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Re: GCC optimizes integer overflow: bug or feature?

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

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