Optimizing out unnecessary allocations

[David Nadlinger]

I recently revived an LDC optimization pass (originally written 
by Frits van Bommel) that can remove dead allocations and promote 
small GC allocations to stack memory by recognizing the related 
druntime calls.

However, I didn't commit it to the main branch yet because it 
causes a test case in the DMD test suite to fail: 
https://github.com/D-Programming-Language/dmd/blob/eaa03fefeb1a698f586d5f5a09068f3433bf4b29/test/runnable/testgc2.d

The test is intended to make sure that the garbage collector 
properly handles out-of-memory situations, but in doing so 
assumes that the compiler can't touch the array allocations. 
However, when building with optimizations on, LDC recognizes the 
fact that the allocations are unused and subsequently removes 
them, causing the "assert(0)"s to be hit:

———
try {
   long[] l = new long[ptrdiff_t.max];
   assert(0);
}
catch (OutOfMemoryError o){}
———

Thus, my question: Is it legal for a D compiler to eliminate dead 
GC allocations (given that there are no side effects due to 
constructors, …)?

I'd strongly argue in favor of such optimizations to be legal, 
because not allowing them would eliminate quite a few 
optimization possibilities. There is also precedent for such 
behavior; many C compilers treat malloc() in a similar fashion.

David