https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82210
Bug ID: 82210
Summary: Having _Alignas in a struct with VLAs causes writing
to one array to overwrite another
Product: gcc
Version: 7.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: gopalsr83 at gmail dot com
Target Milestone: ---
Created attachment 42167
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=42167&action=edit
Preprocessed source that can be used to reproduce the bug
In the following program, writing elements of s.b causes overwrite of the last
few elements of s.a:
void fun(int size)
{
int i;
struct {
_Alignas(16) struct {
short aa;
} a[size];
int b[size];
} s;
for (i = 0; i < size; i++) {
s.a[i].aa = 0x1234;
}
for (i = 0; i < size; i++) {
s.b[i] = 0;
}
for (i = 0; i < size; i++) {
printf("0x%04x ", s.a[i].aa);
}
printf("\n");
}
int main ()
{
fun(15);
}
The above program produces the output:
0x1234 0x1234 0x1234 0x1234 0x1234 0x1234 0x1234 0x1234 0x0000 0x0000 0x0000
0x0000 0x0000 0x0000 0x0000
I have narrowed it down to the use of VLAIS in combination with the _Alignas
specifier.
Removing _Alignas causes the program to execute correctly and produce the
output:
0x1234 0x1234 0x1234 0x1234 0x1234 0x1234 0x1234 0x1234 0x1234 0x1234 0x1234
0x1234 0x1234 0x1234 0x1234
Command line output:
[gopal@localbuild ~]$gcc -v -save-temps -std=c11 -O0 -o test test.c
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0/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++ --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 7.2.0 (GCC)
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c11' '-O0' '-o' 'test'
'-mtune=generic' '-march=x86-64'
/usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0/cc1 -E -quiet -v test.c -mtune=generic
-march=x86-64 -std=c11 -O0 -fpch-preprocess -o test.i
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0/../../../../x86_64-pc-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0/include
/usr/local/include
/usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0/include-fixed
/usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c11' '-O0' '-o' 'test'
'-mtune=generic' '-march=x86-64'
/usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0/cc1 -fpreprocessed test.i -quiet
-dumpbase test.c -mtune=generic -march=x86-64 -auxbase test -O0 -std=c11
-version -o test.s
GNU C11 (GCC) version 7.2.0 (x86_64-pc-linux-gnu)
compiled by GNU C version 7.2.0, GMP version 6.1.2, MPFR version
3.1.5-p2, MPC version 1.0.3, isl version isl-0.18-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C11 (GCC) version 7.2.0 (x86_64-pc-linux-gnu)
compiled by GNU C version 7.2.0, GMP version 6.1.2, MPFR version
3.1.5-p2, MPC version 1.0.3, isl version isl-0.18-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 7c9818d05f48c6fe376de9e4c491b54f
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c11' '-O0' '-o' 'test'
'-mtune=generic' '-march=x86-64'
as -v --64 -o test.o test.s
GNU assembler version 2.29 (x86_64-pc-linux-gnu) using BFD version (GNU
Binutils) 2.29
COMPILER_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/:/usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0/:/usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c11' '-O0' '-o' 'test'
'-mtune=generic' '-march=x86-64'
/usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0/collect2 -plugin
/usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0/liblto_plugin.so
-plugin-opt=/usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0/lto-wrapper
-plugin-opt=-fresolution=test.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 -pie -o test
/usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0/../../../../lib/Scrt1.o
/usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0/../../../../lib/crti.o
/usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0/crtbeginS.o
-L/usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0
-L/usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0/../../../../lib -L/lib/../lib
-L/usr/lib/../lib -L/usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0/../../.. test.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/7.2.0/crtendS.o
/usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0/../../../../lib/crtn.o
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c11' '-O0' '-o' 'test'
'-mtune=generic' '-march=x86-64'
