Re: STUB_MOVE in elfxx-mips.c

[cgd at broadcom dot com]

At Wed, 1 Oct 2003 13:57:12 -0700, Richard Henderson wrote:
> On Wed, Oct 01, 2003 at 11:29:36AM -0700, Ian Lance Taylor wrote:
> > Sure.  As I said, it seemed to me at the time that `add' would never
> > be slower than `or', so it seemed reasonable to always use `add' as
> > the implementation of the `move' pseudo-op.  Which is what the
> > assembler does today.  And it still seems reasonable to me.
> 
> Problem is, that on everyone except one chip, add sign-extends,
> and or doesn't.  Which leads to very subtle bugs.

More generally, of course, on MIPS 32-bit add inputs that aren't
sign-extended 32-bit values has operation that is "unpredictable."

In theory, those inputs shouldn't be generated in 32-bit programs.
But in some (arguably broken, sure) cases it's happened in the past,
and it can be Very Puzzling if 'move' truncates values (as it's
allowed to do in that case).


chris