Created attachment 49477 [details]
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