[LLVMdev] Injecting code before function prolog

Dan Gohman gohman at apple.com
Fri Apr 9 15:40:53 PDT 2010

On Apr 7, 2010, at 10:43 AM, Arlen Cox wrote:

> I'm trying to implement something similar to this:
> http://gcc.gnu.org/wiki/SplitStacks in LLVM.  The reason I want this
> is so that I can have dynamically growing and shrinking stacks in my
> programming language.  In order to do this, I need to be able to check
> for overflow of a stack frame.  The methods of doing this are outlined
> in the link above, but my intention is to pass the current stack limit
> as the first argument to the function.
> What I'm hoping to do is to be able to inject the following code (in
> x86 asm, c calling convention) on entry to each function:
> _foo:
>  lea -frame_size(%esp), %eax
>  cmpl %eax, 4(%esp)
>  jb function_entry
>  // handle overflow
> function_entry:
>  function prolog
>  ...
> The problem I'm encountering is how to force this before the prolog.
> I'm attempting to add a machine function pass after the emit
> prolog/epilog pass that injects this code, but directly injecting x86
> code seems to be very messy as I have to figure out how LLVM encodes
> the addressing modes and instructions specific to x86.  Additionally,
> directly inject x86 code produces an LLVM that is not target
> independent anymore.
> Is there a better way to do this?  Can I maintain target independence?
> All I really need is to be able to access the stack pointer.

Would it make sense to do this within the TargetRegisterInfo::emitPrologue 


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