https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97651
Bug ID: 97651
Summary: abs((int)fabs(0.0/0.0)) results negative
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: olha5b at gmx dot net
Target Milestone: ---
Created attachment 49477
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49477&action=edit
Preprocessing of first sample program
First sample program: i negative after "if (i<0) i=0;" with -O1
===============================================================
In my software I have a combination of fabs, integer conversion, and an
additional check to avoid negative results. But it fails on nans. Here comes a
simplified example program:
#include <math.h>
#include <stdio.h>
int main(int argc __attribute__((unused)), char * argv[]
__attribute__((unused)))
{
int i=(int)fabs(0.0/0.0);
if (i<0) i=0;
printf("RESULT: i=%d\n",i);
return 0;
}
When compiling with -O0 the program prints:
RESULT: i=0
But when compiling with -O1 or higher it prints:
RESULT: i=-2147483648
I don't know, either (int)fabs(0.0/0.0) should not be negative or the compiler
should not optimize out the "if (i<0) i=0;".
I saw the behavior with gcc (Debian 6.3.0-18+deb9u1) 6.3.0 20170516, and I
could reproduce it with gcc 10.2.0,
Second sample program: abs(...) results negative even with -O0
==============================================================
Another example with using abs(..) instead of if (..):
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
int main(int argc __attribute__((unused)), char * argv[]
__attribute__((unused)))
{
int i=abs((int)fabs(0.0/0.0));
printf("RESULT: i=%d\n",i);
return 0;
}
When compiling with -O0 or higher the program prints:
RESULT: i=-2147483648
I think abs(..) should never result negative.
Commands + output
=================
+ gcc -v -save-temps -O1 -Wall -Wextra -fno-strict-aliasing -fwrapv
-fno-aggressive-loop-optimizations -fsanitize=undefined -lm test1.c -o test1
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/extra/gcc-10.2.0/libexec/gcc/x86_64-pc-linux-gnu/10.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ./configure --prefix=/extra/gcc-10.2.0 --disable-multilib
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 10.2.0 (GCC)
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O1' '-Wall' '-Wextra'
'-fno-strict-aliasing' '-fwrapv' '-fno-aggressive-loop-optimizations'
'-fsanitize=undefined' '-o' 'test1' '-mtune=generic' '-march=x86-64'
/extra/gcc-10.2.0/libexec/gcc/x86_64-pc-linux-gnu/10.2.0/cc1 -E -quiet -v
-imultiarch x86_64-linux-gnu test1.c -mtune=generic -march=x86-64 -Wall -Wextra
-fno-strict-aliasing -fwrapv -fno-aggressive-loop-optimizations
-fsanitize=undefined -O1 -fpch-preprocess -o test1.i
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory
"/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../x86_64-pc-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include
/usr/local/include
/extra/gcc-10.2.0/include
/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include-fixed
/usr/include/x86_64-linux-gnu
/usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O1' '-Wall' '-Wextra'
'-fno-strict-aliasing' '-fwrapv' '-fno-aggressive-loop-optimizations'
'-fsanitize=undefined' '-o' 'test1' '-mtune=generic' '-march=x86-64'
/extra/gcc-10.2.0/libexec/gcc/x86_64-pc-linux-gnu/10.2.0/cc1 -fpreprocessed
test1.i -quiet -dumpbase test1.c -mtune=generic -march=x86-64 -auxbase test1
-O1 -Wall -Wextra -version -fno-strict-aliasing -fwrapv
-fno-aggressive-loop-optimizations -fsanitize=undefined -o test1.s
GNU C17 (GCC) version 10.2.0 (x86_64-pc-linux-gnu)
compiled by GNU C version 10.2.0, GMP version 6.1.2, MPFR version
3.1.5, MPC version 1.0.3, isl version none
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C17 (GCC) version 10.2.0 (x86_64-pc-linux-gnu)
compiled by GNU C version 10.2.0, GMP version 6.1.2, MPFR version
3.1.5, MPC version 1.0.3, isl version none
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 44d7e89f433c19bc66ff65a4297e4c94
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O1' '-Wall' '-Wextra'
'-fno-strict-aliasing' '-fwrapv' '-fno-aggressive-loop-optimizations'
'-fsanitize=undefined' '-o' 'test1' '-mtune=generic' '-march=x86-64'
as -v --64 -o test1.o test1.s
GNU assembler version 2.28 (x86_64-linux-gnu) using BFD version (GNU Binutils
for Debian) 2.28
COMPILER_PATH=/extra/gcc-10.2.0/libexec/gcc/x86_64-pc-linux-gnu/10.2.0/:/extra/gcc-10.2.0/libexec/gcc/x86_64-pc-linux-gnu/10.2.0/:/extra/gcc-10.2.0/libexec/gcc/x86_64-pc-linux-gnu/:/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/
LIBRARY_PATH=/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../lib64/:/lib/x86_64-linux-gnu/:/lib/../lib64/:/usr/lib/x86_64-linux-gnu/:/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O1' '-Wall' '-Wextra'
'-fno-strict-aliasing' '-fwrapv' '-fno-aggressive-loop-optimizations'
'-fsanitize=undefined' '-o' 'test1' '-mtune=generic' '-march=x86-64'
/extra/gcc-10.2.0/libexec/gcc/x86_64-pc-linux-gnu/10.2.0/collect2 -plugin
/extra/gcc-10.2.0/libexec/gcc/x86_64-pc-linux-gnu/10.2.0/liblto_plugin.so
-plugin-opt=/extra/gcc-10.2.0/libexec/gcc/x86_64-pc-linux-gnu/10.2.0/lto-wrapper
-plugin-opt=-fresolution=test1.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
--eh-frame-hdr -m elf_x86_64 -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o
test1 /usr/lib/x86_64-linux-gnu/crt1.o /usr/lib/x86_64-linux-gnu/crti.o
/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/10.2.0/crtbegin.o
-L/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/10.2.0
-L/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../lib64
-L/lib/x86_64-linux-gnu -L/lib/../lib64 -L/usr/lib/x86_64-linux-gnu
-L/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../.. -lm test1.o
-lubsan -lgcc --push-state --as-needed -lgcc_s --pop-state -lc -lgcc
--push-state --as-needed -lgcc_s --pop-state
/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/10.2.0/crtend.o
/usr/lib/x86_64-linux-gnu/crtn.o
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O1' '-Wall' '-Wextra'
'-fno-strict-aliasing' '-fwrapv' '-fno-aggressive-loop-optimizations'
'-fsanitize=undefined' '-o' 'test1' '-mtune=generic' '-march=x86-64'
+ ./test1
RESULT: i=-2147483648
+ gcc -v -save-temps -O0 -Wall -Wextra -fno-strict-aliasing -fwrapv
-fno-aggressive-loop-optimizations -fsanitize=undefined -lm test2.c -o test2
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/extra/gcc-10.2.0/libexec/gcc/x86_64-pc-linux-gnu/10.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ./configure --prefix=/extra/gcc-10.2.0 --disable-multilib
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 10.2.0 (GCC)
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O0' '-Wall' '-Wextra'
'-fno-strict-aliasing' '-fwrapv' '-fno-aggressive-loop-optimizations'
'-fsanitize=undefined' '-o' 'test2' '-mtune=generic' '-march=x86-64'
/extra/gcc-10.2.0/libexec/gcc/x86_64-pc-linux-gnu/10.2.0/cc1 -E -quiet -v
-imultiarch x86_64-linux-gnu test2.c -mtune=generic -march=x86-64 -Wall -Wextra
-fno-strict-aliasing -fwrapv -fno-aggressive-loop-optimizations
-fsanitize=undefined -O0 -fpch-preprocess -o test2.i
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory
"/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../x86_64-pc-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include
/usr/local/include
/extra/gcc-10.2.0/include
/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include-fixed
/usr/include/x86_64-linux-gnu
/usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O0' '-Wall' '-Wextra'
'-fno-strict-aliasing' '-fwrapv' '-fno-aggressive-loop-optimizations'
'-fsanitize=undefined' '-o' 'test2' '-mtune=generic' '-march=x86-64'
/extra/gcc-10.2.0/libexec/gcc/x86_64-pc-linux-gnu/10.2.0/cc1 -fpreprocessed
test2.i -quiet -dumpbase test2.c -mtune=generic -march=x86-64 -auxbase test2
-O0 -Wall -Wextra -version -fno-strict-aliasing -fwrapv
-fno-aggressive-loop-optimizations -fsanitize=undefined -o test2.s
GNU C17 (GCC) version 10.2.0 (x86_64-pc-linux-gnu)
compiled by GNU C version 10.2.0, GMP version 6.1.2, MPFR version
3.1.5, MPC version 1.0.3, isl version none
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C17 (GCC) version 10.2.0 (x86_64-pc-linux-gnu)
compiled by GNU C version 10.2.0, GMP version 6.1.2, MPFR version
3.1.5, MPC version 1.0.3, isl version none
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 44d7e89f433c19bc66ff65a4297e4c94
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O0' '-Wall' '-Wextra'
'-fno-strict-aliasing' '-fwrapv' '-fno-aggressive-loop-optimizations'
'-fsanitize=undefined' '-o' 'test2' '-mtune=generic' '-march=x86-64'
as -v --64 -o test2.o test2.s
GNU assembler version 2.28 (x86_64-linux-gnu) using BFD version (GNU Binutils
for Debian) 2.28
COMPILER_PATH=/extra/gcc-10.2.0/libexec/gcc/x86_64-pc-linux-gnu/10.2.0/:/extra/gcc-10.2.0/libexec/gcc/x86_64-pc-linux-gnu/10.2.0/:/extra/gcc-10.2.0/libexec/gcc/x86_64-pc-linux-gnu/:/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/
LIBRARY_PATH=/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../lib64/:/lib/x86_64-linux-gnu/:/lib/../lib64/:/usr/lib/x86_64-linux-gnu/:/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O0' '-Wall' '-Wextra'
'-fno-strict-aliasing' '-fwrapv' '-fno-aggressive-loop-optimizations'
'-fsanitize=undefined' '-o' 'test2' '-mtune=generic' '-march=x86-64'
/extra/gcc-10.2.0/libexec/gcc/x86_64-pc-linux-gnu/10.2.0/collect2 -plugin
/extra/gcc-10.2.0/libexec/gcc/x86_64-pc-linux-gnu/10.2.0/liblto_plugin.so
-plugin-opt=/extra/gcc-10.2.0/libexec/gcc/x86_64-pc-linux-gnu/10.2.0/lto-wrapper
-plugin-opt=-fresolution=test2.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
--eh-frame-hdr -m elf_x86_64 -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o
test2 /usr/lib/x86_64-linux-gnu/crt1.o /usr/lib/x86_64-linux-gnu/crti.o
/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/10.2.0/crtbegin.o
-L/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/10.2.0
-L/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../lib64
-L/lib/x86_64-linux-gnu -L/lib/../lib64 -L/usr/lib/x86_64-linux-gnu
-L/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../.. -lm test2.o
-lubsan -lgcc --push-state --as-needed -lgcc_s --pop-state -lc -lgcc
--push-state --as-needed -lgcc_s --pop-state
/extra/gcc-10.2.0/lib/gcc/x86_64-pc-linux-gnu/10.2.0/crtend.o
/usr/lib/x86_64-linux-gnu/crtn.o
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O0' '-Wall' '-Wextra'
'-fno-strict-aliasing' '-fwrapv' '-fno-aggressive-loop-optimizations'
'-fsanitize=undefined' '-o' 'test2' '-mtune=generic' '-march=x86-64'
+ ./test2
RESULT: i=-2147483648
