Frame pointer register name

[Ian Lance Taylor]

Amittai Aviram <amittai.aviram@yale.edu> writes:

> I am creating a GCC (4.5.2) that uses a modified libgomp.  My libgomp has a file that contains the following line of code:
>
>       asm volatile("mov %%rbp, %0" : "=m"  (rbp) );
>
> I attempted to compile this on a 64-bit machine running under 64-bit Ubuntu:
>
> $ uname -a
>
> Linux dedis 2.6.32-27-server #49-Ubuntu SMP Thu Dec 2 02:05:21 UTC 2010 x86_64 GNU/Linux
>
> But my build failed, with the assembler error that "rbp" was an unrecognized register name.  I changed this to ebp, and it compiled fine.  However, my code then failed, presumably because it was getting an invalid address.
>
> I figured out a workaround that seems fine, but I wanted to know why I was getting this assembler error and what I should have done about it.

You got the assembler error in 32-bit mode because in the x86 assembly
language rbp refers to the 64-bit version of the register and ebp refers
to the 32-bit version (and bp refers to the 16-bit version).  In 32-bit
mode there is no 64-bit version of the register and therefore rbp is
invalid.

Ian