Partial inlining

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

We’ve learned how important inlining is to optimisation, but also that it might sometimes cause code bloat. Inlining doesn’t have to be all-or-nothing!

Let’s look at a simple function that has a fast path and slow path; and then see how the compiler handles it¹.

In this example we have some process function that has a really trivial fast case for numbers in the range 0-100. For other numbers it does something more expensive. Then compute calls process twice (making it less appealing to inline all of process).

Looking at the assembly output, we see what’s happened: The compiler has split process into two functions, a process (part.0) that does the expensive part only. It then rewrites process into the quick check for 100, returning double the value if less than 100. If not, it jumps to the (part.0) function:

process(unsigned int):
  cmp edi, 99                           ; less than or equal to 99?
  jbe .L7                               ; skip to fast path if so
  jmp process(unsigned int) (.part.0)   ; else jump to the expensive path
.L7:
  lea eax, [rdi+rdi]                    ; return `value * 2`
  ret

This first step - extracting the cold path into a separate function - is called function outlining. The original process becomes a thin wrapper handling the hot path, delegating to the outlined process (.part.0) when needed. This split sets up the real trick: partial inlining. When the compiler later inlines process into compute, it inlines just the wrapper whilst keeping calls to the outlined cold path. External callers can still call process and have it work correctly for all values.

Let’s see this optimisation in action in the compute function:

compute(unsigned int, unsigned int):
  cmp edi, 99                   ; is a <= 99?
  jbe .L13                      ; if so, go to the inlined fast path for a
  call process(unsigned int) (.part.0) ; else, call expensive case
  mov r8d, eax                  ; save the result of process(a)
  cmp esi, 99                   ; is b <= 99?
  jbe .L14                      ; if so go to the inlined fast path for b
.L11:
  mov edi, esi                  ; otherwise, call expensive case for b
  call process(unsigned int) (.part.0)
  add eax, r8d                  ; add the two slow cases together
  ret                           ; return

.L13:                           ; case where a is fast case
  lea r8d, [rdi+rdi]            ; process(a) is just a + a
  cmp esi, 99                   ; is b > 99?
  ja .L11                       ; jump to b slow case if so
                                ; (falls through to...)
.L14:                           ; b fast case
  lea eax, [rsi+rsi]            ; double b
  add eax, r8d                  ; return 2*a + 2 * b
  ret

Looking at compute, we can see the benefits of this approach clearly: The simple range check and arithmetic (cmp, lea) are inlined directly, avoiding the function call overhead for the fast path. When a value is 100 or greater, it calls the outlined process (.part.0) function for the more expensive computation.

This is the best of both worlds: we get the performance benefit of inlining the lightweight check and simple arithmetic, whilst avoiding code bloat from duplicating the expensive computation². The original process function remains intact and callable, so external callers still work correctly.

Partial inlining lets the compiler make nuanced trade-offs about what to inline and what to keep shared. The compiler can outline portions of a function based on its heuristics about code size and performance³, giving you benefits of inlining without necessarily paying the full code size cost. In this example, the simple check is duplicated whilst the complex computation stays shared.

As with many optimisations, the compiler’s heuristics⁴ usually make reasonable choices about when to apply partial inlining, but it’s worth checking your hot code paths to see if the compiler has made the decisions you expect. Taking a quick peek in Compiler Explorer is a good way to develop your intuition.

See the video that accompanies this post.

This post is day 18 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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