Re: [boost] Results of Cray SV1 regression tests

From: Matthias Troyer (troyer_at_[hidden])
Date: 2003-02-05 14:04:45

On Wednesday, February 5, 2003, at 04:24 PM, Dave Abrahams wrote:

> On Wednesday, February 05, 2003 10:11 AM [GMT+1=CET],
> Matthias Troyer <troyer_at_[hidden]> wrote:
>
>> I am looking into it but already have a conjecture. When using signed
>> 64 bit integers the Cray compiler (with optimization turned on) uses
>> the floating point units to actually perform the operations in 46 bit
>> arithmetic. This speeds up things but causes compatibility problems. 
>> In
>> the case of the jam sources we could avoid this "optimization" by 
>> using
>> unsigned types. Is it possible to use an unsigned integer here to 
>> avoid
>> similar problems?
>
>
> Just to be clear, we didn't avoid an optimization by using unsigned 
> types.
> We avoided undefined behavior due to overflow in signed integers, or
> conversions from unsigned->signed.

Actually there were two problems. One was the one you mentioned above. 
The other was that we needed either a masking to 32 bits or the type 
made unsigned to prevent an optimization (integer calculations done on 
FPU), which led to non-conforming overflow behavior.