llvm dev - Jan 2013 - [LLVMdev] Stack-based vs. register-based variables and other optimizations

Implementing my little language I'm using clang and the CPP back-end as a
guide for a lot of things. One thing I see is that clang always uses Alloca and
stores parameters on the stack.

Are there LLVM passes that figure this out and put things in registers, skipping
the stack allocations and loads/stores?

Similarly, if I use Alloca to make room for a pointer on the stack, store a
value in it, later load that value and pass it as an argument in a call, can
LLVM choose to optimize that stack allocation away and use a register?

I'm mostly talking about x86 here.

On the other hand, if I just stuck the result of one function call in as an
argument to another function call, or tried to use a parameter value directly
(assuming appropriate casts), will LLVM create intermediate stack-based space if
required?

I'm very unfamiliar with calling conventions on x86 (as compared to PowerPC
or ARM), so I'm hoping I don't have to worry about that stuff much.



-- 
Rick

On Sat, Jan 19, 2013 at 9:03 PM, Rick Mann <rmann at latencyzero.com>
wrote:
> Implementing my little language I'm using clang and the CPP back-end as
a guide for a lot of things. One thing I see is that clang always uses Alloca
and stores parameters on the stack.
>
> Are there LLVM passes that figure this out and put things in registers,
skipping the stack allocations and loads/stores?
>
> Similarly, if I use Alloca to make room for a pointer on the stack, store a
value in it, later load that value and pass it as an argument in a call, can
LLVM choose to optimize that stack allocation away and use a register?
Yep, you might want to look at the bitcode before & after running the
optimizers - it should be easy to observe LLVM doing this. (if you're
curious about where/how it happens, see the mem2reg pass)
>
> I'm mostly talking about x86 here.
>
> On the other hand, if I just stuck the result of one function call in as an
argument to another function call, or tried to use a parameter value directly
(assuming appropriate casts), will LLVM create intermediate stack-based space if
required?
>
> I'm very unfamiliar with calling conventions on x86 (as compared to
PowerPC or ARM), so I'm hoping I don't have to worry about that stuff
much.
>
>
>
> --
> Rick
>
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev