Problem regarding canonicalization

[Ashwin]

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?

Thanks in advance.
Ashwin