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Re: [PATCH] Fix up wi::lrshift (PR c++/69399)
- From: Richard Biener <richard dot guenther at gmail dot com>
- To: Jakub Jelinek <jakub at redhat dot com>
- Cc: Mike Stump <mikestump at comcast dot net>, Richard Sandiford <rdsandiford at googlemail dot com>, "H.J. Lu" <hjl dot tools at gmail dot com>, GCC Patches <gcc-patches at gcc dot gnu dot org>
- Date: Wed, 27 Jan 2016 11:38:33 +0100
- Subject: Re: [PATCH] Fix up wi::lrshift (PR c++/69399)
- Authentication-results: sourceware.org; auth=none
- References: <20160122185533 dot GA3430 at intel dot com> <CAFiYyc3d+rRFi4hOdaDip-c_ohPYOy7keYAWRaE7V-XALnfKwQ at mail dot gmail dot com> <CAMe9rOpAPQendCYodbCC2SjtC3xSGcPz6pfN3LjXNAnq2R2iPQ at mail dot gmail dot com> <CAFiYyc1tGWtw7f5LHKP79v1pKPp8rmJNKRBv52qw_x4Gga=8mg at mail dot gmail dot com> <20160126182148 dot GE3017 at tucnak dot redhat dot com> <4ADD4937-E19F-4CD0-8441-C97935548759 at comcast dot net> <CD95CA3C-11D2-4C9D-A418-BD6DB4DD4B47 at gmail dot com> <20160126212616 dot GK3017 at tucnak dot redhat dot com> <0ADBB08F-103E-4CC7-A7B9-93937E809197 at comcast dot net> <20160126234152 dot GM3017 at tucnak dot redhat dot com>
On Wed, Jan 27, 2016 at 12:41 AM, Jakub Jelinek <jakub@redhat.com> wrote:
> On Tue, Jan 26, 2016 at 01:55:41PM -0800, Mike Stump wrote:
>> On Jan 26, 2016, at 1:26 PM, Jakub Jelinek <jakub@redhat.com> wrote:
>> > will do cc1plus size comparison afterwards.
>>
>> We know the dynamic check is larger. You canât tell the advantage of
>> speed from size. Better would be to time compiling any random large
>> translation unit.
>>
>> Nice to see that only 14 calls remain, thatâs way better than the 34 I
>> thought.
>
> So, it seems probably the PR65656 changes made things actually significantly
> worse, while I see a (small) difference in the generated code between the two
> patches if I compile say tree-ssa-ccp.c with g++ 5.x, in the bootstrapped
> compiler there is no difference at all, the compilers with either patch
> have identical objdump -dr cc1plus. Already at *.gimple time all the
> relevant __builtin_constant_p are resolved and it seems all to 0.
>
> So please agree on one of the two patches (don't care which), and I'll try
> to distill a testcase to look at for PR65656.
I don't care if the wi::lshift by LOG2_BITS_PER_UNIT done by
get_ref_base_and_extent
are compiled to as good quality as before (I suspect it doesn't matter
in this case
as the shift amount is constant, but maybe due to PR65656 the
non-STATIC_CONSTANT_P
variant is better).
Richard.
> Jakub