C++: Undefined behaviour from realloc and the Clang Optimizer

I was wondering around in some system code and found a strange behaviour between two modes in a piece of code, where the code switches from double to triple buffer mode there's a problem, we've already got two buffers of everything we just want to allocate another but the underlying structure for sets of buffers wants to own them all... So the code went from:

SetOfBuffers

{

Buffer* one;

Buffer* two;

}

To:

SetOfBuffers

{

Buffer* one;

Buffer* two;

Buffer* three;

}

Creating the third buffer is fine:

SetOfBuffers::three = malloc(X);

But, the first two is a re-alloc, to reuse the existing buffer:

SetOfBuffers::One = realloc(OldSet::one, X);

SetOfBuffers::Two = realloc(OldSet::two, X);

The problem?  I'd start to modify the values in the new set of buffers, the third buffer being used and present.  Then the first buffer would be changed present... The second buffer changed present and the information is wrong (I over simplify massively here).

Anyway, I was remotely SSH'd into my server for this, so I went to Visual Studio, same code... Worked fine... So I go into my local VM and it's fine too, so I went back to the server and compiled manually and suddenly it's fine too.... WTF.

I literally spent an hour looking at this, the problem?  Well, it appears to be a bug in Clang, the reason the problem disappeared was my Makefile contains a $CC constant for the compiler to use and it was "clang" when I built by hand I used "g++".  Worse still, if I switched to a clang debug build the code worked fine, so this was something about my compilation process not a bug in the code per se.

So, perplexed I went in search of an answer.  And it appeared to be something about the clang optimizer, about which I found this talk from CppCon 2016.

Where there's this example:

#include <cstdlib>

#include <cstdio>

int main ()

{

int* p = (int*)malloc(4); // The original buffer above

int* q = (int*)realloc(p, 4); // The new pointer to the same old buffer

// Allocate a vlaue

*p = 1;

*p = 2;

if ( p == q )

{

printf("%d %d\n", *p, *q);

}

}

What do you expect this code to display?... Well, I expect it to print "2 2".  And it does on VC and G++ and even clang without the optimizer...

But you optimize the compile and its wrong:

Now, this is undefined behaviour and not caused by your code, it's the optimizer and very scary.  Not least as this was identified a while back (the talk along is from 2016) and g++ has solved the problem... Eeeek.