[Bug c++/36115] New: wrong code generated with optimization on x86-64
brett dot polivka at magnetar dot com
gcc-bugzilla@gcc.gnu.org
Fri May 2 22:30:00 GMT 2008
This small program:
// built using g++ -o test -O2 main.cpp
#include <iostream>
struct stuff
{
int x;
};
class MyException : public std::exception
{
public:
MyException() { }
};
// make this global so conditional below doesn't get eliminated
bool should_throw = false;
void calc_x(stuff& s, int n)
{
// set s.x to max(s.x, n, 2)
s.x = std::max(n, s.x);
s.x = std::max(2, s.x);
// bogus throw needed to generate error
if(should_throw)
{
// throw MyException() won't trigger bug - must be separate lines
// also, something like std::runtime_error won't trigger either
MyException ex;
throw ex;
}
}
int main(int argc, char* argv[])
{
stuff s = { 0 };
int n = atoi(argv[1]);
calc_x(s, n);
std::cout << s.x << "\n";
std::cout << (s.x == n ? "SUCCESS" : "FAILURE") << "\n";
}
will fail when passed any value greater than 2.
calc_x should be returning the maximum of s.x, n and 2, but for values of n >
2, always returns the original value of s.x.
Output:
-------------
% ./test 0
2
% ./test 1
2
% ./test 2
2
% ./test 3
0
I've attempted to distill it to a smaller example than this, but eliminating
almost anything causes it to start functioning again.
Looking at the generated assembly, gcc is generating two conditional moves,
corresponding to the two std::max calls. In the bad code, the final move is
moving the address of s.x into a register, which then gets dereferenced and
assigned into s.x. However, the intermediate result of the first comparison was
not stored in s.x, but a scratch temporary on the stack. Therefore, s.x is
being dereferenced and assigned to itself.
movl %esi, 12(%rsp) <--- tmp1 = n
cmpl (%rdi), %esi <--- compare s.x and n
leaq 12(%rsp), %rax <--- rax = &tmp1
cmovl %rdi, %rax <--- rax = &s if n < s.x
movl (%rax), %edx <--- edx = *rax
leaq 28(%rsp), %rax <--- rax = &tmp2
movl $2, 28(%rsp) <--- tmp2 = 2
cmpl $2, %edx
cmovg %rdi, %rax <--- rax = &s.x (!!!) if edx > 2
cmpb $0, should_throw(%rip)
movl (%rax), %eax <--- eax = *rax
movl %eax, (%rdi) <--- s.x = eax
This is using gcc 4.2.3 as distributed with Ubuntu 8.04, however I've also
verified the same results using an unpatched gcc 4.2.3, as well as the latest
gcc-4_2-branch branch from subversion.
Thanks,
Brett Polivka
% g++ -v
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v
--enable-languages=c,c++,fortran,objc,obj-c++,treelang --prefix=/usr
--enable-shared --with-system-zlib --libexecdir=/usr/lib
--without-included-gettext --enable-threads=posix --enable-nls
--with-gxx-include-dir=/usr/include/c++/4.2 --program-suffix=-4.2
--enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --enable-mpfr
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 4.2.3 (Ubuntu 4.2.3-2ubuntu7)
--
Summary: wrong code generated with optimization on x86-64
Product: gcc
Version: 4.2.3
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: brett dot polivka at magnetar dot com
GCC build triplet: x86_64-linux-gnu
GCC host triplet: x86_64-linux-gnu
GCC target triplet: x86_64-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36115
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