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Re: 32-bit libraries in 64-bit build.


On Wed, May 20, 2009 at 10:03:08AM -0700, Bob Plantz wrote:
> On Wed, 2009-05-20 at 12:31 -0400, Michael Meissner wrote:
> 
> > structures as well.  64 and 32-bit programs use a completely different ABI, and
> > the stack is set up differently.
> 
> The main difference being that the first six arguments are passed in
> registers instead of on the stack. It's very "RISC-like."

Also the alignment of the stack, whether or not the implementation can assume
the use of the SSE2 registers, size of long, how 64-bit values are returned,
what the stack unwind tables look like, etc.  Lots of differences.

> >   Finally, some 32-bit instruction encodings
> > are not available when the computer is in 64-bit mode, and others behave
> > differently when the 64-bit registers are enabled.
> > 
> 
> Isn't this true only at the individual instruction level?

If the 32-bit library was compiled with encodings that conflict with the 64-bit
instructions, then instructions will do funky things if you somehow manage to
bolt a 32-bit library into a 64-bit program, because the library will be called
in 64-bit mode.

> For example, you can do
>     movb    %ah, %al    # 32-bit, no REX prefix byte
>     movb    %al, %r8b   # 64-bit, requires REX prefix byte
> 
> but you cannot do
>     movb    %ah, %r8b   # mixed mode, conflict
> 
> The first sequence of instructions will run on my machine under 64-bit
> Ubuntu.
>
> An aside: The gnu assembler, gas, gives an error message about the REX
> prefix being required for the second case. It took me a while to figure
> it out. The error message should really say something about this being
> an attempt to mix 32-bit and 64-bit modes.

Well as they say, patches are welcome.

-- 
Michael Meissner, IBM
4 Technology Place Drive, MS 2203A, Westford, MA, 01886, USA
meissner@linux.vnet.ibm.com


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