https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70514
Bug ID: 70514
Summary: Variable length arrays lead to garbage in another
array
Product: gcc
Version: 5.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: kwpolska at gmail dot com
Target Milestone: ---
Created attachment 38162
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=38162&action=edit
The broken code.
I’m writing a really simple algorithm to find all prime factors of a compound
integer n. factorize() needs to know primes, so it uses a variable-length array
that it passes to the sort_of_sieve() function. It should also make its results
easy to modify, so an array OUT[50] for results is passed.
Desired output of the program:
2
2
2
2
The output of the program is random and might be one of:
a) four twos
b) four twos and a segfault
c) four numbers that are some random stack garbage (and maybe a segfault)
d) a segfault, without any output
e) a floating point exception (?!)
Replacing the primes VLA with
bool *primes = new bool[n];
works.
Compiled with (no error/warning output):
$ g++ -Wall -Wextra vla-bug.cpp -o vla -save-temps
This program compiles with clang++ v3.7.1. MSVC++ does not support VLAs and
thus refuses to compile.
Running on Arch Linux.
$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-unknown-linux-gnu/5.3.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: /build/gcc/src/gcc-5-20160209/configure --prefix=/usr
--libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++ --enable-shared
--enable-threads=posix --enable-libmpx --with-system-zlib --with-isl
--enable-__cxa_atexit --disable-libunwind-exceptions --enable-clocale=gnu
--disable-libstdcxx-pch --disable-libssp --enable-gnu-unique-object
--enable-linker-build-id --enable-lto --enable-plugin
--enable-install-libiberty --with-linker-hash-style=gnu
--enable-gnu-indirect-function --disable-multilib --disable-werror
--enable-checking=release
Thread model: posix
gcc version 5.3.0 (GCC)
