Re: [Qemu-devel] [PATCH v2 6/7] tcg: Introduce zero and sign-extended versions of load helpers

[Aurelien Jarno <aurelien@aurel32.net>]

On Fri, Aug 30, 2013 at 10:20:23AM -0700, Richard Henderson wrote:
> On 08/30/2013 09:55 AM, Aurelien Jarno wrote:
> > While it works for x86 and some other architectures, it makes the
> > assumption that only part of the register can be used later by the TCG
> > code. It won't be the case if we later (and I hope we will) implement a
> > MIPS64 TCG target. In that case, a 32-bit value has to be returned
> > signed extended, which won't be the case for example for a 32-bit guest
> > loading a 16-bit unsigned value.
> 
> This doesn't break the mips64 abi, since we'll be returning a 64-bit value, not
> a 32-bit value that needs sign-extension.
> 
> Given a mips64 host with 32-bit guest, the sign-extension of the 32-bit load
> can either happen by using helper_ret_ldsl_mmu in the table of helper
> functions, or by using an sll insn instead of a move to put the value into
> place at the end of the slow path.

That's indeed a possibility. That said while the MIPS64 ABI is then
still followed, it would have break a MIPS backend as the ABI between
the helper and the TCG code is broken.

I am therefore concerned that we might break some of our 64-bit
backends. x86-64 and ia64 should be fine, I don't know about aarch64,
ppc64, sparc64 or s390x. 

-- 
Aurelien Jarno	                        GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net