llvm dev - Jul 2017 - setting program counter on power8

Hello,
I've been working on powerpc and I'm having some trouble since
there's no
PC register. What I would like to do is to be able to set PC to an
arbitrary address that I want. I know that the only way to do this is to
branch.

I've written the following code to be able to jump to the beginning of foo.
I need to be able to use both immediate values and a register for
introducing the address. I compiled on power8 with
clang -O3 power_pc_write.c -o power_pc_write

The code is working.

#include <stdio.h>
#include <stdint.h>

void* foo(){

 void* addr =  __builtin_return_address(0);

  printf("in foo addr: %p\n", addr);

  return addr;
}

int main(){

  void* addr = foo();
  void* foo_addr = foo;
  printf("foo addr: %p\n", foo_addr);

  // This is for setting PC_IMM
  asm volatile("bl %0" : : "i" (foo) );
  asm volatile("bcl 20,31,%0" : : "i" (foo) );

  // This is for setting PC_REG
  asm volatile("mflr 4; mtlr %0 ; bclrl 20,31"  : : "r"
(foo_addr): "r4" );

  return 0;
}

My concerns are as follows:
I'm not sure if there's a specific way of setting PC to an arbitrary
address other than branching. Am I doing this the right way?
I realized that bl instruction jumps to foo+8. But bcl does not. I couldn't
figure out why.
The ISA specification says that for gettin the the next instruction we can
use this:
  bcl 20,31,$+4
  So I tried bcl first. But this is a conditional branch instruction. It's
not really clear to me why I need to use a conditional branch here.
  Also I assumed that $ give me some sort of address. But I wasn't able to
use "$" in another way. Could you provide me hints on this?

I appreciate if you can help me clear these in my mind.

*P.S. I'm using llvm 3.7. The ISA specification I'm currently workign
with
is PowerISA Version 3.0 B.*

Thanks!
-- 
Buse
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20170727/0a537d2c/attachment.html>

Setting PC to an arbitrary value _is_ branching.

The $ in the "bcl 20,31,$+4" means "current program
counter". Since bl
and bcl use relative addressing, in inline assembly you could just write 
"bcl 20,31,4".

There are branch instructions that use absolute address as the immediate 
operand, but they don't cover the entire address space. If you want to 
be able to branch to anywhere, use the LR or CTR as the branch target.

-Krzysztof


On 7/27/2017 11:42 AM, Buse Yilmaz via llvm-dev wrote:
> Hello,
> I've been working on powerpc and I'm having some trouble since
there's
> no PC register. What I would like to do is to be able to set PC to an 
> arbitrary address that I want. I know that the only way to do this is to 
> branch.
> 
> I've written the following code to be able to jump to the beginning of 
> foo. I need to be able to use both immediate values and a register for 
> introducing the address. I compiled on power8 with
> clang -O3 power_pc_write.c -o power_pc_write
> 
> The code is working.
> 
> #include <stdio.h>
> #include <stdint.h>
> 
> void* foo(){
> 
>   void* addr =  __builtin_return_address(0);
> 
>    printf("in foo addr: %p\n", addr);
> 
>    return addr;
> }
> 
> int main(){
> 
>    void* addr = foo();
>    void* foo_addr = foo;
>    printf("foo addr: %p\n", foo_addr);
> 
>    // This is for setting PC_IMM
>    asm volatile("bl %0" : : "i" (foo) );
>    asm volatile("bcl 20,31,%0" : : "i" (foo) );
> 
>    // This is for setting PC_REG
>    asm volatile("mflr 4; mtlr %0 ; bclrl 20,31"  : :
"r" (foo_addr): "r4" );
> 
>    return 0;
> }
> 
> My concerns are as follows:
> I'm not sure if there's a specific way of setting PC to an
arbitrary
> address other than branching. Am I doing this the right way?
> I realized that bl instruction jumps to foo+8. But bcl does not. I 
> couldn't figure out why.
> The ISA specification says that for gettin the the next instruction we 
> can use this:
>    bcl 20,31,$+4
>    So I tried bcl first. But this is a conditional branch instruction. 
> It's not really clear to me why I need to use a conditional branch
here.
>    Also I assumed that $ give me some sort of address. But I wasn't
able
> to use "$" in another way. Could you provide me hints on this?
> 
> I appreciate if you can help me clear these in my mind.
> 
> *P.S. I'm using llvm 3.7. The ISA specification I'm currently
workign
> with is PowerISA Version 3.0 B.*
> 
> Thanks!
> -- 
> Buse
> 
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 
-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, 
hosted by The Linux Foundation