https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85957
Bug ID: 85957
Summary: i686: Integers appear to be different, but compare as
equal
Product: gcc
Version: 7.3.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: lukeshu at lukeshu dot com
Target Milestone: ---
Created attachment 44200
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44200&action=edit
The preprocessed source
This is a bug that at first looks a bit like a "problems with floating
point numbers" bug. However, my problem is with integers (calculated
from float types) behave inconsistently.
a6 = a.dbl * 1e6;
b6 = b.dbl * 1e6;
printf ("a6 = %llu\n", a6); // prints "1"
printf ("b6 = %llu\n", b6); // prints "0"
printf ("(a6 == b6) = %s\n", (a6 == b6) ? "true" : "false"); // prints
"true"
I understand why floating point math could result in a6 and b6 being
different; my concern is that a6 and b6 (which are integer types)
appear to be different, yet compare as being equal.
This happens on i686 with -O1 and -O2 (but not -O0), and not on
x86-64.
I apologize that my minimal testcase makes use of the glib-2.0
library; I'm having a hard time replicating the problem without it; it
seems GCC optimizing out a variable is key; and removing the library
use makes it not optimize it out.
Here is the output of gcc, including the appropriate version information:
$ gcc -v -save-temps -O1 $(pkg-config --libs --cflags glib-2.0) demo.c -o
demo
Using built-in specs.
COLLECT_GCC=/usr/bin/gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i686-pc-linux-gnu/7.3.1/lto-wrapper
Target: i686-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure --prefix=/usr
--libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++ --enable-shared
--enable-threads=posix --enable-libmpx --with-system-zlib --with-isl
--enable-__cxa_atexit --disable-libunwind-exceptions --enable-clocale=gnu
--disable-libstdcxx-pch --disable-libssp --enable-gnu-unique-object
--enable-linker-build-id --enable-lto --enable-plugin
--enable-install-libiberty --with-linker-hash-style=gnu
--enable-gnu-indirect-function --disable-multilib --disable-werror
--enable-checking=release --enable-default-pie --enable-default-ssp
Thread model: posix
gcc version 7.3.1 20180312 (GCC)
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O1' '-I' '/usr/include/glib-2.0'
'-I' '/usr/lib/glib-2.0/include' '-o' 'demo' '-mtune=generic'
'-march=pentiumpro'
/usr/lib/gcc/i686-pc-linux-gnu/7.3.1/cc1 -E -quiet -v -I
/usr/include/glib-2.0 -I /usr/lib/glib-2.0/include demo.c -mtune=generic
-march=pentiumpro -O1 -fpch-preprocess -o demo.i
ignoring nonexistent directory
"/usr/lib/gcc/i686-pc-linux-gnu/7.3.1/../../../../i686-pc-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/include/glib-2.0
/usr/lib/glib-2.0/include
/usr/lib/gcc/i686-pc-linux-gnu/7.3.1/include
/usr/local/include
/usr/lib/gcc/i686-pc-linux-gnu/7.3.1/include-fixed
/usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O1' '-I' '/usr/include/glib-2.0'
'-I' '/usr/lib/glib-2.0/include' '-o' 'demo' '-mtune=generic'
'-march=pentiumpro'
/usr/lib/gcc/i686-pc-linux-gnu/7.3.1/cc1 -fpreprocessed demo.i -quiet
-dumpbase demo.c -mtune=generic -march=pentiumpro -auxbase demo -O1 -version -o
demo.s
GNU C11 (GCC) version 7.3.1 20180312 (i686-pc-linux-gnu)
compiled by GNU C version 7.3.1 20180312, GMP version 6.1.2, MPFR
version 4.0.1, MPC version 1.1.0, isl version isl-0.18-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C11 (GCC) version 7.3.1 20180312 (i686-pc-linux-gnu)
compiled by GNU C version 7.3.1 20180312, GMP version 6.1.2, MPFR
version 4.0.1, MPC version 1.1.0, isl version isl-0.18-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: b94f7ca39249d495c6913c6ded8c0b64
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O1' '-I' '/usr/include/glib-2.0'
'-I' '/usr/lib/glib-2.0/include' '-o' 'demo' '-mtune=generic'
'-march=pentiumpro'
as -v -I /usr/include/glib-2.0 -I /usr/lib/glib-2.0/include --32 -o demo.o
demo.s
GNU assembler version 2.30 (i686-pc-linux-gnu) using BFD version (GNU
Binutils) 2.30
COMPILER_PATH=/usr/lib/gcc/i686-pc-linux-gnu/7.3.1/:/usr/lib/gcc/i686-pc-linux-gnu/7.3.1/:/usr/lib/gcc/i686-pc-linux-gnu/:/usr/lib/gcc/i686-pc-linux-gnu/7.3.1/:/usr/lib/gcc/i686-pc-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/i686-pc-linux-gnu/7.3.1/:/usr/lib/gcc/i686-pc-linux-gnu/7.3.1/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O1' '-I' '/usr/include/glib-2.0'
'-I' '/usr/lib/glib-2.0/include' '-o' 'demo' '-mtune=generic'
'-march=pentiumpro'
/usr/lib/gcc/i686-pc-linux-gnu/7.3.1/collect2 -plugin
/usr/lib/gcc/i686-pc-linux-gnu/7.3.1/liblto_plugin.so
-plugin-opt=/usr/lib/gcc/i686-pc-linux-gnu/7.3.1/lto-wrapper
-plugin-opt=-fresolution=demo.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 --build-id
--eh-frame-hdr --hash-style=gnu -m elf_i386 -dynamic-linker /lib/ld-linux.so.2
-pie -o demo /usr/lib/gcc/i686-pc-linux-gnu/7.3.1/../../../Scrt1.o
/usr/lib/gcc/i686-pc-linux-gnu/7.3.1/../../../crti.o
/usr/lib/gcc/i686-pc-linux-gnu/7.3.1/crtbeginS.o
-L/usr/lib/gcc/i686-pc-linux-gnu/7.3.1
-L/usr/lib/gcc/i686-pc-linux-gnu/7.3.1/../../.. -lglib-2.0 demo.o -lgcc
--as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed
/usr/lib/gcc/i686-pc-linux-gnu/7.3.1/crtendS.o
/usr/lib/gcc/i686-pc-linux-gnu/7.3.1/../../../crtn.o
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O1' '-I' '/usr/include/glib-2.0'
'-I' '/usr/lib/glib-2.0/include' '-o' 'demo' '-mtune=generic'
'-march=pentiumpro'
Attached is the preprocessed demo.i file.
