llvm dev - Jun 2007 - [LLVMdev] Tail calls

The ref manual states: The optional "tail" marker indicates whether
the callee function accesses any allocas or varargs in the caller. If
the "tail" marker is present, the function call is eligible for tail
call optimization. Note that calls may be marked "tail" even if they
do not occur before a ret instruction.

Does "eligible" in the above mean that some analysis is performed, and
if the call is found to meet certain criteria it is turned into a tail
call? Or is it guaranteed to perform a tail call if 'tail' is
specified?

The last statement of that paragraph implies that some analysis is
involved, but [1] states that there is no analysis performed.

Sandro

[1] http://nondot.org/sabre/LLVMNotes/GuaranteedEfficientTailCalls.txt

On Wed, 20 Jun 2007, Sandro Magi wrote:
> Does "eligible" in the above mean that some analysis is
performed, and
> if the call is found to meet certain criteria it is turned into a tail
> call? Or is it guaranteed to perform a tail call if 'tail' is
> specified?
>
> The last statement of that paragraph implies that some analysis is
> involved, but [1] states that there is no analysis performed.
Eligible means that, if followed by a return and if the caller and callee 
are both "fastcc"you should be guaranteed to get a tail call.  In
practice
however, tail calls aren't implemented by any of the targets, so you 
won't.

I consider this a serious bug, and welcome anyone interested in helping to 
improve this.

-Chris

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