[Bug middle-end/106495] New: [13 Regression] Build fails gcc/tree-ssa-threadbackward.cc:22: gcc/vec.h:890:19: error: array subscript 4294967294 is above array bounds of 'basic_block_def* [1]' [-Werror=array-bounds]

["slyfox at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106495

            Bug ID: 106495
           Summary: [13 Regression] Build fails
                    gcc/tree-ssa-threadbackward.cc:22: gcc/vec.h:890:19:
                    error: array subscript 4294967294 is above array
                    bounds of 'basic_block_def* [1]'
                    [-Werror=array-bounds]
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: slyfox at gcc dot gnu.org
                CC: rguenth at gcc dot gnu.org
  Target Milestone: ---
              Host: i686-unknown-linux-gnu
            Target: i686-unknown-linux-gnu

This week's snapshot fails to build due to -Werror on i686-unknown-linux-gnu
as:

configure flags: --prefix=/<<NIX>>/gcc-13.0.0
--with-gmp-include=/<<NIX>>/gmp-with-cxx-stage3-6.2.1-dev/include
--with-gmp-lib=/<<NIX>>/gmp-with-cxx-stage3-6.2.1/lib
--with-mpfr-include=/<<NIX>>/mpfr-stage3-4.1.0-dev/include
--with-mpfr-lib=/<<NIX>>/mpfr-stage3-4.1.0/lib
--with-mpc=/<<NIX>>/libmpc-stage3-1.2.1 --with-libelf=/<<NIX>>/libelf-0.8.13
--with-native-system-header-dir=/<<NIX>>/glibc-2.35-163-dev/include
--with-build-sysroot=/ --program-prefix= --enable-lto --disable-libstdcxx-pch
--without-included-gettext --with-system-zlib --enable-static
--enable-languages=c\,c++ --disable-multilib --enable-plugin
--with-isl=/<<NIX>>/isl-stage3-0.20 --with-arch=i686
--build=i686-unknown-linux-gnu --host=i686-unknown-linux-gnu
--target=i686-unknown-linux-gnu

/build/build/./prev-gcc/xg++ -B/build/build/./prev-gcc/
-B/nix/store/n7mwmdj0lp1p5dih5irc2ibpzrbmdcaf-gcc-13.0.0/i686-unknown-linux-gnu/bin/
-nostdinc++ -B/build/build/prev-i686-unknown-linux-gnu/libstdc++-v3/src/.libs
-B/build/build/prev-i686-unknown-linux-gnu/libstdc++-v3/libsupc++/.libs 
-I/build/build/prev-i686-unknown-linux-gnu/libstdc++-v3/include/i686-unknown-linux-gnu
 -I/build/build/prev-i686-unknown-linux-gnu/libstdc++-v3/include 
-I/build/gcc-13-20220731/libstdc++-v3/libsupc++
-L/build/build/prev-i686-unknown-linux-gnu/libstdc++-v3/src/.libs
-L/build/build/prev-i686-unknown-linux-gnu/libstdc++-v3/libsupc++/.libs 
-fno-PIE -c   -O2
-I/nix/store/iwn78y2lccsmribf58z84nqkb6prsxg8-glibc-2.35-163-dev/include
-B/nix/store/p3w78y574wiwy8zg8mh3vg5mw4ddlqjh-glibc-2.35-163/lib/ -idirafter
/nix/store/iwn78y2lccsmribf58z84nqkb6prsxg8-glibc-2.35-163-dev/include
-idirafter
/nix/store/n5k6i1bs84fyb8p5191mzjzb89qd0h6s-bootstrap-tools/lib/gcc/i686-unknown-linux-gnu/8.3.0/include-fixed
-Wl,-rpath,/nix/store/sj99b1mb1wg0dq0aih6vnmn8flshzf5d-gcc-13.0.0-lib/lib
-Wl,-L/nix/store/p3w78y574wiwy8zg8mh3vg5mw4ddlqjh-glibc-2.35-163/lib -Wl,-rpath
-Wl,/nix/store/p3w78y574wiwy8zg8mh3vg5mw4ddlqjh-glibc-2.35-163/lib
-Wl,-dynamic-linker=/nix/store/p3w78y574wiwy8zg8mh3vg5mw4ddlqjh-glibc-2.35-163/lib/ld-linux.so.2
-fno-checking -gtoggle -DIN_GCC     -fno-exceptions -fno-rtti
-fasynchronous-unwind-tables -W -Wall -Wno-narrowing -Wwrite-strings
-Wcast-qual -Wmissing-format-attribute -Woverloaded-virtual -pedantic
-Wno-long-long -Wno-variadic-macros -Wno-overlength-strings -Werror -fno-common
 -DHAVE_CONFIG_H -I. -I. -I../../gcc-13-20220731/gcc
-I../../gcc-13-20220731/gcc/. -I../../gcc-13-20220731/gcc/../include
-I../../gcc-13-20220731/gcc/../libcpp/include
-I../../gcc-13-20220731/gcc/../libcody
-I/nix/store/1iwmdhhdxyxv5dyaj61qb1vbyammbb7z-gmp-with-cxx-stage3-6.2.1-dev/include
-I/nix/store/py5l6yngaiw6zdz5rml85nxvxs32hs0x-mpfr-stage3-4.1.0-dev/include
-I/nix/store/bqrl2cd92fb6hnxfs9m1yrx1yxn2qydk-libmpc-stage3-1.2.1/include 
-I../../gcc-13-20220731/gcc/../libdecnumber
-I../../gcc-13-20220731/gcc/../libdecnumber/bid -I../libdecnumber
-I../../gcc-13-20220731/gcc/../libbacktrace
-I/nix/store/whk7v7bwhrmpgi3am9r8gmwl9k2h86m3-isl-stage3-0.20/include  -o
tree-ssa-threadbackward.o -MT tree-ssa-threadbackward.o -MMD -MP -MF
./.deps/tree-ssa-threadbackward.TPo
../../gcc-13-20220731/gcc/tree-ssa-threadbackward.cc

In file included from ../../gcc-13-20220731/gcc/hash-table.h:248,
                 from ../../gcc-13-20220731/gcc/coretypes.h:486,
                 from ../../gcc-13-20220731/gcc/tree-ssa-threadbackward.cc:22:
In member function 'T& vec<T, A, vl_embed>::operator[](unsigned int) [with T =
basic_block_def*; A = va_heap]',
    inlined from 'const T& vec<T>::operator[](unsigned int) const [with T =
basic_block_def*]' at ../../gcc-13-20220731/gcc/vec.h:1486:20,
    inlined from 'bool back_threader_profitability::profitable_path_p(const
vec<basic_block_def*>&, tree, edge, bool*)' at
../../gcc-13-20220731/gcc/tree-ssa-threadbackward.cc:781:37:
../../gcc-13-20220731/gcc/vec.h:890:19: error: array subscript 4294967294 is
above array bounds of 'basic_block_def* [1]' [-Werror=array-bounds]
  890 |   return m_vecdata[ix];
      |          ~~~~~~~~~^
../../gcc-13-20220731/gcc/vec.h: In member function 'bool
back_threader_profitability::profitable_path_p(const vec<basic_block_def*>&,
tree, edge, bool*)':
../../gcc-13-20220731/gcc/vec.h:635:5: note: while referencing
'vec<basic_block_def*, va_heap, vl_embed>::m_vecdata'
  635 |   T m_vecdata[1];
      |     ^~~~~~~~~
cc1plus: all warnings being treated as errors
make[3]: *** [Makefile:1146: tree-ssa-threadbackward.o] Error 1

Could be a side-effect of r13-1891 ?