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

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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Tobias,
I've looked into this a bit more.
You are right.
The confusion arose as I have two versions of my compiler:
The ahead-of-time compiler uses symbolic info and does the right thing.
The JIT compiler uses runtime addresses (in effect integers) and
when I examined the code in the debugger I found that LLVM produces 
indirect calls, like this:
mov    $0x8153c8c,%eax
call   *%eax

Sadly, however, I have no idea how to fix this :(
but I will try and investigate.

Do you have any ideas yet?

Mark.

Tobias wrote:
> 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.
> 
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>