Re: Re: [gui-dev] Re: Faster SHA1 and Tiger MessageDigestSPI's (version1.11)

> Message du 01/07/04 22:10
> > Can you give details about what is reported by the GNU Java compiler 
> > as an "undefined" operation on some value?
> 
> Uh no, not the GNU Java compiler (gcj). I'm talking about gcc...
> 
> d += ((e << 5) | (e >>> 27)) + 0x5a827999 // K16
> + ((a & ((b = (b << 30) | (b >>> 2)) ^ c)) ^ c) // Ch(a,b,c)
> + (i00 = ((i00 ^= i02 ^ i08 ^ i13) << 1) | (i00 >>> 31)); // W16
> 
> operation on `b' may be undefined
> operation on `i00' may be undefined

Thanks for correcting me, I thought you were speaking about the GNU compiler for Java.
Yes I know that in C and C++ the evaluation order is defined as much as precisely as in Java, so it's true that any subexpression that has a side effect on one of its variables is not guaranteed to be compiled and executed in the same order. For example the following C/C++ expressions has an undefined value for c:

int data[] = {1,2,3,4};
int *p = data;
int c = ((*p++)<<24) | ((*p++) <<16) | ((*p++)<<8) | (*p++);

because the side-effect of the incrementation of p is not guaranteed to be executed before the next indirection of p is executed...

So it's true that in C/C++, you need to decompose these expressions with side-effects which are only correct in Java, as documented and guaranteed in the Java specification.
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