https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97456
Bug ID: 97456
Summary: An incorrect optimization causes a function to always
return the same value when using -flto
Product: gcc
Version: 10.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: pqfrml at gmail dot com
Target Milestone: ---
https://godbolt.org/z/f65cTb < Minimal example
To the best of my knowledge this is incorrect behavior and it has not yet been
reported.
This is my minimal example:
A floating point reference is passed twice to a function,
the function constructs one std::complex(called a) from the reference,
it constructs another zeroed std::complex(called b),
it assigns b + a to b,
it has a branch, where if b is equal to zero, the function returns 1,
if not it continues and prints b.imag() before returning 0.
For this to work the compiler cannot know of input value in advance which is
why I have used a random number in the example. It can also be user input or
something else opaque at compile time.
The function must be called at least twice for the behavior to appear.
-flto must be on and -O2 or -O3
-fsanitize=undefined causes the bug to disappear, -fno-strict-aliasing -fwrapv
-fno-aggressive-loop-optimizations doesn't make a difference and -Wall and
-Wextra gives no warnings.
The behavior appeared in 10.1 and is present in gcc trunk on godolt.
It is consistent on godbolt, my stationary PC and my laptop.
The files I will give you are from my stationary.
The minimum compilation command is:
g++ -O2 -flto -std=c++17 bug.cpp
and the files I will give you used:
g++ -v -save-temps -O2 -flto -std=c++17 bug.cpp -Wall -Wextra \
-fno-strict-aliasing -fwrapv -fno-aggressive-loop-optimizations
The unexpected behaviour is that it always returns the 1 from the branch, even
when it is impossible for it to evaluate to true, as it should be in my
example.
b needs to be used somehow after the branch, and the easiest way I have found
is console output.
My stationary computer is using Manjaro Linux and I am using the default gcc
from the package manager.
Compiler output:
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/lto-wrapper
Target: x86_64-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++,d --with-isl
--with-linker-hash-style=gnu --with-system-zlib --enable-__cxa_atexit
--enable-cet=auto --enable-checking=release --enable-clocale=gnu
--enable-default-pie --enable-default-ssp --enable-gnu-indirect-function
--enable-gnu-unique-object --enable-install-libiberty --enable-linker-build-id
--enable-lto --enable-multilib --enable-plugin --enable-shared
--enable-threads=posix --disable-libssp --disable-libstdcxx-pch
--disable-libunwind-exceptions --disable-werror
gdc_include_dir=/usr/include/dlang/gdc
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 10.2.0 (GCC)
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O2' '-flto' '-std=c++17' '-Wall'
'-Wextra' '-fno-strict-aliasing' '-fwrapv' '-fno-aggressive-loop-optimizations'
'-shared-libgcc' '-mtune=generic' '-march=x86-64'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/cc1plus -E -quiet -v -D_GNU_SOURCE
bug.cpp -mtune=generic -march=x86-64 -std=c++17 -Wall -Wextra -flto
-fno-strict-aliasing -fwrapv -fno-aggressive-loop-optimizations -O2
-fpch-preprocess -o bug.ii
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../x86_64-pc-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../include/c++/10.2.0
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../include/c++/10.2.0/x86_64-pc-linux-gnu
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../include/c++/10.2.0/backward
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include
/usr/local/include
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/include-fixed
/usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O2' '-flto' '-std=c++17' '-Wall'
'-Wextra' '-fno-strict-aliasing' '-fwrapv' '-fno-aggressive-loop-optimizations'
'-shared-libgcc' '-mtune=generic' '-march=x86-64'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/cc1plus -fpreprocessed bug.ii -quiet
-dumpbase bug.cpp -mtune=generic -march=x86-64 -auxbase bug -O2 -Wall -Wextra
-std=c++17 -version -flto -fno-strict-aliasing -fwrapv
-fno-aggressive-loop-optimizations -o bug.s
GNU C++17 (GCC) version 10.2.0 (x86_64-pc-linux-gnu)
compiled by GNU C version 10.2.0, GMP version 6.2.0, MPFR version
4.1.0, MPC version 1.1.0, isl version isl-0.21-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C++17 (GCC) version 10.2.0 (x86_64-pc-linux-gnu)
compiled by GNU C version 10.2.0, GMP version 6.2.0, MPFR version
4.1.0, MPC version 1.1.0, isl version isl-0.21-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 2affcd71b0797e2421aae734ab600c81
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O2' '-flto' '-std=c++17' '-Wall'
'-Wextra' '-fno-strict-aliasing' '-fwrapv' '-fno-aggressive-loop-optimizations'
'-shared-libgcc' '-mtune=generic' '-march=x86-64'
as -v --64 -o bug.o bug.s
GNU assembler version 2.35 (x86_64-pc-linux-gnu) using BFD version (GNU
Binutils) 2.35
COMPILER_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O2' '-flto' '-std=c++17' '-Wall'
'-Wextra' '-fno-strict-aliasing' '-fwrapv' '-fno-aggressive-loop-optimizations'
'-shared-libgcc' '-mtune=generic' '-march=x86-64'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/collect2 -plugin
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/liblto_plugin.so
-plugin-opt=/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/lto-wrapper
-plugin-opt=-fresolution=bug.res -plugin-opt=-pass-through=-lgcc_s
-plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lc
-plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lgcc -flto
--build-id --eh-frame-hdr --hash-style=gnu -m elf_x86_64 -dynamic-linker
/lib64/ld-linux-x86-64.so.2 -pie
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../lib/Scrt1.o
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../lib/crti.o
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/crtbeginS.o
-L/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0
-L/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../lib -L/lib/../lib
-L/usr/lib/../lib -L/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../.. bug.o
-lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/crtendS.o
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../lib/crtn.o
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/lto-wrapper -fresolution=bug.res
-flinker-output=pie bug.o
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/lto-wrapper -fresolution=bug.res
-flinker-output=pie bug.o
g++ @/tmp/ccZAfZSN
Using built-in specs.
COLLECT_GCC=g++
Target: x86_64-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++,d --with-isl
--with-linker-hash-style=gnu --with-system-zlib --enable-__cxa_atexit
--enable-cet=auto --enable-checking=release --enable-clocale=gnu
--enable-default-pie --enable-default-ssp --enable-gnu-indirect-function
--enable-gnu-unique-object --enable-install-libiberty --enable-linker-build-id
--enable-lto --enable-multilib --enable-plugin --enable-shared
--enable-threads=posix --disable-libssp --disable-libstdcxx-pch
--disable-libunwind-exceptions --disable-werror
gdc_include_dir=/usr/include/dlang/gdc
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 10.2.0 (GCC)
COLLECT_GCC_OPTIONS='-c' '-fno-openmp' '-fno-openacc' '-fPIC' '-O2' '-v'
'-save-temps' '-O2' '-Wextra' '-fno-strict-aliasing' '-fwrapv'
'-fno-aggressive-loop-optimizations' '-shared-libgcc' '-mtune=generic'
'-march=x86-64' '-fltrans-output-list=/tmp/ccFLP8uN.ltrans.out' '-fwpa'
'-fresolution=bug.res' '-flinker-output=pie' '-shared-libgcc'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/lto1 -quiet -dumpbase bug.o
-mtune=generic -march=x86-64 -auxbase bug -O2 -O2 -Wextra -version -fno-openmp
-fno-openacc -fPIC -fno-strict-aliasing -fwrapv
-fno-aggressive-loop-optimizations
-fltrans-output-list=/tmp/ccFLP8uN.ltrans.out -fwpa -fresolution=bug.res
-flinker-output=pie @/tmp/ccq1CCRU
GNU GIMPLE (GCC) version 10.2.0 (x86_64-pc-linux-gnu)
compiled by GNU C version 10.2.0, GMP version 6.2.0, MPFR version
4.1.0, MPC version 1.1.0, isl version isl-0.21-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU GIMPLE (GCC) version 10.2.0 (x86_64-pc-linux-gnu)
compiled by GNU C version 10.2.0, GMP version 6.2.0, MPFR version
4.1.0, MPC version 1.1.0, isl version isl-0.21-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
COMPILER_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../lib/../lib/:/lib/../lib/../lib/:/usr/lib/../lib/../lib/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../:/lib/:/usr/lib/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-c' '-fno-openmp' '-fno-openacc' '-fPIC' '-O2' '-v'
'-save-temps' '-O2' '-Wextra' '-fno-strict-aliasing' '-fwrapv'
'-fno-aggressive-loop-optimizations' '-shared-libgcc' '-mtune=generic'
'-march=x86-64' '-fltrans-output-list=/tmp/ccFLP8uN.ltrans.out' '-fwpa'
'-fresolution=bug.res' '-flinker-output=pie' '-shared-libgcc'
[Leaving LTRANS /tmp/ccFLP8uN.ltrans.out]
g++ @/tmp/ccRp94jQ
Using built-in specs.
COLLECT_GCC=g++
Target: x86_64-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++,d --with-isl
--with-linker-hash-style=gnu --with-system-zlib --enable-__cxa_atexit
--enable-cet=auto --enable-checking=release --enable-clocale=gnu
--enable-default-pie --enable-default-ssp --enable-gnu-indirect-function
--enable-gnu-unique-object --enable-install-libiberty --enable-linker-build-id
--enable-lto --enable-multilib --enable-plugin --enable-shared
--enable-threads=posix --disable-libssp --disable-libstdcxx-pch
--disable-libunwind-exceptions --disable-werror
gdc_include_dir=/usr/include/dlang/gdc
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 10.2.0 (GCC)
COLLECT_GCC_OPTIONS='-c' '-fno-openmp' '-fno-openacc' '-fPIC' '-O2' '-v'
'-save-temps' '-O2' '-Wextra' '-fno-strict-aliasing' '-fwrapv'
'-fno-aggressive-loop-optimizations' '-shared-libgcc' '-mtune=generic'
'-march=x86-64' '-fltrans-output-list=/tmp/ccFLP8uN.ltrans.out' '-fltrans' '-o'
'/tmp/ccFLP8uN.ltrans0.ltrans.o' '-shared-libgcc'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/lto1 -quiet -dumpbase
ccFLP8uN.ltrans0.o -mtune=generic -march=x86-64 -auxbase-strip
/tmp/ccFLP8uN.ltrans0.ltrans.o -O2 -O2 -Wextra -version -fno-openmp
-fno-openacc -fPIC -fno-strict-aliasing -fwrapv
-fno-aggressive-loop-optimizations
-fltrans-output-list=/tmp/ccFLP8uN.ltrans.out -fltrans @/tmp/ccePO4q3 -o
ccFLP8uN.ltrans0.s
GNU GIMPLE (GCC) version 10.2.0 (x86_64-pc-linux-gnu)
compiled by GNU C version 10.2.0, GMP version 6.2.0, MPFR version
4.1.0, MPC version 1.1.0, isl version isl-0.21-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU GIMPLE (GCC) version 10.2.0 (x86_64-pc-linux-gnu)
compiled by GNU C version 10.2.0, GMP version 6.2.0, MPFR version
4.1.0, MPC version 1.1.0, isl version isl-0.21-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
COLLECT_GCC_OPTIONS='-c' '-fno-openmp' '-fno-openacc' '-fPIC' '-O2' '-v'
'-save-temps' '-O2' '-Wextra' '-fno-strict-aliasing' '-fwrapv'
'-fno-aggressive-loop-optimizations' '-shared-libgcc' '-mtune=generic'
'-march=x86-64' '-fltrans-output-list=/tmp/ccFLP8uN.ltrans.out' '-fltrans' '-o'
'/tmp/ccFLP8uN.ltrans0.ltrans.o' '-shared-libgcc'
as -v --64 -o /tmp/ccFLP8uN.ltrans0.ltrans.o ccFLP8uN.ltrans0.s
GNU assembler version 2.35 (x86_64-pc-linux-gnu) using BFD version (GNU
Binutils) 2.35
COMPILER_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../lib/../lib/:/lib/../lib/../lib/:/usr/lib/../lib/../lib/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../:/lib/:/usr/lib/:/usr/lib/gcc/x86_64-pc-linux-gnu/10.2.0/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-c' '-fno-openmp' '-fno-openacc' '-fPIC' '-O2' '-v'
'-save-temps' '-O2' '-Wextra' '-fno-strict-aliasing' '-fwrapv'
'-fno-aggressive-loop-optimizations' '-shared-libgcc' '-mtune=generic'
'-march=x86-64' '-fltrans-output-list=/tmp/ccFLP8uN.ltrans.out' '-fltrans' '-o'
'/tmp/ccFLP8uN.ltrans0.ltrans.o' '-shared-libgcc'
[Leaving LTRANS /tmp/ccFLP8uN.ltrans0.o]
[Leaving a.out.lto_wrapper_args]
[Leaving /tmp/ccFLP8uN.ltrans0.ltrans.o]
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-O2' '-flto' '-std=c++17' '-Wall'
'-Wextra' '-fno-strict-aliasing' '-fwrapv' '-fno-aggressive-loop-optimizations'
'-shared-libgcc' '-mtune=generic' '-march=x86-64'
