llvm dev - Aug 2006 - [LLVMdev] problems defining the stack layout

I am having some problems defining the stack layout on ARM. It should look like

Incoming vars
-----------------
Saved Link Register
-----------------
Local vars
-----------------

The problem is that the the first local variable is being assigned to
the same location as the link register. I tried to use the
OffsetOfLocalArea option, but do to alignment restrictions this
increased the stack size.

The offset of the local vars is set in calculateFrameObjectOffsets, so
I added the following hack to eliminateFrameIndex:

if (Offset < 0) //local variable
    Offset -= 4;

Is there a better way?

Thanks,
Rafael

On Tue, 15 Aug 2006, [UTF-8] Rafael Esp?ndola wrote:
> I am having some problems defining the stack layout on ARM. It should look 
> like
>
> Incoming vars
> -----------------
> Saved Link Register
> -----------------
> Local vars
> -----------------
>
> The problem is that the the first local variable is being assigned to
> the same location as the link register. I tried to use the
> OffsetOfLocalArea option, but do to alignment restrictions this
> increased the stack size.
>
> The offset of the local vars is set in calculateFrameObjectOffsets, so
> I added the following hack to eliminateFrameIndex:
>
> if (Offset < 0) //local variable
>     Offset -= 4;
>
> Is there a better way?
I'm not sure I follow the question.  It sounds like you should use the 
standard model, where local vars are allocated using variable frame 
indexes and the incoming vars are fixed frame regs.  The link register 
gets spilled to a fixed location (e.g. see the PPC backend's handling of 
the LR register).

I'm not familiar with the ARM ISA, so I don't know the right answer. 
The
OffsetOfLocalArea is used for ISAs (like X86) where the call sequence 
modifies the stack pointer (in the case of X86, the call instruction 
pushes a 4/8-byte return value).  If ARM is like PPC, where the return 
address is implicitly copied into a physreg, then you should probably use 
OffsetOfLocalArea=0 and just ask for LR to be spilled to a specific 
location (e.g. see the PPC backend).

I feel like I'm not really understanding the question though, can you 
rephrase it?

-Chris

-- 
http://nondot.org/sabre/
http://llvm.org/

> I'm not familiar with the ARM ISA, so I don't know the right
answer.  The
> OffsetOfLocalArea is used for ISAs (like X86) where the call sequence
> modifies the stack pointer (in the case of X86, the call instruction
> pushes a 4/8-byte return value).  If ARM is like PPC, where the return
> address is implicitly copied into a physreg, then you should probably use
> OffsetOfLocalArea=0 and just ask for LR to be spilled to a specific
> location (e.g. see the PPC backend).
The problem was that I was generating hard coded loads and stores for
the link register. I was trying to shift the local variables, but this
isn't the correct solution. I have now implemented
getCalleeSaveSpillSlots, so that the link register is treated like any
other callee save register.

Thanks,
Rafael