[LLVMdev] interesting possible compiler bug

Nick Lewycky nicholas at mxc.ca
Tue Oct 2 02:12:14 PDT 2012

David Chisnall wrote:
> On 2 Oct 2012, at 03:40, Nick Lewycky wrote:
>> As far as I know, this optimization is legal. Fix the test with a volatile pointer:
> Why would that be required?

It isn't. My suggestion for fixing the test is to make use of the 
returned pointer in a fashion that the compiler is forbidden to elide 
it: make it an observable side-effect using volatile. Another way would 
be to take the pointer and print it to file or stdout. Perhaps you can 
think of others.

   malloc() is defined by the standard to return a pointer that is 
distinct from any other valid pointer, or NULL.  Any optimisation that 
makes any assumptions about its next value is invalid.

Nowhere do we make assumptions about malloc's next value.

This is a straight-forward application of the as-if rule. The malloc 
call behaves as-if it allocated memory. Because we prove that the code 
doesn't use that memory, we can get away with allocating no memory at 
all and not change the behaviour of the program.

But we did change the behaviour of the program, didn't we? Well, we 
haven't changed the behaviour of the program in any way that is 
observable through a well-defined mechanism. Crashes, running out of 
memory, or asking the kernel for your processes' memory usage isn't 
behaviour you get to rely on. The first two really aren't defined in the 
language, and the last one goes through I/O which is permitted to do its 
own thing. (For instance, we don't forbid constant-folding "1+1" because 
a program may fopen and disassemble itself to look for the "add 1, 1" 

>> int main() {
>>    volatile char *curr;
>>    do {
>>      curr = malloc(1);
>>      int i = *curr;
> This, in particular, looks very wrong.  If curr is void, then you are dereferencing an invalid pointer, and so you are doing something undefined.

Do you mean, if curr is NULL? It's a char*, not void*.

In fact, this version of the code is completely free to elide the 
conditional loop, because by dereferencing the pointer you are asserting 
that it is not NULL (or, at least, that if it is then after this point 
the program is in an undefined state and so any behaviour is legal) and 
so it is completely free to generate the code that it in fact does 
generate without this test.  So here we have another bug, because the 
testq in your output is redundant after the movb.

Yes, good point, I totally missed the NULL dereference. I haven't 
checked what happens if you write:

   curr = malloc(1);
   if (curr)
     int i = *curr;

but I expect that would work.


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