std.math performance (SSE vs. real)

[via Digitalmars-d]

On Tuesday, 1 July 2014 at 16:58:55 UTC, Walter Bright wrote:
> Click on "compiling for strict IEEE conformance"

It also states that compliance affect performance.

>> not IEEE754 compliant. VPF is almost compliant, but does not 
>> support subnormal
>> numbers and flush them to zero. Which can be a disaster…
>
> It wouldn't be any different if the D spec says "floating point 
> is, ya know, whatevah". You can't fix stuff like this in the 
> spec.

Well, the difference is that you can make VPF mostly compliant, 
but that means you trap subnormal numbers and fix it in software. 
Which affects performance.

If the specs require IEEE754 compliance then you default to 
software emulation and have to turn it off through compiler 
switches.

Here is another example: the parallella Coprocessor from Adapteva:

«The CPU is compatible with the IEEE-754 single-precision format, 
with the following
exceptions:
- No support for inexact flags.
- NAN inputs generate an invalid exception and return a quiet 
NAN. When one or both of the
inputs are NANs, the sign bit of the operation is set as an XOR 
of the signs of the input sign
bits.
- Denormal operands are flushed to zero when input to a 
computation unit and do not generate
an underflow exception. Any denormal or underflow result from an 
arithmetic operation is
flushed to zero and an underflow exception is generated.
- Round-to-±infinity is not supported.»

> Besides, Java and Javascript, for example, both require IEEE 
> conformance.

But they aren't system level programming languages… and we 
probably cannot expect future many-core processors or 
transputer-like processors to waste die space in order to conform.

So sure you can specify the IEEE-754 spec and just make D run at 
full rate on typical CISC CPUs, but the alternative is to 
constrain the promise of compliance to what is typical for 
efficient CPUs. Then have a "-strict" flag for the few 
applications that are scientific in nature. That would be more in 
line with system level programming.