http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56360
Bug #: 56360
Summary: Loop invariant motion can introduce speculative store
Classification: Unclassified
Product: gcc
Version: 4.8.0
Status: UNCONFIRMED
Keywords: wrong-code
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: i...@airs.com
When I compile this C++ program with -O3 -std=gnu+11 on x86_64:
#include<mutex>
#include<thread>
extern int a;
std::mutex m;
void nop2(int*);
// this function is ONLY called with i=0.
void foo(int i)
{
if (i) m.lock();
nop2(&i); // Only here to prevent optimiser from creating one big if().
for (int c = 0; c < 10000; ++c) {
if (i) {
++a; // Thus this is never executed.
}
}
nop2(&i); // Only here to prevent optimiser from creating one big if().
if (i) m.unlock();
}
I see this in _Z3fooi:
movl a(%rip), %eax
movl 12(%rsp), %ecx
leaq 12(%rsp), %rdi
leal 10000(%rax), %edx
testl %ecx, %ecx
cmovne %edx, %eax
movl %eax, a(%rip)
This unconditionally stores a value in a. This is a speculative store, which
is invalid according to the C++ memory model.
This does not happen with -O2.
>From looking at the dumps, the bug appears to be in the loop invariant motion
pass.
