[LLVMdev] Broke my tail (call)

[Arnold Schwaighofer]

Hello Duncan and Jon,

I am the criminal responsible for the tail call implementation in the backends.

On Mon, Feb 23, 2009 at 9:17 AM, Duncan Sands <baldrick at free.fr> wrote:
> Hi Jon,
>
>> >From what I have understood of the LLVM docs about when tail calls get
>> eliminated on x86 and x64 it should be a tail call, yes.
See below.

> this list is for the code generator, and it seems obviously incomplete: it makes
> no mention of local variables (alloca).  Probably it is implicitly assuming that
> the call was marked "tail call" by the LLVM optimizers.  So you also need to check
> under what conditions the LLVM optimizers do that.

The backend does not check the following condition but assumes it to be true.

>From <http://llvm.org/docs/LangRef.html#i_call>:

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.

If the llvm IR is generated by a front end (HLVM) the front end must
guarantee this condition.


>> > I didn't look closely but at a glance it seems to be passing a local stack
>> > variable as a call parameter.
To me, also only glancing over the code and with a bit rusty llvm ir
knowledge, it seems that %14 (the value passed to the tail call)is
allocated in the stack frame  (alloca) of the calling function. So the
above condition is violated.

%3   = alloca { { i8*, i8* }*, i8* }
%13 = getelementptr { { i8*, i8* }*, i8* }* %3, i32 0
%14 = load { { i8*, i8* }*, i8* }* %13
%17 = tail call fastcc { { i8*, i8* }*, i8* }
     @init({ { i8*, i8* }*,i8*} %14, i32 %16)

>> Is this a bug in LLVM?
> Could be, but first I'd like to be sure that you are not misusing tail calls.

Assuming that i am right and a pointer to alloca'ed memory is passed
this is not a bug but a misuse of the 'tail' instruction.

Maybe we should add a link or a note on
<http://llvm.org/docs/CodeGenerator.html#tailcallopt> to
<http://llvm.org/docs/LangRef.html#i_call> that explains that the call
can only be marked as 'tail' if no pointers to alloca'ed memory are
passed.

regards arnold