Compiler is ignoring my C code!

Sometimes you come across a weird bug where the output seems to be completely impossible.  And it’s extremely hard to debug or search on google for, if you don’t know about this:

If your code has Undefined Behaviour, the compiler is allowed to assume it won’t happen, and can ‘optimize out’ chunks of your code.

Here’s a code snippet in a real bug I found yesterday:


#define FOO_SIZE 10
int foo2[FOO_SIZE];
int n=0;
do {
    p->foo[n] = foo2[n];
} while( (foo2[n] != 0) && (n < FOO_SIZE) );
printk("n is %d\n", n);



This printed out:

   n is 165

and the system crashed.

But how can n become larger than FOO_SIZE=10 ?  Because the compiler ‘optimized’ away the check.  Here’s the asm:

} while( (foo2[n] != 0) && (n < FOO_SIZE) );
   21a14:       f813 2f01       ldrb.w  r2, [r3, #1]!
   21a18:       2a00            cmp     r2, #0
   21a1a:       d1f8            bne.n   21a0e <NV_Get+0x82>
   21a1c:       e7ca            b.n     219b4 <NV_Get+0x28>

What we are seeing here is that the “n < FOO_SIZE”  check has been completely removed by the compiler.  Why?

Because in the check, if n == FOO_SIZE, we would be checking if:  foo2[FOO_SIZE] != 0.  But this is out of bounds for foo.  The compiler knows that this is out of bounds, and so knows that this is undefined behaviour.  But the compiler is allowed to assume that undefined behaviour doesn’t happen, and so it assumes that n can NEVER be >= FOO_SIZE.  Thus it can remove the n < FOO_SIZE check.

This can be fixed by switching the order of the && like:

} while( (n < FOO_SIZE) && (foo2[n] != 0) );

Or, by checking n-1 instead  (which is slightly different behaviour, but good enough for me.  I was changing a lot of code with this bug.)

} while( (foo2[n-1] != 0) && (n < FOO_SIZE) );
   21a26:       7809            ldrb    r1, [r1, #0]
   21a28:       2900            cmp     r1, #0
   21a2a:       d0c3            beq.n   219b4 <NV_Get+0x28>
   21a2c:       429a            cmp     r2, r3
   21a2e:       d1f5            bne.n   21a1c <NV_Get+0x90>
   21a30:       e7c0            b.n     219b4 <NV_Get+0x28>

Now we see the second comparison in the asm.


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