On Wed, Jan 30, 2013 at 3:22 PM, Andrew Haley <a...@redhat.com> wrote: > Hi, > > On 01/30/2013 02:18 PM, Michael Matz wrote: > >> On Wed, 30 Jan 2013, Andrew Haley wrote: >> >>> I'm looking at Section 3.2.3, Parameter Passing. >>> http://artfiles.org/kernel.org/pub/scm/devel/binutils/hjl/x86-64-psabi.git/ >>> >>> I still cannot tell whether parameters should or should not be sign- or >>> zero-extended when they are moved into registers at a call. I'm guessing >>> not. >> >> It's intentionally unspecified. > > Aha! It would have been nice if the psABI said so explicitly. Quite > a few people have spent time trying to find this information. > >>> Which is it? This is important for interworking. >> >> How? You aren't allowed to access the bits outside the specified argument >> type (which must match on caller and callee side), so you can't observe >> them, so it's not required to specify their content. > > OK, thanks. It's clear now. > > The problem is that LLVM assumes that values are extended at a call. GCC > does that, but libffi doesn't. So, calls via libffi to LLVM don't work > correctly.
It's an optimization to do so to avoid partial register stalls. Richard. > Andrew. >
