llvm dev - Feb 2009 - [LLVMdev] direct calls to inttoptr constants

I'm compiling code which contains indirect function calls
via their absolute addresses, which are known/fixed at compile-time:

pseudo c code:
int main() {
int (* f)(int) = (int (*)(int))12345678;
return (*f)(0);
}

the IR looks like:
define i32 @main() nounwind {
entry:
%0 = tail call i32 inttoptr (i64 12345678 to i32 (i32)*)(i32 0) nounwind
ret i32 %0
}

on X86 llc 2.4 compiles this to:
 .text
 .align 16
 .globl main
 .type main, at function
main:
 subl $4, %esp
 movl $0, (%esp)
 movl $12345678, %eax
 call *%eax
 addl $4, %esp
 ret
 .size main, .-main

 .section .note.GNU-stack,"", at progbits

take a look at:
movl $12345678, %eax
call *%eax

does anyone know a way to cause llc to call the address directly?
hints where to start patching the codegen are also welcome.

expected assembly:
call *12345678

best regards
tobias

Tobias,

I'm doing something similar (I use LLVM as part of my JIT compiler) and
if I remember correctly, LLVM does the correct thing.

I think you need to try changing the i64 value to an i32 value.
If that doesn't work you could also try replacing the tail call with a 
normal call.


Mark.


Tobias wrote:
> I'm compiling code which contains indirect function calls
> via their absolute addresses, which are known/fixed at compile-time:
> 
> pseudo c code:
> int main() {
> int (* f)(int) = (int (*)(int))12345678;
> return (*f)(0);
> }
> 
> the IR looks like:
> define i32 @main() nounwind {
> entry:
> %0 = tail call i32 inttoptr (i64 12345678 to i32 (i32)*)(i32 0) nounwind
> ret i32 %0
> }
> 
> on X86 llc 2.4 compiles this to:
>  .text
>  .align 16
>  .globl main
>  .type main, at function
> main:
>  subl $4, %esp
>  movl $0, (%esp)
>  movl $12345678, %eax
>  call *%eax
>  addl $4, %esp
>  ret
>  .size main, .-main
> 
>  .section .note.GNU-stack,"", at progbits
> 
> take a look at:
> movl $12345678, %eax
> call *%eax
> 
> does anyone know a way to cause llc to call the address directly?
> hints where to start patching the codegen are also welcome.
> 
> expected assembly:
> call *12345678
> 
> best regards
> tobias
> 
> 
> _______________________________________________
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> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>

Hello Mark,

I've followed your advice and changed the IR to:
%0 = call i32 inttoptr (i32 12345678 to i32 (i32)*)(i32 0) nounwind
the call is still indirect.

IMHO llc does not call it directly because the address is neither
a globalvalue (JIT) nor a external symbol.
That's why it uses a fallback mechanism to call it indirectly
assuming the address is not constant and is calculated at runtime.

tobias

Mark wrote:
> I'm doing something similar (I use LLVM as part of my JIT compiler) and
> if I remember correctly, LLVM does the correct thing.
> 
> I think you need to try changing the i64 value to an i32 value.
> If that doesn't work you could also try replacing the tail call with a 
> normal call.
> 
> 
> Mark.