https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102655
Bug ID: 102655
Summary: wrong code from optimizer for simple loop
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: wildebeest at av8n dot com
Target Milestone: ---
Created attachment 51568
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51568&action=edit
precompiled .i file to demonstrate bug
In the doit() function of the attached gccbug.i file, there is a simple loop
that does some arithmetic.
-- Optimization level 0 seems to produce the correct code.
-- Any higher optimization produces wildly wrong behavior. The loop is executed
far too few times.
-- Unsurprisingly, both g++ and gcc produce the same misbehavior.
-- This is observed with the latest gcc-snapshot (10.0.1) and also with older
versions (9.3.0).
-- Debugging is difficult. Small changes to the program, even when they are far
outside the doit() function, cause the behavior to change. To exhibit this, I
will precompile some of the mutant versions and put the .i files in the
comments.
The exact command used to create the main .i file was:
/usr/lib/gcc-snapshot/bin/gcc -v -save-temps -g -O1 -Dgxx=gcc -DbldOpts="-O1
-Dgxx=gcc" -Wall ./gccbug.c -o ./gccbug
The console output from the compilation step will be attached in the comments.
If you compile the .i file with -O1, the bug appears. If you recompile with
-O0, the bug is not observed.
Platform:
:; uname -srmo
Linux 5.13.1+ x86_64 GNU/Linux
:; lsb_release -a
LSB Version:
core-11.1.0ubuntu2-noarch:cxx-3.0-amd64:cxx-3.0-noarch:cxx-3.1-amd64:cxx-3.1-noarch:cxx-3.2-amd64:cxx-3.2-noarch:cxx-4.0-amd64:cxx-4.0-noarch:cxx-4.1-amd64:cxx-4.1-noarch:graphics-2.0-amd64:graphics-2.0-noarch:graphics-3.0-amd64:graphics-3.0-noarch:graphics-3.1-amd64:graphics-3.1-noarch:graphics-3.2-amd64:graphics-3.2-noarch:graphics-4.0-amd64:graphics-4.0-noarch:graphics-4.1-amd64:graphics-4.1-noarch:printing-11.1.0ubuntu2-noarch:security-11.1.0ubuntu2-noarch
Distributor ID: Ubuntu
Description: Ubuntu 20.04.3 LTS
Release: 20.04
Codename: focal
:; /usr/lib/gcc-snapshot/bin/gcc --version
gcc (Ubuntu 20200418-1ubuntu1) 10.0.1 20200418 (experimental) [master revision
b57e1621eb7:e6f26876f0c:baf3b9b2e5259558ef86bd62398e2ccecd7a4a4c]
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
:; /usr/lib/gcc-snapshot/bin/gcc -v
Using built-in specs.
COLLECT_GCC=/usr/lib/gcc-snapshot/bin/gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc-snapshot/libexec/gcc/x86_64-linux-gnu/10/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa:hsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
20200418-1ubuntu1' --with-bugurl=file:///usr/share/doc/gcc-snapshot/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++
--prefix=/usr/lib/gcc-snapshot --with-gcc-major-version-only --program-prefix=
--enable-shared --enable-linker-build-id --disable-nls --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-plugin --with-system-zlib
--enable-libphobos-checking=release --with-target-system-zlib=auto
--enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686
--with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib
--with-tune=generic --enable-offload-targets=nvptx-none,amdgcn-amdhsa,hsa
--without-cuda-driver --enable-checking=yes --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 10.0.1 20200418 (experimental) [master revision
b57e1621eb7:e6f26876f0c:baf3b9b2e5259558ef86bd62398e2ccecd7a4a4c] (Ubuntu
20200418-1ubuntu1)
