https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84471
Bug ID: 84471
Summary: Instruction reordering happens in lambdas even with
-O0
Product: gcc
Version: 7.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: jengelh at inai dot de
Target Milestone: ---
Expected behavior:
Objects built with -O0 ought not cause gdb to spuriously "go backwards" in
source (for reasons other than the end of a loop).
Observed behavior:
Despite being explicitly built with no optimizations, I observe that gdb jumps
around inside C++ programs' lambdas that capture something.
#include <cstring>
#include <utility>
#define N 256
int main()
{
const char key[] = "keykeykeykeykeykey";
[&]() {
unsigned char S[N];
int len = strlen(key);
int j = 0;
for (int i = 0; i < N; ++i)
j = (j + S[i] + key[i%len]) % N;
}();
}
Using built-in specs.
COLLECT_GCC=g++-7
COLLECT_LTO_WRAPPER=/usr/lib64/gcc/x86_64-suse-linux/7/lto-wrapper
OFFLOAD_TARGET_NAMES=hsa:nvptx-none
Target: x86_64-suse-linux
Configured with: ../configure --prefix=/usr --infodir=/usr/share/info
--mandir=/usr/share/man --libdir=/usr/lib64 --libexecdir=/usr/lib64
--enable-languages=c,c++,objc,fortran,obj-c++,ada,go
--enable-offload-targets=hsa,nvptx-none=/usr/nvptx-none, --without-cuda-driver
--enable-checking=release --disable-werror
--with-gxx-include-dir=/usr/include/c++/7 --enable-ssp --disable-libssp
--disable-libvtv --disable-libcc1 --enable-plugin
--with-bugurl=http://bugs.opensuse.org/ --with-pkgversion='SUSE Linux'
--with-slibdir=/lib64 --with-system-zlib --enable-__cxa_atexit
--enable-libstdcxx-allocator=new --disable-libstdcxx-pch
--enable-version-specific-runtime-libs --with-gcc-major-version-only
--enable-linker-build-id --enable-linux-futex --enable-gnu-indirect-function
--program-suffix=-7 --without-system-libunwind --enable-multilib
--with-arch-32=x86-64 --with-tune=generic --build=x86_64-suse-linux
--host=x86_64-suse-linux
Thread model: posix
gcc version 7.3.0 (SUSE Linux)
$ g++-7 x.cpp -O0 -ggdb3 -fno-reorder-blocks -fno-reorder-blocks-and-partition
-fno-reorder-functions
$ gdb a.out
GNU gdb (GDB; openSUSE Tumbleweed) 8.0.1
(gdb) b main
Breakpoint 1 at 0x4005c0: file x.cpp, line 6.
(gdb) r
Breakpoint 1, main () at x.cpp:6
6 const char key[] = "keykeykeykeykeykey";
(gdb) s
13 }();
(gdb)
<lambda()>::operator()(void) const (__closure=0x7fffffffdd98) at x.cpp:9
9 int len = strlen(key);
(gdb) n
10 int j = 0;
(gdb)
11 for (int i = 0; i < N; ++i)
(gdb)
12 j = (j + S[i] + key[i%len]) % N;
(gdb)
9 int len = strlen(key);
(gdb)
12 j = (j + S[i] + key[i%len]) % N;
Expected behavior:
Never reach line 9 again.
