llvm dev - Jul 2013 - [LLVMdev] A problem on returning value for functions

Hi,

I write a backend and come cross an abnormal problem. Here I give a example
to describe it:

///////////////////////////////////////////////////////////////////////////////////////////
// A simple C function
int  foo()
{
  return 1234;
}
/////////////////////////////////////////////////////////////////////////////////////////

When compiling foo() into my target ISA, I would expect codes like:

/////////////////////////////////////////////////////////////////////////////////////////
...
movi  r0, #1234  // prepare r0 to return value 1234.
...
ret     // return to caller.
////////////////////////////////////////////////////////////////////////////////////////

The headache is when I pass option -O0 to llc, the generated codes are
correct. However, if I omit -O0 and use default compiling options, the
instruction "movi  r0, #1234" does not show.

I have already checked DAGs using options -view-xxxxx-dags and they all
seem OK.

I do not have any clue right now. Please help me.

Thank you.

-- 
杨勇勇 (Yang Yong-Yong)
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Hi,
> The headache is when I pass option -O0 to llc, the generated codes are
> correct. However, if I omit -O0 and use default compiling options, the
> instruction "movi  r0, #1234" does not show.
It's probably being eliminated as dead code. You want to make sure
that during ISelLowering your RET instruction has %R0 as one of its
operands (check in the -view-isel-dags step).

That's the most likely cause anyway. If not, posting the DAG might
help, or the output of "llc -debug" on an equivalent .ll file.

Cheers.

Tim.

Thank you, this is very instructive. I soon realized I forgot to add
SDNPVariadic in my node definition of return operator.
And thus even though the  LowerReturn() is implemented properly, the
instructions for passing return value is eliminated.

Regards.

2013/7/9 Tim Northover <t.p.northover at gmail.com>
> Hi,
>
> > The headache is when I pass option -O0 to llc, the generated codes are
> > correct. However, if I omit -O0 and use default compiling options, the
> > instruction "movi  r0, #1234" does not show.
>
> It's probably being eliminated as dead code. You want to make sure
> that during ISelLowering your RET instruction has %R0 as one of its
> operands (check in the -view-isel-dags step).
>
> That's the most likely cause anyway. If not, posting the DAG might
> help, or the output of "llc -debug" on an equivalent .ll file.
>
> Cheers.
>
> Tim.
>


-- 
杨勇勇 (Yang Yong-Yong)
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