On Tue, Jan 26, 2021 at 7:45 PM Luc Van Oostenryck
<luc.vanoostenryck@xxxxxxxxx> wrote:
>
> This series fixes and improves the simplification and the
> canonicalization of signed compares.
Hmm. Sorry for not replying earlier, but I just checked the most
common simplification of signed compares, and it didn't work.
This:
_Bool test(int a)
{
return a >=0 && a < 16;
}
should simplify to be the same as
_Bool test(int a)
{
return (unsigned)a < 16;
}
but it doesn't. It generates the silly - but straightforward - "two
comparisons and a 'and' of the result".
In fact, the recent canonicalizations means that the compare against
zero is actually pessimised, and ">= 0" becomes "> 0xffffffff", which
is often a much more expensive operation.
This came up because I was looking at some kernel code that did
exactly that "check that a signed value is within proper bounds", and
the zero check is a very common bound.
Linus
