Re: differences between implementations of IEEE-754 basic operators

[Vincent Lefevre <vincent@xxxxxxxxxx>]

On 2008-07-04 10:35:56 +0200, David Monniaux wrote:
> My understanding is that as long as:
> * One uses IEEE-754 single and double precision formats (this leaves out  
> the x87 extended-precision registers).

and rounding directly to the same format.

> * One does not set trap handlers except for invalid operation.
> * One sets the trap handler for invalid operation to abort the program.
> * One does not use the "flush denormals to zero" and similar flags from  
> certain FPUs (PowerPC).
> Then all computations on any IEEE-754 compatible system (let's say,  
> Intel SSE and PowerPC) give exactly the same result: either a trap for  
> invalid operation, either a same non-NaN result.
>
> Am I correct or is there still leeway for differences?

You should not look at the memory representation either (due to the
differences in the endianness).

And as you said:

> * More generally, how trap handlers are set, when they are triggered, is
> unspecified.

In particular, there are differences between x87 (perhaps not SSE,
though) and PowerPC[*].

[*] http://gcc.gnu.org/ml/gcc/2005-06/msg00559.html

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Vincent Lefèvre <vincent@xxxxxxxxxx> - Web: <http://www.vinc17.org/>
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