Re: GCC optimizes integer overflow: bug or feature?

[Robert Dewar]

Paul Brook wrote:
> On Friday 22 December 2006 00:58, Denis Vlasenko wrote:
> > On Tuesday 19 December 2006 23:39, Denis Vlasenko wrote:
> > > There are a lot of 100.00% safe optimizations which gcc
> > > can do. Value range propagation for bitwise operations, for one
> > Or this, absolutely typical C code. i386 arch can compare
> > 16 bits at a time here (luckily, no alighment worries on this arch):
> >
> > int f(char *p)
> > {
> >     if (p[0] == 1 && p[1] == 2) return 1;
> >     return 0;
> > }
>
> Definitely not 100% safe. p may point to memory that is sensitive to the 
> access width and/or number of accesses. (ie. memory mapped IO).

A program that depends on this is plain wrong. There is no guarantee
that memory references are as they appear in the program. For a
non-volatile variable, any such optimization is valid. For instance
if the flow can be used to prove that p[0] is already 1, then there
is no need to repeat the read.
> Paul