llvm dev - Mar 2013 - [LLVMdev] Calling with register indirect reference instead of memory indirect reference.

Hi,
I am working on a small optimization feature to replace the calls with indirect
reference using  a memory with an indirect reference using register. The purpose
of this feature is to improve the performance of calls to functions referred to
by function pointers. The motivation behind this work is that gcc does this
optimization.
Here is a small test case, that will generate an indirect call with memory
reference:
int main()
{
    extern void (*foo)();
    foo();
    return 0;
}
And the corresponding assembly output is:
...
main:                                   # @main
               .cfi_startproc
# BB#0:                                 # %entry
               pushq    %rax
.Ltmp1:
               .cfi_def_cfa_offset 16
               movl      $2, %edi
               callq       *foo(%rip)
               xorl        %eax, %eax
               popq      %rdx
               ret
...

The patch aims to make the memory reference to use a register reference by
moving the address in *foo(%rip) into a register and use it for the call. The
updated assembly output is:
...
main:                                   # @main
               .cfi_startproc
# BB#0:                                 # %entry
               pushq    %rax
.Ltmp1:
               .cfi_def_cfa_offset 16
               movq     foo(%rip), %rax
               movl      $0, 4(%rsp)
               callq       *%rax
               xorl        %eax, %eax
               popq      %rdx
               ret
...


However, I am unable to proceed with this fix because it modifies the
SelectionDAG to insert a CopyFromReg and a CopyToReg nodes to do this operation.
I tried to use a slightly modified test case, using arguments to function
instead of void, and it fails. Here is the modified test case:
int main()
{
    extern void (*foo)(int);
    foo(2);
    return 0;
}
And the problem is that I am seeing an assertion failure with respect to the DAG
structure.
                             .file         "<stdin>"
llc: ~/llvm/lib/CodeGen/ScheduleDAG.cpp:510: void
llvm::ScheduleDAGTopologicalSort::InitDAGTopologicalSorting(): Assertion
`Node2Index[SU->NodeNum] > Node2Index[I->getSUnit()->NodeNum]
&& "Wrong topological sorting"' failed.
0  llc             0x000000000117ef3a llvm::sys::PrintStackTrace(_IO_FILE*) + 38

I am wondering if the modification made to the DAG is causing a problem, and can
it be done at all? If I cannot do this, is there any other place I can look at,
to make this work.
--
Sriram Murali
SSG/DPD/ECDL/DMP
+1 (519) 772 - 2579

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Hello
> I am wondering if the modification made to the DAG is causing a problem,
and
> can it be done at all? If I cannot do this, is there any other place I can
> look at, to make this work.
It's hard to tell w/o seeing the exact code / DAG. Note, however, that
this assertion is seen on simple LLVM IR:
http://llvm.org/bugs/show_bug.cgi?id=15053

So, it might be not your bug after all.

--
With best regards, Anton Korobeynikov
Faculty of Mathematics and Mechanics, Saint Petersburg State University

Hi Anton, 
I am attaching the Selection DAG before legalize stage (pre and post application
of the patch). The test case with one int argument (fn_ptr_int_..) fails and the
test case with no argument( fn_ptr_void...) doesn’t error out.  I also attached
the LLVM IR for the test cases if that might be of some help.

Thanks
Ram

-----Original Message-----
From: Anton Korobeynikov [mailto:anton at korobeynikov.info] 
Sent: Friday, March 01, 2013 2:56 AM
To: Murali, Sriram
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Calling with register indirect reference instead of
memory indirect reference.

Hello
> I am wondering if the modification made to the DAG is causing a 
> problem, and can it be done at all? If I cannot do this, is there any 
> other place I can look at, to make this work.
It's hard to tell w/o seeing the exact code / DAG. Note, however, that this
assertion is seen on simple LLVM IR:
http://llvm.org/bugs/show_bug.cgi?id=15053

So, it might be not your bug after all.

--
With best regards, Anton Korobeynikov
Faculty of Mathematics and Mechanics, Saint Petersburg State University
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