Re: c/10339: strncmp generates imPure code

[Falk Hueffner <falk dot hueffner at student dot uni-tuebingen dot de>]

Wolfgang Bangerth <bangerth at ices dot utexas dot edu> writes:

> > It generates a memcmp of 4 bytes when one of the arguments
> > is 3 bytes long.
> 
> You're a little terse in your descriptions of the problem :-]
> 
> I think I understand now what you mean: with -O, gcc optimizes the strncmp 
> away and replaces it by a call to memcmp. However, it doesn't take into 
> account that strncmp compares _at most_ n  characters, but that the actual 
> number may be less than n if one of the two strings is shorter. memcmp on 
> the other hand always does n bytes.

Well, just like in real life, we can do anything we like as long as
nobody notices. This optimiziation would only be invalid if

* it gives a different result,
* or the memory read straddles a page boundary and faults.

I suspect the extra byte read is actually not relevant for the result,
and because of alignment, gcc knows the second problem cannot occur,
but I have neither a SPARC nor SPARC knowledge to test that.

-- 
	Falk