[LLVMdev] Understanding tail calls

[Chris Lattner]

On Mar 16, 2010, at 6:46 PM, Scott Ricketts wrote:

> I have some code generated with llvm-g++ and llvm-link that includes a tail call that is confusing me for two reasons:
> 
> 1) I am not sure why it is a tail call (i.e. it does not look like it is in the tail position)

The "tail" marker has a very specific description in LLVM IR: it says that the caller does not access the callee's stack:
http://llvm.org/docs/LangRef.html#i_call


> 2) When I instrument the code using my opt pass, none of the instrumentation functions in the callee get called, leading me to believe that some funny business is going on.

I don't know about this.

-Chris

> 
> Below I have pasted the tail call in question and surrounding code. My instrumentation pass instruments every llvm instruction. When I run it and just print out debug info, I see all the instructions up to the tail call, the tail call instruction itself, and then the icmp following the tail call. But none of the debug info from within the callee gets printed.
> 
> I do not use "-tailcallopt" as an option for llc when I generate the code.
> 
> It seems like I am misunderstanding something about tail calls and tail call optimization. Any insight might help me debug this issue.
> 
> Thanks,
> Scott
> 
> define linkonce_odr i64 @_ZN16FCEmailInputFile4openERKSsR11FCIdManager(%struct.FCEmailInputFile* %this, %"struct.std::basic_string<char,std::char_traits<char>,std::allocator<char> >"* nocapture %fn, %struct.FCIdManager* nocapture %idm) align 2 {
> entry:
>   %0 = getelementptr inbounds %"struct.std::basic_string<char,std::char_traits<char>,std::allocator<char> >"* %fn, i64 0, i32 0, i32 0 ; <i8**> [#uses=1]
>   %1 = load i8** %0, align 8                      ; <i8*> [#uses=1]
>   %2 = getelementptr inbounds %struct.FCEmailInputFile* %this, i64 0, i32 1 ; <%"struct.std::ifstream"*> [#uses=1]
>   %3 = getelementptr inbounds %struct.FCEmailInputFile* %this, i64 0, i32 1, i32 1 ; <%"struct.std::basic_filebuf<char,std::char_traits<char> >"*> [#uses=1]
>   %4 = tail call %"struct.std::basic_filebuf<char,std::char_traits<char> >"* @_ZNSt13basic_filebufIcSt11char_traitsIcEE4openEPKcSt13_Ios_Openmode(%"struct.std::basic_filebuf<char,std::char_traits<char> >"* %3, i8* %1, i32 8) ; <%"struct.std::basic_filebuf<char,std::char_traits<char> >"*> [#uses=1]
>   %5 = icmp eq %"struct.std::basic_filebuf<char,std::char_traits<char> >"* %4, null ; <i1> [#uses=1]
>   %6 = getelementptr inbounds %struct.FCEmailInputFile* %this, i64 0, i32 1, i32 0, i32 0 ; <i32 (...)***> [#uses=1]
>   %7 = load i32 (...)*** %6, align 8              ; <i32 (...)**> [#uses=1]
>   %8 = getelementptr inbounds i32 (...)** %7, i64 -3 ; <i32 (...)**> [#uses=1]
>   %9 = bitcast i32 (...)** %8 to i64*             ; <i64*> [#uses=1]
>   %10 = load i64* %9, align 8                     ; <i64> [#uses=1]
>   %11 = ptrtoint %"struct.std::ifstream"* %2 to i64 ; <i64> [#uses=1]
>   %12 = add i64 %10, %11                          ; <i64> [#uses=1]
>   %13 = inttoptr i64 %12 to %"struct.std::basic_ios<char,std::char_traits<char> >"* ; <%"struct.std::basic_ios<char,std::char_traits<char> >"*> [#uses=6]
>   br i1 %5, label %bb.i, label %bb1.i
> 
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