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Diffstat (limited to 'docs/LangRef.rst')
| -rw-r--r-- | docs/LangRef.rst | 41 |
1 files changed, 29 insertions, 12 deletions
diff --git a/docs/LangRef.rst b/docs/LangRef.rst index 752e357822..a6595f7fd0 100644 --- a/docs/LangRef.rst +++ b/docs/LangRef.rst @@ -6161,7 +6161,7 @@ Syntax: :: - <result> = [tail] call [cconv] [ret attrs] <ty> [<fnty>*] <fnptrval>(<function args>) [fn attrs] + <result> = [tail | musttail] call [cconv] [ret attrs] <ty> [<fnty>*] <fnptrval>(<function args>) [fn attrs] Overview: """"""""" @@ -6173,17 +6173,34 @@ Arguments: This instruction requires several arguments: -#. The optional "tail" marker indicates that the callee function does - not access any allocas or varargs in the caller. Note that calls may - be marked "tail" even if they do not occur before a - :ref:`ret <i_ret>` instruction. If the "tail" marker is present, the - function call is eligible for tail call optimization, but `might not - in fact be optimized into a jump <CodeGenerator.html#tailcallopt>`_. - The code generator may optimize calls marked "tail" with either 1) - automatic `sibling call - optimization <CodeGenerator.html#sibcallopt>`_ when the caller and - callee have matching signatures, or 2) forced tail call optimization - when the following extra requirements are met: +#. The optional ``tail`` and ``musttail`` markers indicate that the optimizers + should perform tail call optimization. The ``tail`` marker is a hint that + `can be ignored <CodeGenerator.html#sibcallopt>`_. The ``musttail`` marker + means that the call must be tail call optimized in order for the program to + be correct. The ``musttail`` marker provides these guarantees: + + #. The call will not cause unbounded stack growth if it is part of a + recursive cycle in the call graph. + #. Arguments with the :ref:`inalloca <attr_inalloca>` attribute are + forwarded in place. + + Both markers imply that the callee does not access allocas or varargs from + the caller. Calls marked ``musttail`` must obey the following additional + rules: + + - The call must immediately precede a :ref:`ret <i_ret>` instruction, + or a pointer bitcast followed by a ret instruction. + - The ret instruction must return the (possibly bitcasted) value + produced by the call or void. + - The caller and callee prototypes must match. Pointer types of + parameters or return types may differ in pointee type, but not + in address space. + - The calling conventions of the caller and callee must match. + - All ABI-impacting function attributes, such as sret, byval, inreg, + returned, and inalloca, must match. + + Tail call optimization for calls marked ``tail`` is guaranteed to occur if + the following conditions are met: - Caller and callee both have the calling convention ``fastcc``. - The call is in tail position (ret immediately follows call and ret |