On Tue, 24 May 2011, Andrew Stubbs wrote: > I've created this new, simpler patch that converts > > (extend (mult a b)) > > into > > (mult (extend a) (extend b)) > > regardless of what 'a' and 'b' might be. (These are then simplified and > superfluous extends removed, of course.)
Are there some missing conditions here? The two aren't equivalent in general - (extend:SI (mult:HI a b)) multiplies the HImode values in HImode (with modulo arithmetic on overflow) before extending the possibly wrapped result to SImode. You'd need a and b themselves to be extended from narrower modes in such a way that if you interpret the extended values in the signedness of the outer extension, the result of the multiplication is exactly representable in the mode of the multiplication. (For example, if both values are extended from QImode, and all extensions have the same signedness, that would be OK. There are cases that are OK where not all extensions have the same signedness, e.g. (sign_extend:DI (mult:SI a b)) where a and b are zero-extended from HImode or QImode, at least one from QImode, though there the outer extension is equivalent to a zero-extension.) -- Joseph S. Myers jos...@codesourcery.com
