Floating point support in 64-bit compiler

Question

What should we expect from the floating point support in 64-bit Delphi compiler?

• Will 64-bit compiler use SSE to implement floating point arithmetic?

• Will 64-bit compiler support the current 80-bit floating type (Extended)?

These questions are closely related, so I ask them as a single question.

Answer 1

For the double=extended bit:

Read ALlen Bauer's Twitter account Kylix_rd:

http://twitter.com/kylix_rd

In hindsight logical, because while SSE2 regs are 128 bit, they are used as two 64-bit doubles.

Answer 2

I made two posts on the subject (here and there), to summarize, yes, the 64bit compiler uses SSE2 (double precision), but it doesn't use SSE (single precision). Everything is converted to double precision floats, and computed using SSE2 (edit: however there is an option to control that)

This means f.i. that if Maths on double precision floats is fast, maths on single precision is slow (lots of redundant conversions between single and double precisions are thrown in), "Extended" is aliased to "Double", and intermediate computations precision is limited to double precision.

Edit: There was an undocumented (at the time) directive that controls SSE code generation, {$EXCESSPRECISION OFF} activates SSE code generation, which brings back performance within expectations.

Answer 3

According to Marco van de Voort in his answer to: How should I prepare my 32-bit Delphi programs for an eventual 64-bit compiler:

x87 FPU is deprecated on x64, and in general SSE2 will be used for florating point. so floating point and its exception handling might work slightly differently, and extended might not be 80-bit (but 64-bit or, less likely 128-bit). This also relates to the usual rounding (copro controlwork) changes when interfacing wiht C code that expects a different fpu word.

PHis commented on that answer with:

I wouldn't say that the x87 FPU is deprecated, but it is certainly the case that Microsoft have decided to do their best to make it that way (and they really don't seem to like 80-bit FP values), although it is clearly technically possible to use the FPU/80-bit floats on Win64.

Answer 4

I just posted an answer to your other question, but I guess it actually should go here:

Obviously, nobody except for Embarcadero can answer this for sure before the product is released.

It is very likely that any decent x64 compiler will use the SSE2 instruction set as a baseline and therefore attempt to do as much floating point computation using SSE features as possible, minimising the use of the x87 FPU. However, it should also be said that there is no technical reason that would prevent the use of the x87 FPU in x64 application code (despite rumours to the contrary which have been around for some time; if you want more info on that point, please have a look at Agner Fog's Calling Convention Manual, specifically chapter 6.1 "Can floating point registers be used in 64-bit Windows?").

Edit 1: Delphi XE2 Win64 indeed does not support 80-bit floating-point calculations out of the box (see e.g. discussuion here (although it allows one to read/write such values). One can bring such capabilities back to Delphi Win64 using a record + class operators, as is done in this TExtendedX87 type (although caveats apply).

Answer 5

We won't know for sure how the 64-bit Delphi compiler will implement floating point arithmetic until Embarcadero actually ships it. Anything prior to that is just speculation. But once we know for sure it'll be too late to do anything about it.

Allen Bauer's tweets do seem to indicate that they'll use SSE2 and that the Extended type may be reduced to 64 bits instead of 80 bits. I think that would be a bad idea, for a variety of reasons. I've written up my thoughts in a QualityCentral report Extended should remain an 80-bit type on 64-bit platforms

If you don't want your code to drop from 80-bit precision to 64-bit precision when you move to 64-bit Delphi, click on the QualityCentral link and vote for my report. The more votes, the more likely Embarcadero will listen. If they do use SSE2 for 64-bit floating point, which makes sense, then adding 80-bit floating point using the FPU will be extra work for Embarcadero. I doubt they'll do that work unless a lot of developers ask for it.

Answer 6

If you really need it, then you can use the TExtendedX87 unit by Philipp M. Schlüter (PhiS on SO) as mentioned in this Embarcadero forum thread.

@PhiS: when you update your answer with the info from mine, I'll remove mine.