Re: [PATCH][vectorizer][2/2] Hook up mult synthesis logic into vectorisation of mult-by-constant

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

On Wed, 15 Jun 2016, Kyrill Tkachov wrote:

> This is a respin of https://gcc.gnu.org/ml/gcc-patches/2016-06/msg00952.html 
> following feedback.
> I've changed the code to cast the operand to an unsigned type before applying 
> the multiplication algorithm
> and cast it back to the signed type at the end.
> Whether to perform the cast is now determined by the function 
> cast_mult_synth_to_unsigned in which I've implemented
> the cases that Marc mentioned in [1]. Please do let me know
> if there are any other cases that need to be handled.

Ah, I never meant those cases as an exhaustive list, I was just looking 
for examples showing that the transformation was unsafe, and those 2 came 
to mind:

- x*15 -> x*16-x the second one can overflow even when the first one 
doesn't.

- x*-2 -> -(x*2) can overflow when the result is INT_MIN (maybe that's 
redundant with the negate_variant check?)

On the other hand, as long as we remain in the 'positive' operations, 
turning x*3 to x<<1+x seems perfectly safe. And even x*30 to (x*16-x)*2 
cannot cause spurious overflows. But I didn't look at the algorithm 
closely enough to characterize the safe cases. Now if you have done it, 
that's good :-) Otherwise, we might want to err on the side of caution.

--
Marc Glisse