https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85992
Bug ID: 85992
Summary: Invalid optimization with atanf
Product: gcc
Version: 8.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: mpeddie at gmail dot com
Target Milestone: ---
Created attachment 44209
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44209&action=edit
Preprocessed source file
I apologize again if I've chosen the wrong category for this bug; I don't know
what's the correct choice.
The output from my small test program varies depending on whether I enable
optimizations with -O. The un-preprocessed program is the following
(preprocessed output attached):
#include <stdio.h>
#include <math.h>
static float example(const int i, const float f)
{
return atanf(i == 0 ? f : 3.141593F);
}
int main(void)
{
printf("atanf(3.141593F) = %.8f\n", example(1, 0.0F));
return 0;
}
With optimizations enabled, this prints
atanf(3.141593F) = 1.26262724
Without optimizations, it prints
atanf(3.141593F) = 1.26262736
I'm not certain which of these is correct, but I expect them to be the same.
(GNU MPFR tells me the result should be 1.2626272439 for a 24-bit significand,
which is close to that of the optimized version.) The results differ by one
bit; the optimized program's output value is 0x3FA19DC6, and the unoptimized
program's output is 0x3FA19DC5. If I move the call to atanf() into the main
function, the program is consistent, always outputting 1.26262724 (like the
optimized version of the program above).
The attached file test.i is the preprocessed source file. The test program
compiles without errors or warnings and never triggers the undefined-behavior
sanitizer. I've observed the same problem with gcc version 7.3.0, 6.4.0 and
5.5.0 for x86_64. Below is the command-line invocation of gcc along with its
complete output.
gcc-8 -v -save-temps -O -Wall -Wextra -Werror -fsanitize=undefined
-fno-strict-aliasing -fwrapv -fno-aggressive-loop-optimizations -lm test.c -o
test
Using built-in specs.
COLLECT_GCC=gcc-8
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/8/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 8.1.0-3'
--with-bugurl=file:///usr/share/doc/gcc-8/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++ --prefix=/usr
--with-gcc-major-version-only --program-suffix=-8
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-libmpx --enable-plugin --enable-default-pie
--with-system-zlib --with-target-system-zlib --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 --without-cuda-driver
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 8.1.0 (Debian 8.1.0-3)
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O' '-Wall' '-Wextra' '-Werror'
'-fsanitize=undefined' '-fno-strict-aliasing' '-fwrapv'
'-fno-aggressive-loop-optimizations' '-o' 'test' '-mtune=generic'
'-march=x86-64'
/usr/lib/gcc/x86_64-linux-gnu/8/cc1 -E -quiet -v -imultiarch x86_64-linux-gnu
test.c -mtune=generic -march=x86-64 -Wall -Wextra -Werror -fsanitize=undefined
-fno-strict-aliasing -fwrapv -fno-aggressive-loop-optimizations -O
-fpch-preprocess -o test.i
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-linux-gnu/8/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/gcc/x86_64-linux-gnu/8/include
/usr/local/include
/usr/lib/gcc/x86_64-linux-gnu/8/include-fixed
/usr/include/x86_64-linux-gnu
/usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O' '-Wall' '-Wextra' '-Werror'
'-fsanitize=undefined' '-fno-strict-aliasing' '-fwrapv'
'-fno-aggressive-loop-optimizations' '-o' 'test' '-mtune=generic'
'-march=x86-64'
/usr/lib/gcc/x86_64-linux-gnu/8/cc1 -fpreprocessed test.i -quiet -dumpbase
test.c -mtune=generic -march=x86-64 -auxbase test -O -Wall -Wextra -Werror
-version -fsanitize=undefined -fno-strict-aliasing -fwrapv
-fno-aggressive-loop-optimizations -o test.s
GNU C17 (Debian 8.1.0-3) version 8.1.0 (x86_64-linux-gnu)
compiled by GNU C version 8.1.0, GMP version 6.1.2, MPFR version 4.0.1,
MPC version 1.1.0, isl version isl-0.19-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C17 (Debian 8.1.0-3) version 8.1.0 (x86_64-linux-gnu)
compiled by GNU C version 8.1.0, GMP version 6.1.2, MPFR version 4.0.1,
MPC version 1.1.0, isl version isl-0.19-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: bbc65a5a9118b9b79402871f4ead4543
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O' '-Wall' '-Wextra' '-Werror'
'-fsanitize=undefined' '-fno-strict-aliasing' '-fwrapv'
'-fno-aggressive-loop-optimizations' '-o' 'test' '-mtune=generic'
'-march=x86-64'
as -v --64 -o test.o test.s
GNU assembler version 2.30 (x86_64-linux-gnu) using BFD version (GNU Binutils
for Debian) 2.30
COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/8/:/usr/lib/gcc/x86_64-linux-gnu/8/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/8/:/usr/lib/gcc/x86_64-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/8/:/usr/lib/gcc/x86_64-linux-gnu/8/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/8/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/8/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O' '-Wall' '-Wextra' '-Werror'
'-fsanitize=undefined' '-fno-strict-aliasing' '-fwrapv'
'-fno-aggressive-loop-optimizations' '-o' 'test' '-mtune=generic'
'-march=x86-64'
/usr/lib/gcc/x86_64-linux-gnu/8/collect2 -plugin
/usr/lib/gcc/x86_64-linux-gnu/8/liblto_plugin.so
-plugin-opt=/usr/lib/gcc/x86_64-linux-gnu/8/lto-wrapper
-plugin-opt=-fresolution=test.res -plugin-opt=-pass-through=-lgcc
-plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lc
-plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lgcc_s --sysroot=/
--build-id --eh-frame-hdr -m elf_x86_64 --hash-style=gnu -dynamic-linker
/lib64/ld-linux-x86-64.so.2 -pie -o test
/usr/lib/gcc/x86_64-linux-gnu/8/../../../x86_64-linux-gnu/Scrt1.o
/usr/lib/gcc/x86_64-linux-gnu/8/../../../x86_64-linux-gnu/crti.o
/usr/lib/gcc/x86_64-linux-gnu/8/crtbeginS.o -L/usr/lib/gcc/x86_64-linux-gnu/8
-L/usr/lib/gcc/x86_64-linux-gnu/8/../../../x86_64-linux-gnu
-L/usr/lib/gcc/x86_64-linux-gnu/8/../../../../lib -L/lib/x86_64-linux-gnu
-L/lib/../lib -L/usr/lib/x86_64-linux-gnu -L/usr/lib/../lib
-L/usr/lib/gcc/x86_64-linux-gnu/8/../../.. -lm test.o -lubsan -lgcc
--push-state --as-needed -lgcc_s --pop-state -lc -lgcc --push-state --as-needed
-lgcc_s --pop-state /usr/lib/gcc/x86_64-linux-gnu/8/crtendS.o
/usr/lib/gcc/x86_64-linux-gnu/8/../../../x86_64-linux-gnu/crtn.o
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O' '-Wall' '-Wextra' '-Werror'
'-fsanitize=undefined' '-fno-strict-aliasing' '-fwrapv'
'-fno-aggressive-loop-optimizations' '-o' 'test' '-mtune=generic'
'-march=x86-64'
