Re: [PATCH][GCC] Simplification of 1U << (31 - x)

[Segher Boessenkool <segher@kernel.crashing.org>]

On Wed, Apr 12, 2017 at 08:59:34PM +0200, Jakub Jelinek wrote:
> On Wed, Apr 12, 2017 at 01:15:56PM -0500, Segher Boessenkool wrote:
> > On Wed, Apr 12, 2017 at 07:06:38PM +0200, Jakub Jelinek wrote:
> > > On Wed, Apr 12, 2017 at 09:29:55AM +0000, Sudi Das wrote:
> > > > This is a fix for PR 80131 
> > > > Currently the code A << (B - C) is not simplified.
> > > > However at least a more specific case of 1U << (C -x) where C = precision(type) - 1 can be simplified to (1 << C) >> x.
> > > 
> > > Is that always a win though?
> > > Some constants have higher costs than others on various targets, some
> > > significantly higher.  This change might be beneficial only
> > > if if C is as expensive as 1, then you get rid of a one (typically cheap)
> > > operation.
> > > Which makes me wonder whether this should be done at GIMPLE time and not
> > > at RTL time (expansion or combine etc.) when one can compare the RTX costs.
> > 
> > Yeah, either combine or simplify-rtx I'd say.
> > 
> > The transform    A << (B - C)  --->   (A << B) >> C
> > only works if A << B does not overflow but A << (B + 1) does (and then
> 
> Is that really true?  The A << B does not overflow is obvious precondition.
> 
> But consider say A 5U, B 29 and C (not compile time known) -2.

Ah yes, A unsigned.  Bah.

> 5U << 31 is valid 0x80000000U, but (5U << 29) >> (-2) is UB.
> Isn't the other condition instead that either C must be non-negative, or
> B must be number of bits in A's type - 1, i.e. that for negative C
> A << (B - C) would already be always UB?
> But then unless we know C is non-negative, A must be really just 1,
> otherwise A << B overflows.

Yeah.


Segher