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Add 'musttail' marker to call instructions

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This is similar to the 'tail' marker, except that it guarantees that
tail call optimization will occur.  It also comes with convervative IR
verification rules that ensure that tail call optimization is possible.

Reviewers: nicholas

Differential Revision: http://llvm-reviews.chandlerc.com/D3240

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207143 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Reid Kleckner
2014-04-24 20:14:34 +00:00
parent 870200a833
commit 710c1a449d
21 changed files with 334 additions and 30 deletions
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41
docs/LangRef.rst
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@@ -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