llvm dev - Nov 2013 - [LLVMdev] "-relocation-model=static" does not work in llc compiler ?

Hi LLVM,

When I compile code using llc with options "
-relocation-model=static", the generated code still has
relative addressing using *rip* frame pointer. May I know how to get rid of
frame pointer and totally
using absolute addressing ? 

E.g. 
    callq   *addr(%rip).  
Here *addr* is a global variable, and I want the global address shows up here, 
instead of relative addressing. 
In this sense, can we say " -relocation-model=static" doesn't work
?

One more thing, 
When I compile with O0, O1, the generated code is absolute addressing. like,
callq   *addr.
but when compiling with O2, O3, the generated code is relative addressing, callq
*addr(%rip).

How could I make it always absolute addressing ? 

Thanks, 

- Pengcheng

Hi Pengcheng,
> When I compile code using llc with options "
-relocation-model=static", the generated code still has
> relative addressing using *rip* frame pointer. May I know how to get rid of
frame pointer and totally
> using absolute addressing ?
>
> E.g.
>     callq   *addr(%rip).
> Here *addr* is a global variable, and I want the global address shows up
here,
> instead of relative addressing.
> In this sense, can we say " -relocation-model=static" doesn't
work ?
I think you may be misinterpreting "static". It doesn't mean
"only use
absolute relocations" but rather "you don't *have* to use
position-independent relocations" (as PIC requires to make shared
libraries work).

In this case, the default ("small") code model in x86-64 guarantees
that all code and data will be in the low 2GB of the address space so
it's more efficient for the compiler to use PC-relative addressing
anyway.

If you also give "-code-model=large" then LLVM won't assume that
"addr" is nearby so it'll use a movabsq with a static relocation.

Cheers.

Tim.

> If you also give "-code-model=large" then LLVM won't assume
that
> "addr" is nearby so it'll use a movabsq with a static
relocation.
Actually, it seems our own Eli has written rather a good description
of these code models:
http://eli.thegreenplace.net/2012/01/03/understanding-the-x64-code-models/.

Well worth the read (the default models he talks about at the top are
essentially "static").

Cheers.

Tim.