Aliasing

Written by me, proof-read by an LLM.
Details at end.

Yesterday we ended on a bit of a downer: aliasing stopped optimisations dead in their tracks. I know this is supposed to be the Advent of Compiler Optimisations, not the Advent of Compiler Giving Up! Knowing why your compiler can’t optimise is just as important as knowing all the clever tricks it can pull off.

Let’s take a simple example of a counter class¹. It accumulates integers into a member variable total. I’ve used C++ templates to show two versions of the code: one that accumulates in an int and one that accumulates in an long.

At first glance, the two loop bodies look almost identical, as you might expect. In one case we’ll accumulate in eax, and the other in rax, right? The truth is more subtle. Let’s first look at the int case:

  mov eax, DWORD PTR [rdi]      ; eax = total
.L3:
  add eax, DWORD PTR [rsi]      ; add element to total
  add rsi, 4                    ; move to next element
  mov DWORD PTR [rdi], eax      ; total = eax
  cmp rsi, rdx                  ; are we finished?
  jne .L3                       ; loop if not
  ret

Looks pretty reasonable, right? Now let’s look at the long version:

  mov rax, QWORD PTR [rdi]      ; rax = total
.L9:
  movsx rdx, DWORD PTR [rsi]    ; read & sign extend next element
  add rsi, 4                    ; move to next element
  add rax, rdx                  ; rax += element
  cmp rcx, rsi                  ; are we finished?
  jne .L9                       ; loop if not
  mov QWORD PTR [rdi], rax      ; total = rax
  ret                           ; return

The first change from the 32-bit case you’ll notice is the movsx to turn the 32-bit signed integer into a 64-bit signed integer. That’s all but free on modern CPUs, and so while it looks like the loop is doing more work than the 32-bit version, it’s not as bad as it seems.

The important difference here is the update of total: In the first version, each loop iteration updates the member variable total. In the second version everything remains in registers until the end of the loop, and then total is only updated at the end. CPU caches are super fast, but it’s still best to avoid redundant stores in hot loops!

So, why this difference? Of course it’s aliasing: In the int version the compiler can’t be sure that the span passed in to count doesn’t cover the Counter’s member variable total. They are the same type, and so that would be perfectly OK by the type-based aliasing rules of C++.

In the long version, the types differ (int vs long), and under C++’s strict aliasing rules, it would be undefined behaviour for them to overlap in memory. Since the compiler can assume the program doesn’t invoke undefined behaviour, it knows they don’t alias and can safely optimise. That lets it cache the total in a register and only update the member variable at the end. As we’ll see later in the series, being aliasing-free helps other optimisations too, like vectorisation.

To fix this issue, we have a couple of choices. The easy way would be to rewrite to accumulate in a local variable, and then update the total at the end: Using total += std::accumulate(span.begin(), span.end(), 0) would fix this and be more intention-revealing.

The other, non-standard way to work around this issue is to use __restrict. This GNU extension (borrowed from C) lets us decorate pointers and essentially promises that this pointer uniquely refers to the object it points at. In our case, the thing we need to prove is unique is the Counter’s “this pointer” itself. Adding __restrict after the parameter list (where you would add const for a const member function) works. But again - this is very non-standard, so use at your peril².

Aliasing is one of C++’s sneakier gotchas, especially when you’re working with base types like int and float - you can’t avoid using them, and they’re prime candidates for aliasing with each other³. It’s perfectly legal to have overlapping same-type pointers, so the compiler assumes the worst and peppers your hot loops with memory accesses. The fix may be as simple as only using local variables within your loop - but first you have to spot it. Fire up Compiler Explorer, look for those unexpected writes to memory when you’d expect everything to stay in registers, and you’ll know when aliasing is holding you back!

See the video that accompanies this post.

This post is day 15 of Advent of Compiler Optimisations 2025, a 25-day series exploring how compilers transform our code.

This post was written by a human (Matt Godbolt) and reviewed and proof-read by LLMs and humans.

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