[C11-atomic] test invalid hoist across and acquire load

[Aldy Hernandez <aldyh at redhat dot com>]

In the test below, we cannot cache either [x] or [y] neither before the 
load of flag1 nor the load of flag2.  This is because the corresponding 
store/release can flush a different value of x or y:

+  if (__atomic_load_n (&flag1, __ATOMIC_ACQUIRE))
+    i = x + y;
+
+  if (__atomic_load_n (&flag2, __ATOMIC_ACQUIRE))
+    a = 10;
+  j = x + y;

For example, on x86-64, we are hoisting "x" and "y" before the load of 
flag2:

        movl    flag1(%rip), %eax
        movl    x(%rip), %edx		<-- hoist of X
        testl   %eax, %eax
        movl    y(%rip), %eax		<-- hoist of Y
        je      .L2
        leal    (%edx,%eax), %ecx
        movl    %ecx, i(%rip)
.L2:
        movl    flag2(%rip), %ecx
        testl   %ecx, %ecx
        je      .L3
        movl    $10, a(%rip)
.L3:
        addl    %edx, %eax		<-- x/y may have changed by the
					    acquire of flag2.
        movl    %eax, j(%rip)
        ret

(For that matter, we are also hoisting "x" before the actual test of 
flag1 as well, but I believe this is allowed since flag1 has already 
been loaded.)

The pass at fault here is the combine stack adjustment RTL pass.  I have 
not looked into why this is happening, but I wanted to get this test 
into the branch lest we forget about it.

Is this OK for the branch?  Is my understanding correct?

Aldy

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