https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97783
Bug ID: 97783
Summary: Optimizer assumes global static variable cannot be
updated by external function, even though function is
passed address of local functions
Product: gcc
Version: 10.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: quentin at armitage dot org.uk
Target Milestone: ---
Created attachment 49541
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49541&action=edit
The preprocessed glob_err.i file
The attached code, when compiled with -O1 outputs:
glob(/tmp/zz{a,b}.conf) returned 3, missing files 0
glob(/tmp/zz{a,b}.conf) returned 3, missing files 2
When compiled with -O0, the following is output:
glob(/tmp/zz{a,b}.conf) returned 3, missing files 2
glob(/tmp/zz{a,b}.conf) returned 3, missing files 2
Since the two fprintf statements (lines 40 and 41) generating the two lines of
output are consecutive statements, and no variables are altered in the calls to
fprintf, the output when using -O1 is incorrect.
It appears that when compiled with -O1 or higher levels of optimisation, if the
missing_files variable is accessed prior to another function call following the
return from the call to open_and_check_glob(), then the value used is 0; if
there is any intermediate function call before missing_files is accessed, then
the correct value is used.
If missing_files is not declared static, or if it is declared static volatile,
then the problem does not occur. The problem also does not occur if the
statement (at line 36)
missing_files = 0;
is deleted.
The versions of gcc given below are using the latest version I have installed.
If have also tested this on Fedora 31 x86_64 with gcc version 9.3.1, Debian 10
(Buster) 32 bit armv7l, and Debian 10 aarch64 (both with gcc version 8.3.0),
and all exhibit exactly the same behaviour.
the exact version of GCC
the system type
the options given when GCC was configured/built
================================================
This is from a Fedora 33 x86_64 system:
Output of gcc -v:
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/10/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-redhat-linux
Configured with: ../configure --enable-bootstrap
--enable-languages=c,c++,fortran,objc,obj-c++,ada,go,d,lto --prefix=/usr
--mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-shared
--enable-threads=posix --enable-checking=release --enable-multilib
--with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions
--enable-gnu-unique-object --enable-linker-build-id
--with-gcc-major-version-only --with-linker-hash-style=gnu --enable-plugin
--enable-initfini-array --with-isl --enable-offload-targets=nvptx-none
--without-cuda-driver --enable-gnu-indirect-function --enable-cet
--with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 10.2.1 20201016 (Red Hat 10.2.1-6) (GCC)
the complete command line that triggers the bug
===============================================
Ensure that /tmp/azz.conf and /tmp/bzz.conf do not exist:
./glob_err "/tmp/{a,b}zz.conf"
the compiler output (error messages, warnings, etc.)
====================================================
None
the preprocessed file (*.i*) that triggers the bug
==================================================
Attached
