[LLVMdev] Reference Manual Clarifications 2

Jon Sargeant delta17 at cox.net
Tue Apr 1 12:07:24 PDT 2008

Chris Lattner wrote:
> On Mon, 31 Mar 2008, Jon Sargeant wrote:
>> The fptrunc instruction states "If the value cannot fit within the 
>> destination type, ty2, then the results are undefined."  This is fine, but 
>> what about other floating-point operations that can overflow?  For example, 
>> does 'mul double 1.0e300, 1.0e300' produce +infinity or is it undefined?
> It is defined by IEEE to be inf.

Yes.  AFAIK, IEEE754 defines floating-point truncation also.  If a 
double-precision value is too large for single precision, the result
is infinity (not undefined).

>> The manual needs to state the rounding mode of the fptrunc, uitofp, and 
>> sitofp instruction (if only to say it's undefined).  I recommend 
>> round-to-nearest.
> Everything is implicitly assumed to be in the default rounding mode.

After doing a quick search on Google, it looks like the default rounding 
mode is round to nearest.  Would the default rounding mode ever be 
different than round to nearest?

Best Regards,

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