https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77436
Bug ID: 77436
Summary: Incorrect constant result for summing loop inserted
Product: gcc
Version: 6.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: gcc-bugzilla at lucaswerkmeister dot de
Target Milestone: ---
Created attachment 39529
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=39529&action=edit
Preprocessed version of a small C program that sums up all integers from
-(INT_MAX-1) to INT_MAX-1 (including both limits).
The attached program computes the sum of all integers from -(INT_MAX-1) and
INT_MAX-1. Since the lower limit is the additive inverse of the upper limit,
all summands must cancel each other out and the result must be zero (this is
still true with overflow). When compiled with -O0, that result is indeed
printed. With -O1 and -O2, gcc optimizes the constant calculation and fills in
the printf() call with the known result (though the loop is only removed on
-O2); however, the inserted result is incorrect: instead of 0x00000000, it is
0x80000000 – the highest bit has been flipped.
The bug appears to depend on the width of the summation window. Starting at
INT_MIN + 32768 + 8192 + 4096 + 1024 + 256 + 2 and summing up to INT_MAX - 1
still exhibits the same bug; starting one later (+ 1) no longer does.
Subtracting the same constant value from start and end does not change the
result (provided the “shift” doesn’t cause overflow at the start).
Output of gcc -v -save-temps -O1 gcc-bug.c:
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/6.1.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc-multilib/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 --enable-multilib --disable-werror
--enable-checking=release
Thread model: posix
gcc version 6.1.1 20160802 (GCC)
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O1' '-mtune=generic' '-march=x86-64'
/usr/lib/gcc/x86_64-pc-linux-gnu/6.1.1/cc1 -E -quiet -v gcc-bug.c
-mtune=generic -march=x86-64 -O1 -fpch-preprocess -o gcc-bug.i
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-pc-linux-gnu/6.1.1/../../../../x86_64-pc-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/gcc/x86_64-pc-linux-gnu/6.1.1/include
/usr/local/include
/usr/lib/gcc/x86_64-pc-linux-gnu/6.1.1/include-fixed
/usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O1' '-mtune=generic' '-march=x86-64'
/usr/lib/gcc/x86_64-pc-linux-gnu/6.1.1/cc1 -fpreprocessed gcc-bug.i -quiet
-dumpbase gcc-bug.c -mtune=generic -march=x86-64 -auxbase gcc-bug -O1 -version
-o gcc-bug.s
GNU C11 (GCC) version 6.1.1 20160802 (x86_64-pc-linux-gnu)
compiled by GNU C version 6.1.1 20160802, GMP version 6.1.1, MPFR
version 3.1.4-p1, MPC version 1.0.3, isl version 0.15
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C11 (GCC) version 6.1.1 20160802 (x86_64-pc-linux-gnu)
compiled by GNU C version 6.1.1 20160802, GMP version 6.1.1, MPFR
version 3.1.4-p1, MPC version 1.0.3, isl version 0.15
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 0344da0e3652e90e95fdb0223debdba9
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O1' '-mtune=generic' '-march=x86-64'
as -v --64 -o gcc-bug.o gcc-bug.s
GNU assembler version 2.27 (x86_64-pc-linux-gnu) using BFD version (GNU
Binutils) 2.27
COMPILER_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/6.1.1/:/usr/lib/gcc/x86_64-pc-linux-gnu/6.1.1/:/usr/lib/gcc/x86_64-pc-linux-gnu/:/usr/lib/gcc/x86_64-pc-linux-gnu/6.1.1/:/usr/lib/gcc/x86_64-pc-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/6.1.1/:/usr/lib/gcc/x86_64-pc-linux-gnu/6.1.1/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-pc-linux-gnu/6.1.1/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O1' '-mtune=generic' '-march=x86-64'
/usr/lib/gcc/x86_64-pc-linux-gnu/6.1.1/collect2 -plugin
/usr/lib/gcc/x86_64-pc-linux-gnu/6.1.1/liblto_plugin.so
-plugin-opt=/usr/lib/gcc/x86_64-pc-linux-gnu/6.1.1/lto-wrapper
-plugin-opt=-fresolution=gcc-bug.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_x86_64 -dynamic-linker
/lib64/ld-linux-x86-64.so.2
/usr/lib/gcc/x86_64-pc-linux-gnu/6.1.1/../../../../lib/crt1.o
/usr/lib/gcc/x86_64-pc-linux-gnu/6.1.1/../../../../lib/crti.o
/usr/lib/gcc/x86_64-pc-linux-gnu/6.1.1/crtbegin.o
-L/usr/lib/gcc/x86_64-pc-linux-gnu/6.1.1
-L/usr/lib/gcc/x86_64-pc-linux-gnu/6.1.1/../../../../lib -L/lib/../lib
-L/usr/lib/../lib -L/usr/lib/gcc/x86_64-pc-linux-gnu/6.1.1/../../.. gcc-bug.o
-lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s
--no-as-needed /usr/lib/gcc/x86_64-pc-linux-gnu/6.1.1/crtend.o
/usr/lib/gcc/x86_64-pc-linux-gnu/6.1.1/../../../../lib/crtn.o
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O1' '-mtune=generic' '-march=x86-64'
I am running on Arch Linux, which ships gcc 6.1.1, release 5. Unfortunately, I
have not been able to test this on gcc 6.2.0, so this might be fixed already;
if that is the case, I’m sorry. (Debian’s 4.9.2-10 does not apply this
optimization at all and prints the correct result.)
