[llvm-dev] Tail call optimization is getting affected due to local function related optimization with IPRA

[vivek pandya via llvm-dev]

On Sat, Jun 25, 2016 at 11:03 PM, vivek pandya <vivekvpandya at gmail.com>
wrote:

> Hello LLVM Community,
>
> To improve Interprocedural Register Allocation (IPRA) we are trying to
> force caller
> saved registers for local functions (which has likage type local). To
> achive it
> I have modified TargetFrameLowering::determineCalleeSaves() to return
> early for
> function which satisfies if (F->hasLocalLinkage() &&
> !F->hasAddressTaken()) and
> also reflecting the fact that for local function there are no caller saved
> registers
> I am also changing RegUsageInfoCollector.cpp to not to mark regiseters as
> callee
> saved in RegMask due to CC with follwoing change in code:
>
> if (!F->hasLocalLinkage() || F->hasAddressTaken()) {
>       const uint32_t *CallPreservedMask =
>         TRI->getCallPreservedMask(MF, MF.getFunction()->getCallingConv());
>       // Set callee saved register as preserved.
>       for (unsigned i = 0; i < RegMaskSize; ++i)
>         RegMask[i] = RegMask[i] | CallPreservedMask[i];
>     }
>
> For more details please follow following link.
> https://groups.google.com/d/msg/llvm-dev/XRzGhJ9wtZg/bYFMzppXEwAJ
>
> Now consider following bug due to forcing caller saved registers for local
> function
> when IPRA enable:
>
> void makewt(int nw, int *ip, double *w) {
>   ...
>   bitrv2(nw, ip, w);
> }
>
> here bitrv2 is local fuction and for that when IPRA enable callee saved
> registers
> are set to none. So for that function following is set of collbered
> register as
> per regmaks collected by RegUsageInfoCollector pass.
>
> Function Name : bitrv2
> Clobbered Registers:
> AH AL AX BH BL BP BPL BX CH CL CX DI DIL EAX EBP EBX ECX EDI EFLAGS ESI
> ESP RAX
> RBP RBX RCX RDI RSI RSP SI SIL SP SPL R8 R9 R10 R11 R12 R13 R14 R15 R8B
> R9B R10B
> R11B R12B R13B R14B R15B R8D R9D R10D R11D R12D R13D R14D R15D R8W R9W
> R10W R11W
> R12W R13W R14W R15W
>
> How ever caller of bitrv2, makewt has callee saved registers as per CC,
> but this
> code results in segmentation fault when compliled with O1 because makewt
> has value
> of *ip in R14 register and that is stored and restore by makewt at
> begining of call
> but due to tail call optimization following code is generated and here
> bitrv2 does
> not preserve R14 so whwn execution returns to main (which is caller of
> makewt)
> value of *ip is gone from R14 (which sould not) and when main calls makewt
> again
> then value of *ip (R14) is wrong and result into segmentation fault.
>
> Assembly code of makewt:
>   _makewt:
>   ...
>   popq  %rbx
>   popq  %r12
>   popq  %r13
>   popq  %r14
>   popq  %r15
>   popq  %rbp
>   jmp _bitrv2                 ## TAILCALL
>

A very naive solution to this problem come to me is to convert above code
to following:

 _makewt:
  ...
  jmp _bitrv2                 ## TAILCALL
  popq  %rbx
  popq  %r12
  popq  %r13
  popq  %r14
  popq  %r15
  popq  %rbp

So that when _bitrv2 returns caller will over write callee saved register (
as per CC of that function ) to correct values.
I wanted to try it out but I am not able to find correct code where I can
do that.
-Vivek

>
> There is one more case of faluire due to local function related
> optimization.
> I am analysing that (sorry for taking more time but I am not much good at
> assembly).
>
> I need some hints for how to solve this. If you feel some problem with my
> analyses
> please let me know if you want me to send generated .s file and source .c
> file.
>
> Sincerely,
> Vivek
>
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