger numbers, so I don't understand why 
> this problem exists./
>
> /Is there a summary of why this is a problem anywhere I can refer to 
> so I can understand why this happens, and what (if anything) can be 
> done to solve it?/
>
> /I've always been fascinated by compilers, though I've never actually 
> written anything except an assembler for dos several years ago, I was 
> never able to extend that to other languages, because of lack of 
> knowledge of how the cpu does things, but this is still an interesting 
> topic for me, and I honestly can't figure out why there would be an 
> issue at all./
>
> /I'm not doubting there is one, I'm just missing a piece or two to 
> understand it./
>
> /Any help would be appreciated.
> /
>
The i386 compiler uses the x87 FPU for floating point. The x87 supports 
the 32-bit single precision floating point type, the 64-bit double 
precision floating point type and the 80-bit extended precision extended 
floating point type. So, the best precision you get is 80-bit.

The x86_64 compiler uses SSE2 instructions for floating point. However, 
the SSE2 instructions only support 32-bit and 64-bit float types.

X86_64 processors still have the FPU, and it still functions in 64-bit 
mode, so the x86_64 compiler can still use the FPU for the 80-bit 
extended type. In fact, this is why the linux x86_64 compiler works 
without limitations. However, Windows doesn't guarantee future 
compatibility if the FPU is used.

https://docs.microsoft.com/en-us/windows/win32/dxtecharts/sixty-four-bit-programming-for-game-developers?redirectedfrom=MSDN#Porting_to_64bit

Quote:

"The x87, MMX, and 3DNow! instruction sets are deprecated in 64-bit 
modes. The instructions sets are still present for backward 
compatibility for 32-bit mode; however, to avoid compatibility issues in 
the future, their use in current and future projects is discouraged."

A logical question that follows is how can they break it in future 
Windows versions, if it's supported by the CPU. The answer is, since 
Windows is a multitasking operating system, they might stop saving the 
FPU registers when switching between 64-bit tasks. So, the only 
guarantee that x87 FPU code will continue to work in future Windows 
versions is if it's in a 32-bit process.

It kinda sucks, but it is what it is. Microsoft have decided that the 
FPU is considered "legacy" and "deprecated". However, the replacement 
(SSE2) doesn't have all the capabilities that the FPU has, namely it 
doesn't have 80-bit extended precision. I guess, you can complain to 
Microsoft about that and not to Free Pascal developers, but I doubt that 
they will care. :)


Nikolay
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