llvm dev - Mar 2012 - [LLVMdev] A question about DBG_VALUE and Frame Index

Hi,

I have a case that is causing me grief in the form of an assert. The prolog
Epilog inserter tries to remove Frame Index references. I have a DBG_VALUE
instruction that looks like this (alongwith the Frame Index). This is for
the Hexagon backend.
**************************
fi#2: size=4, align=4, at location [SP-84]
DBG_VALUE <fi#2>, 0, !"fooBar"; line no:299
**************************

Clearly, the FI in question is at an offset of -84 from the SP at entry to
the function i.e. FP - 84. So I remove the FI by changing the instruction
to.
**************************
DBG_VALUE %R30, -84, !"fooBar"; line no:299
**************************
(R30 is the frame pointer register in Hexagon.)


So, logically we have moved from frame indices to actually base + offset
representation. However the assembly printer, while trying to emit debug
info, sticks to the frame index representation and looks for a base+offset
reference for -84 !!
This is at DwarfCompileUnit.cpp:1334
**************************
     int Offset 
            TFI->getFrameIndexReference(*Asm->MF,

                                        DVInsn->getOperand(1).getImm(),

                                        FrameReg);  
**************************

In my view we have lost information that (R30-84) is <fi#2>. The above
statement is asking the Frame Lowering Information to give it a base+offset
pair for the frame index -84.

I do not think this is correct or am I missing something here ? For the sake
of completeness, I must mention that Hexagon uses the base class version of
getFrameIndexReference and does not provide its own. 

Pranav

Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.

On Mar 7, 2012, at 5:19 PM, Pranav Bhandarkar <pranavb at codeaurora.org>
wrote:
> Hi,
> 
> I have a case that is causing me grief in the form of an assert. The prolog
> Epilog inserter tries to remove Frame Index references. I have a DBG_VALUE
> instruction that looks like this (alongwith the Frame Index). This is for
> the Hexagon backend.
> **************************
> fi#2: size=4, align=4, at location [SP-84]
> DBG_VALUE <fi#2>, 0, !"fooBar"; line no:299
> **************************
> 
> Clearly, the FI in question is at an offset of -84 from the SP at entry to
> the function i.e. FP - 84. So I remove the FI by changing the instruction
> to.
> **************************
> DBG_VALUE %R30, -84, !"fooBar"; line no:299
> **************************
> (R30 is the frame pointer register in Hexagon.)
The offset field on a DBG_VALUE instruction refers to the user variable, not the
first register argument.

Your DBG_VALUE above is saying that fooBar[-84] can be found in %R30.

You want something like:

  DBG_VALUE %R30, -84, 0, !"fooBar"

That is a target-dependent DBG_VALUE, you will need to implement the target
hooks to create and parse it. Target-dependent DBG_VALUE instrs are recognized
by having more than 3 operands.

/jakob

> The offset field on a DBG_VALUE instruction refers to the user
> variable, not the first register argument.
> 
> Your DBG_VALUE above is saying that fooBar[-84] can be found in %R30.
> 
> You want something like:
> 
>   DBG_VALUE %R30, -84, 0, !"fooBar"
> 
> That is a target-dependent DBG_VALUE, you will need to implement the
> target hooks to create and parse it. Target-dependent DBG_VALUE instrs
> are recognized by having more than 3 operands.
> 
> /jakob
Right, thanks for the information Jakob.

I'll dig through the source code / documentation, looking for the necessary
target hooks and bother folks on this list, if I have questions regarding
this.

Thanks again,
Pranav