[Bug c++/98126] New: -Wsequence-point is non-linear for certain cases

[Bug c++/98126] New: -Wsequence-point is non-linear for certain cases

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98126

            Bug ID: 98126
           Summary: -Wsequence-point is non-linear for certain cases
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mpolacek at gcc dot gnu.org
  Target Milestone: ---

This script generates C++ programs that show that -Wsequence-point exhibits
non-linear behavior:

#!/bin/bash
n=$1

printf "struct T { bool operator==(const T &ot) const; };
struct C {
  bool operator==(const C &ot) const {
    return\n"
for i in $(seq $n); do printf "      t$i == ot.t$i &&\n"; done
printf "      true; 
  }
"
for i in $(seq $n); do printf "  T t$i;\n"; done
printf "};\n"

$ ./gen.sh 100 > gen.cc && time -p ./cc1plus -quiet -fsyntax-only -Wall gen.cc
real 0.60
$ ./gen.sh 200 > gen.cc && time -p ./cc1plus -quiet -fsyntax-only -Wall gen.cc
real 4.50
$ ./gen.sh 300 > gen.cc && time -p ./cc1plus -quiet -fsyntax-only -Wall gen.cc
real 14.62
$ ./gen.sh 400 > gen.cc && time -p ./cc1plus -quiet -fsyntax-only -Wall gen.cc
real 34.46
$ ./gen.sh 500 > gen.cc && time -p ./cc1plus -quiet -fsyntax-only -Wall gen.cc
real 67.27
$ ./gen.sh 600 > gen.cc && time -p ./cc1plus -quiet -fsyntax-only -Wall gen.cc
real 117.33
$ ./gen.sh 700 > gen.cc && time -p ./cc1plus -quiet -fsyntax-only -Wall gen.cc
real 179.78
$ ./gen.sh 800 > gen.cc && time -p ./cc1plus -quiet -fsyntax-only -Wall gen.cc
real 278.54

Note that
$ ./gen.sh 800 > gen.cc && time -p ./cc1plus -quiet -fsyntax-only -Wall gen.cc
-Wno-sequence-point
real 0.14

[Bug c++/98126] -Wsequence-point is non-linear for certain cases

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98126

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2020-12-03
             Status|UNCONFIRMED                 |ASSIGNED

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
I have a (simple, stupid) patch.

[Bug c++/98126] -Wsequence-point is non-linear for certain cases

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98126

--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:1cac89da2cb1f2a7c2d93f7f325484c2d1619ca8

commit r11-5826-g1cac89da2cb1f2a7c2d93f7f325484c2d1619ca8
Author: Marek Polacek <polacek@redhat.com>
Date:   Thu Dec 3 18:35:00 2020 -0500

    c-family: Fix hang with -Wsequence-point [PR98126]

    verify_sequence_points uses verify_tree to recursively walk the
    subexpressions of an expression, and while recursing, it also
    keeps lists of expressions found after/before a sequence point.
    For a large expression, the list can grow significantly.  And
    merge_tlist is at least N(n^2): for a list of length n it will
    iterate n(n -1) times, and call candidate_equal_p each time, and
    that can recurse further.  warn_for_collision also has to go
    through the whole list.  With a large-enough expression, the
    compilation can easily get stuck here for 24 hours.

    This patch is a simple kludge: if we see that the expression is
    overly complex, don't even try.

    gcc/c-family/ChangeLog:

            PR c++/98126
            * c-common.c (verify_tree_lim_r): New function.
            (verify_sequence_points): Use it.  Use nullptr instead of 0.

    gcc/testsuite/ChangeLog:

            PR c++/98126
            * g++.dg/warn/Wsequence-point-4.C: New test.

[Bug c++/98126] -Wsequence-point is non-linear for certain cases

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98126

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed.  This *could* be backported to GCC 10, if someone wants to.