On Wed, Jul 27, 2011 at 1:25 PM, Paolo Bonzini <bonz...@gnu.org> wrote: > On 07/27/2011 07:29 PM, H.J. Lu wrote: >> >> If IRNORE_ADDRESS_WRAP_AROUND is TRUE, we >> + also permute the conversion and addition of a constant. It is used to >> + optimize cases where overflow of base + constant offset won't happen >> or >> + its behavior is implementation-defined for a given target. */ > > Regarding correctness: you're converting a SImode operation to DImode by > "pushing in" the zero_extend operation. What makes you think that base + > constant offset won't overflow in any case? > > And also: what are you gaining by allowing the wrap around? I don't need to > know what ignore_address_wrap_around does, I need to know _why_ it is > necessary. >
We have (zero_extend:DI (plus:SI (FOO:SI) (const_int Y))) I want to convert it to (plus:DI (zero_extend:DI (FOO:SI)) (const_int Y)) There is no zero-extend on (const_int Y). if FOO == 0xfffffffc and Y = 8, (zero_extend:DI (plus:SI (FOO:SI) (const_int Y))) gives 0x4 and (plus:DI (zero_extend:DI (FOO:SI)) (const_int Y)) gives 0x100000004. If (plus:SI (FOO:SI) (const_int Y)) won't overflow or its behavior is implementation-defined, the conversion is safe. If it isn't the case, we should just drop it. -- H.J.
