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Re: [PATCH] Minor optimisations in operator new(size_t, align_val_t)
On Fri, 17 Aug 2018, Jonathan Wakely wrote:
On 17/08/18 19:28 +0200, Marc Glisse wrote:
On Mon, 13 Aug 2018, Jonathan Wakely wrote:
Thanks to Lars for the suggestions.
* libsupc++/new_opa.cc (operator new(size_t, align_val_t)): Use
__is_pow2 to check for valid alignment. Avoid branching when rounding
size to multiple of alignment.
Tested x86_64-linux, committed to trunk.
Are you getting better code with __is_pow2 on many platforms? As far as I
can tell from a quick look at the patch (I didn't actually test it, I could
be completely off), this replaces (x&(x-1))==0 with popcount(x)==1. On a
basic x86_64, popcount calls into libgcc, which doesn't seem so good. On a
more recent x86_64 (BMI1), x&(x-1) is a single instruction that sets a flag
when the result is 0, that's hard to beat.
Then shouldn't we do that in __ispow2?
Even better would be a peephole optimisation to turn
__builtin_popcount(x)==1 into that.
There is the complication of how to handle 0. For x=0, ispow2(x) is false
while (x&(x-1))==0 is true. So the transformation corresponds to
__builtin_popcount(x)<=1, and for ==1 we need more complications unless we
somehow know that x cannot be 0.
Or was the goal to accept an alignment of 0, and not an optimization?
Accepting alignment of 0 isn't the goal :-)
std::ispow2 should be the best way to check if an unsigned integer is
a power of two, so I wanted to use that instead of manual bit
Makes sense. The best way to test this may not be the same if we know that
the number cannot be 0, but I guess that if we don't find a better way it
would be possible to use __builtin_constant_p to select between x&(x-1)
and popcount==1. Using range information to enable the compiler
transformation is also possible. Both may require an explicit if(align==0)
in operator new (whether it replaces align with another value or calls
__builtin_unreachable() probably doesn't matter).
I hope that check will go away soon,
In that case, please ignore my comments on the speed of the check.