Re: [PATCH] match.pd: optimize unsigned mul overflow check

[Marc Glisse <marc dot glisse at inria dot fr>]

On Sat, 28 May 2016, Alexander Monakov wrote:

> For unsigned A, B, 'A > -1 / B' is a nice predicate for checking whether 'A*B'
> overflows (or 'B && A > -1 / B' if B may be zero).  Let's optimize it to an
> invocation of __builtin_mul_overflow to avoid the divide operation.

I forgot to ask earlier: what does this give for modes / platforms where 
umulv4 does not have a specific implementation? Is the generic 
implementation worse than A>-1/B, in which case we may want to check 
optab_handler before doing the transformation? Or is it always at least as 
good?

(I didn't ask because I was assuming the latter, but I am not 100% 
certain)

--
Marc Glisse