[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=28364 --- Comment #35 from Richard Biener --- Author: rguenth Date: Wed Dec 19 11:10:08 2018 New Revision: 267262 URL: https://gcc.gnu.org/viewcvs?rev=267262=gcc=rev Log: 2018-12-19 Richard Biener PR tree-optimization/88533 Revert

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=28364 Richard Biener changed: What|Removed |Added Status|ASSIGNED|RESOLVED Known to work|

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=28364 --- Comment #33 from Richard Biener --- Author: rguenth Date: Mon Apr 30 07:23:36 2018 New Revision: 259754 URL: https://gcc.gnu.org/viewcvs?rev=259754=gcc=rev Log: 2018-04-30 Richard Biener PR

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=28364 amker at gcc dot gnu.org changed: What|Removed |Added CC||amker at gcc dot gnu.org ---

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=28364 bin.cheng changed: What|Removed |Added CC||amker.cheng at gmail dot com --- Comment

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=28364 --- Comment #30 from Zack Weinberg --- It's been a very long time and I don't know exactly what changed, but GCC 7.3 generates essentially the same code for both of the functions in the "C test case" and I would not describe that code as "bad".

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=28364 Eric Gallager changed: What|Removed |Added CC||egallager at gcc dot gnu.org ---

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #28 from rakdver at gcc dot gnu dot org 2006-08-16 21:14 --- Subject: Bug 28364 Author: rakdver Date: Wed Aug 16 21:14:11 2006 New Revision: 116189 URL: http://gcc.gnu.org/viewcvs?root=gccview=revrev=116189 Log: PR tree-optimization/28364 *

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #27 from rakdver at gcc dot gnu dot org 2006-07-26 19:38 --- Patch for the wrong choice of induction variable: http://gcc.gnu.org/ml/gcc-patches/2006-07/msg01125.html -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28364

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #26 from rakdver at gcc dot gnu dot org 2006-07-25 15:20 --- A patch for the return in the middle of the loop problem: http://gcc.gnu.org/ml/gcc-patches/2006-07/msg00893.html (to be commited once mainline is open). -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28364

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #25 from rakdver at gcc dot gnu dot org 2006-07-18 00:45 --- Created an attachment (id=11906) -- (http://gcc.gnu.org/bugzilla/attachment.cgi?id=11906action=view) A patch for loop header selection. I tried improving the heuristics for the selection of the loop header,

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

-- rakdver at gcc dot gnu dot org changed: What|Removed |Added AssignedTo|unassigned at gcc dot gnu |rakdver at gcc dot gnu dot |dot org

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #18 from rakdver at atrey dot karlin dot mff dot cuni dot cz 2006-07-13 07:58 --- Subject: Re: poor optimization choices when iterating over a std::string (probably not c++-specific) I-cache. this only matters if this increase in code size will make the code go out

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #19 from rakdver at atrey dot karlin dot mff dot cuni dot cz 2006-07-13 08:01 --- Subject: Re: poor optimization choices when iterating over a std::string (probably not c++-specific) I-cache. this only matters if this increase in code size will make the code go out

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #20 from zackw at panix dot com 2006-07-13 08:25 --- Subject: Re: poor optimization choices when iterating over a std::string (probably not c++-specific) However, ch isn't just copying the loop header; it is also copying the *entire body of the loop*, which nothing can

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #21 from zackw at panix dot com 2006-07-13 08:28 --- Subject: Re: poor optimization choices when iterating over a std::string (probably not c++-specific) on your real program, how much performance do you gain by hand-rewriting the assembler to look the way you like?

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #22 from rguenth at gcc dot gnu dot org 2006-07-13 08:28 --- For practical purposes (determining the loop runs at least once) it needs to duplicate the exit condition. Which happens to be difficult here, as there are multiple loop exits. --

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #23 from rakdver at atrey dot karlin dot mff dot cuni dot cz 2006-07-13 09:00 --- Subject: Re: poor optimization choices when iterating over a std::string (probably not c++-specific) on your real program, how much performance do you gain by hand-rewriting the

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #24 from rakdver at atrey dot karlin dot mff dot cuni dot cz 2006-07-13 09:03 --- Subject: Re: poor optimization choices when iterating over a std::string (probably not c++-specific) However, ch isn't just copying the loop header; it is also copying the *entire body

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #1 from zackw at panix dot com 2006-07-12 22:33 --- Created an attachment (id=11874) -- (http://gcc.gnu.org/bugzilla/attachment.cgi?id=11874action=view) assembly output (bad on top, hand-corrected below) -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28364

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #2 from pinskia at gcc dot gnu dot org 2006-07-12 22:41 --- Loop-Copy header is doing it Which means there is a confusion at what is the real header of the loop here. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28364

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #3 from zackw at panix dot com 2006-07-12 22:42 --- I should mention that the exact command line flags were -O2 -fomit-frame-pointer -march=pentium4, and that I hand-tweaked the label numbers for ease of reading. Also, -fno-tree-ch does suppress this bad optimization, but

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #4 from zackw at panix dot com 2006-07-12 22:48 --- I remembered that I had a build of 4.2 from last week lying around. It generates even worse code - still with the duplication of most of the loop, plus a bunch of unnecessary register fiddling and bad addressing mode

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #5 from pinskia at gcc dot gnu dot org 2006-07-12 22:52 --- For me on PPC-darwin, it generates pretty good code at just -O2 even though there is a duplicated header. The loop is pretty good at scheduling the code too: L9: lbz r0,0(r3) cmplwi cr7,r0,31

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #6 from rakdver at gcc dot gnu dot org 2006-07-12 23:13 --- Loop header copying is OK; the result is the one I would expect, it certainly does not make the code worse (unless you are optimizing for code size), and in many cases may make it better. I will have a look at the

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #7 from zackw at panix dot com 2006-07-12 23:19 --- Created an attachment (id=11875) -- (http://gcc.gnu.org/bugzilla/attachment.cgi?id=11875action=view) C test case (with interesting implications) I've found a plain C test case. In the process, I've found that the way

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #8 from zackw at panix dot com 2006-07-12 23:21 --- Zdenek: I don't see how you can say that what we get is optimal code unless optimizing for size. The code generated *will* be slower than the alternative. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28364

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #9 from rakdver at atrey dot karlin dot mff dot cuni dot cz 2006-07-12 23:30 --- Subject: Re: poor optimization choices when iterating over a std::string (probably not c++-specific) Zdenek: I don't see how you can say that what we get is optimal code unless optimizing

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #10 from zackw at panix dot com 2006-07-12 23:33 --- I-cache. Also, more iterations before the branch predictors figure out what's going on. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28364

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #11 from rakdver at atrey dot karlin dot mff dot cuni dot cz 2006-07-12 23:39 --- Subject: Re: poor optimization choices when iterating over a std::string (probably not c++-specific) I-cache. this only matters if this increase in code size will make the code go out of

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #12 from zackw at panix dot com 2006-07-13 03:09 --- Subject: Re: poor optimization choices when iterating over a std::string (probably not c++-specific) I-cache. this only matters if this increase in code size will make the code go out of instruction cache. The real

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #13 from pinskia at gcc dot gnu dot org 2006-07-13 03:37 --- Hmm, for some reason I don't like the idea of using std::string in the inner loop :). Even the C testcase does not seem like a good inner loop in general anyways as there is no caculation going on here. To me

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #14 from zackw at panix dot com 2006-07-13 03:40 --- Subject: Re: poor optimization choices when iterating over a std::string (probably not c++-specific) It's a validation routine, yes, which is run over every pathname the program is working on, and there can be hundreds

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #15 from pinskia at gcc dot gnu dot org 2006-07-13 03:45 --- One more comment, a loop with an early exit is whole issue and that is the reason why all of the code in the loop is considered the header. There are a couple of loop headers in this case, one for each exit which

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #16 from pinskia at gcc dot gnu dot org 2006-07-13 04:01 --- If this is really a program's inner most loop, then the program is messed up as there is no caculation going on here at all. What type of program is this? Do you cache the result of this function? Maybe

[Bug tree-optimization/28364] poor optimization choices when iterating over a std::string (probably not c++-specific)

--- Comment #17 from zackw at panix dot com 2006-07-13 04:23 --- Subject: Re: poor optimization choices when iterating over a std::string (probably not c++-specific) One more comment, a loop with an early exit is whole issue and that is the reason why all of the code in the loop is