"Richard Guenther" <[EMAIL PROTECTED]> writes: | On 30 Dec 2006 23:55:46 +0100, Gabriel Dos Reis | <[EMAIL PROTECTED]> wrote: | > [EMAIL PROTECTED] (Richard Kenner) writes: | > | > | > Here's an example from the intprops module of gnulib. | > | | > | These are interesting case. | > | | > | Note that all the computations are constant-folded. | > | | > | And I think this points the fact that we can "have our cake and eat it too" | > | in many cases. Everything we're seeing points to the fact that the cases | > | where assuming undefined behavior for overflow will help optimizations are | > | cases where few if any sane programmers would write the code depending on | > | wrap semantics (e.g., loop variables). And nearly all the cases where wrap | > | semantics are expected (e.g., the above) are such that there's no reasonable | > | optimization benefit in assuming they're undefined. | > | | > | Take constant folding: if we were pendantic about it, we could say, "this | > | folded expression overflows and so is undefined, so let's set its value to be | > | whatever constant would yield the most efficient code in that case". | > | | > | Such behavior would be standard-compliant, but as unfriendly as possible | > | because it wouldn't "optimize" any real code, just break common idioms. | > | I doubt anybody would suggest not implementing wrapping semantics in | > | constant folding. | > | > As I'm looking into the VRP and CHREC codes to implement the | > -Wundefined warning, I came across this: | > | > /* Wrapper around int_const_binop. If the operation overflows and we | > are not using wrapping arithmetic, then adjust the result to be | > -INF or +INF depending on CODE, VAL1 and VAL2. */ | > | > static inline tree | > vrp_int_const_binop (enum tree_code code, tree val1, tree val2)
[...] | > What would you suggest this function to do, based on your comments? | | The function should be looked at in the context of the few callers - this | is really one of the more ugly and tricky parts of VRP. I've done that; I do not see an obvious way to make everybody happy -- except issueing a warning (which I've done). That is why I was asking since you raised that particular point. Maybe VRP experts may have opinions... The heavy (and sole) user of vrp_int_const_binop() is extract_range_from_binary_expr(). -- Gaby
