llvm dev - Aug 2009 - [LLVMdev] Stack Management in LLVM

On Thu, Jul 23, 2009 at 4:06 PM, Joshua
Moore-Oliva<llvm-dev at chatgris.com> wrote:
> If I could reduce my requirements to the ability to the following:
>    * Ability to know how much stack space a function requires
>    * Move the stack pointer arbitrarily (the ability to write my own
function
> prologue and epilogue, would need to be able to insert assembly/low level
> code here) before and after a method call
>
> could LLVM do that for me, using the system stack, (not continuations with
> malloc)?  Or am I barking up the wrong tree with this tool?
Messing with the prologue and epilogue should be feasible, although
you'll have to modify the code, and it's very platform-specific; see
X86RegisterInfo::emitPrologue in lib/Target/X86/X86RegisterInfo.cpp in
the source tree.

-Eli

First off, thanks for the help so far.
>From what I have been able to tell, emitPrologue kicks in after the
arguments
for the function have been copied.  For example, consider the function

int testfunc( int foo, int bar );

Emitting assembly code from the llvm-gcc frontend in a small test program 
gives the following for the call to testfunc

movl    $1338, (%esp)
movl    $1339, 4(%esp)
call    testfunc

Correct me if I am wrong, but I believe that the hook emitPrologue gives is 
before "call testfunc".  The issue I have is that if I am going to
move the
stack pointer, I need to do it before "movl    $1338, (%esp)" so that
the
called function can access its arguments where it expects them to be.

Is there anywhere in the compiler I could feasibly hook before arguments are 
pushed to the stack for a function call?

Thanks, Josh

On July 23, 2009 08:03:23 pm Eli Friedman wrote:
> Messing with the prologue and epilogue should be feasible, although
> you'll have to modify the code, and it's very platform-specific;
see
> X86RegisterInfo::emitPrologue in lib/Target/X86/X86RegisterInfo.cpp in
> the source tree.
>
> -Eli

On Wed, Aug 5, 2009 at 3:56 AM, Joshua Moore-Oliva<llvm-dev at
chatgris.com> wrote:
> First off, thanks for the help so far.
>
> From what I have been able to tell, emitPrologue kicks in after the
arguments
> for the function have been copied.  For example, consider the function
>
> int testfunc( int foo, int bar );
>
> Emitting assembly code from the llvm-gcc frontend in a small test program
> gives the following for the call to testfunc
>
> movl    $1338, (%esp)
> movl    $1339, 4(%esp)
> call    testfunc
>
> Correct me if I am wrong, but I believe that the hook emitPrologue gives is
> before "call testfunc".  The issue I have is that if I am going
to move the
> stack pointer, I need to do it before "movl    $1338, (%esp)" so
that the
> called function can access its arguments where it expects them to be.
>
> Is there anywhere in the compiler I could feasibly hook before arguments
are
> pushed to the stack for a function call?
EmitPrologue is not even related to that location; it's at the
beginning and end of a function.

Basically the only place you can catch the manipulations for a call is
X86TargetLowering::LowerCall.

-Eli