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

[Marc Glisse]

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.

Hmm, that division by zero thing is a good point. I may be confusing with 
dereferencing a null pointer, but I believe that some languages catch the 
corresponding signal, so by removing that division you would be changing 
the behavior. I wish we had a -fno-divisions-by-zero or equivalent, but 
otherwise this may require an extra check like tree_expr_nonzero_p, 
although we are quite inconsistent about this (we don't simplify x/x to 1, 
but we do simplify 0%x to 0 if x is not (yet) known to be the constant 0). 
We'll see what the reviewers think...

Any plan on optimizing the 'B && ovf' form?

-- 
Marc Glisse