Ashwin writes:
> I have a combiner pattern that converts a sub-cmp pattern to a cmp insn,
> something like this -
> "if (a-1 < 0)"
> is converted to
> "if (a<1)"
>
> Now consider the following test case -
>
> <snip>
> f(long a){return (--a > 0);}
> main(){if(f(0x80000000L)==0)abort();exit(0);}
> </snip>
>
> The compiler generates the following code for f()
> cmp r0, 1 ;;canonicalized
> mov r0,1
> sub.le r0,r0,r0
>
> This works fine under normal circumstances. However, in the testcase,
> the least negative no. i.e. 0x80000000 (hereafer referred to as MIN) is
> passed. When 1 is subtracted from MIN, by --a, it becomes positive and
> the conditions get reversed thus leading to failure during execution.
>
> Similar problem seems to arise when MAX is passed to a function that
> does "return (++a < 0).
>
> How do I tackle this problem? Anything I may be missing?
Yes. Signed integer overflow is undefined in C, so this is not a
well-defined C program. For example, this program may trap.
So, the semantics of the C language allow this optimization. See 6.5
Para 5.
For unsigned types C is much more specific, and you must not do any
optimizations that do not return the correct result modulo 2**N, where
N is the wordlength. See 6.2.5 Para 9.
Some other languges targeted by gcc, such as the Java langauge, are
much more strict about the behaviour of integer overflow.
Andrew.
