On Fri, Aug 3, 2012 at 10:02 AM, Richard Guenther <richard.guent...@gmail.com> wrote: > On Fri, Aug 3, 2012 at 9:24 AM, Uros Bizjak <ubiz...@gmail.com> wrote: >> On Fri, Aug 3, 2012 at 8:40 AM, Uros Bizjak <ubiz...@gmail.com> wrote: >> >>> We can implement atomic_fetch_sub with atomic_fetch_add and inverted >>> operand 2. However, we have to prevent overflows with negation, so >>> only const_int operand 2 is allowed in the expander. > > Why do you need to prevent overflows?
Because we change (x)sub to (x)add, so for SImode operation - without overflow check - sub 0x80000000, %eax gets changed to add 0x80000000, %eax. The same reasoning goes for dynamic negation: for neg %eax,%eax value 0x80000000 stays the same, but we have changed (x)sub to an (x)add in the code stream. Uros.
