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Re: Worse code after bbro?
- From: Senthil Kumar Selvaraj <senthilkumar dot selvaraj at microchip dot com>
- To: Segher Boessenkool <segher at kernel dot crashing dot org>
- Cc: Richard Biener <rguenther at suse dot de>, <gcc at gcc dot gnu dot org>, Jan Hubicka <hubicka at ucw dot cz>
- Date: Fri, 6 Jan 2017 12:17:17 +0530
- Subject: Re: Worse code after bbro?
- Authentication-results: sourceware.org; auth=none
- References: <firstname.lastname@example.org> <alpine.LSU.email@example.com> <20170104104656.GE28613@gate.crashing.org>
Segher Boessenkool writes:
> On Wed, Jan 04, 2017 at 10:05:49AM +0100, Richard Biener wrote:
>> > The code size is identical, but the trunk version executes one more
>> > instruction everytime the loop runs (explicit jump to .L5 with trunk vs
>> > fallthrough with 4.8) - it's faster only if the loop never runs. This
>> > happens irrespective of the memory clobber inline assembler statement.
> With -Os you've asked for smaller code, not faster code.
> All of the block reordering is based on heuristics -- there is no polynomial
> time and space algorithm to do it optimally, let alone the linear time and
> space we need in GCC -- so there always will be cases we do not handle
> optimally. -Os does not get as much attention as -O2 etc., as well.
> OTOH this seems to be a pretty common case that we could handle. Please
> open a PR to keep track of this?
Filed PR 79012 (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79012)