Better X87

[Thomas Jakway]

OK, makes sense, thanks.

On 10/09/2016 02:14 PM, Ben Gamari wrote:
> Hi Thomas!
>
>
> Thomas Jakway <tjakway at nyu.edu> writes:
>
>> I was looking through compiler/nativeGen/X86/Instr.hs
>> <https://github.com/ghc/ghc/blob/master/compiler/nativeGen/X86/Instr.hs#L71>
>> and it's pretty hard not to notice the (hilarious) diatribe about the
>> horror that is x87.
>>
> Reading these notes is one of the joys of working on GHC.
>
>> git log -p says this was apparently written in 2009 by
>> Ben.Lippmeier at anu.edu.au (92ee78e03c3670f56ebbbbfb0f67a00f9ea1305f).
>>
>> Since this has survived in X86/ all this time I'm guessing this is still
>> an issue (another guess: we've gotten by because of SSE?).  Is there any
>> interest in improving x87 code generation? And if so, has anyone tried
>> before?
>>
> As far as I know this is indeed still an issue, but one that (I would
> guess) relatively few people really feel. There are two reasons for
> this,
>
>   * We have the LLVM backend which users needing high performance
>     numerics tend to gravitate towards
>
>   * We have -msse2 which is used by default on x86_64 (which is most
>     users as this point).
>
> My impression is that this is still a "problem" but, unless you are
> yourself actively affected by it, there are probably more important
> ways to contribute (e.g. fix up the graph coloring register allocator).
>
> If you did want to fix up x87 support, I think it would preferable to do
> so in a way that avoids complicating the register allocator; some day
> the monster that is x87 will die and we'd prefer not to have to rip out
> more of its tentacles from the code generator than necessary. I think
> the "more clever" approach described in the note would probably be
> a good start: retain the virtual "registers" but try to be more clever
> about assigning them to stack entries by looking at more than one
> instruction at once.
>
> Cheers,
>
> - Ben
>