On 01/30/2013 03:46 PM, Richard Biener wrote: > 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.
Well, it's hardly an optimization if it's incorrect, and it seems to be incorrect. As the old saying goes, I can make your code infinitely fast if you don't care about the results. Andrew.