3.6.1.11 Relax Option

These optimizations include replacement of the long-form call and jump instructions with shorter and/or faster relative calls and jumps when the relative forms of the instructions can be determined to be in range of their destination. For example, code such as:

  call foo
  ...

foo:
  ...

can be replaced with:

  rcall foo
  ...

foo:
  ...

when foo is within range of the relative call. (For more information, see 4.3.6.3 Function Pointers).

When relative calls would be out of range, the compiler will attempt to replace them with rcall instructions to a jmp instruction that will 'trampoline' execution to the required address. For example, code such as:

  call foo
  ...
  call foo
  ...
  call foo

can be changed to:

  rcall tramp_foo
  ...
  rcall tramp_foo
  ...
  rcall tramp_foo
  ...

tramp_foo:
  jmp foo

provided the trampoline is within range of the relative calls. The compiler will re-use an existing jmp instruction to the required destination if possible, to further reduce code size. Such transformations will slow execution speed but can improve code size if there multiple calls that can be replaced.

Additionally, redundant return instructions are removed from functions with a tail call. For example, code such as:

  ...
  call last
  return
last:
  ...
  return

can be replaced with:

  ...
  jmp last
last:
  ...
  return