Re: [boost] Boost SIMD beta release

Subject: Re: [boost] Boost SIMD beta release
From: Thorsten Ottosen (thorsten.ottosen_at_[hidden])
Date: 2012-12-20 08:49:56

On 20-12-2012 12:52, Mathias Gaunard wrote:

> You can choose which extension to generate code for at compile time.
>
> The approach we recommend is to compile various versions of your
> function with different settings, and then choose the right one at
> runtime depending on the host capabilities.
> We provide functions to easily check whether an extension is supported.

That seems cool. Can illegal instructions be in a binary as long as they 
are not exeuted?

>> B. Floating point precision is crucial in some of our software. Maybe
>> you could provide kahan_sum and high-precision normalization functions?
>> Do you know if there are any problems about the precision in SIMD code?
>
> We took great care of guaranteeing high precision with all our
> functions, along with nan/inf support and, when reasonably supported by
> the hardware, denormal support. Some of these have performance costs,
> and can be disabled.
>
> The main difference when writing SSE code is that float operations are
> really done in single-precision and double in double-precision, which is
> not the case when using the x87 FPU for example, which may use up to 80
> bits of precision for intermediate computations.

Sure, the 80 bits are one reason that a naive sum is often as precise as 
a kahan sum on x87.

Let me ask another way: compilers often provide flags that affect how 
code is generate for fp. For example, we use fp:precise on VS. Do 
fp:precise affect SIMD code or?

-Thorsten