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

[Richard Henderson <rth@twiddle.net>]

On 08/30/2013 12:12 PM, Aurelien Jarno wrote:
> 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.

How's that?  We're passing a value extended to tcg_target_ulong.

For the 32-bit mips backend running o32 (or even a theoretical n32 backend),
that type is uint32_t.  That gets returned from C exactly how C returns that
type.  For o32 it's the full width of the register, full stop.  For n32, it
would be returned sign-extended in the 64-bit register.

Please explain exactly the failure mode you imagine, because I don't think
there is one.

> 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. 

Nope, all 4 of those will be fine.  Not least of which because the later 3
are still using the original helper functions, not the new helper_ret_* ones.

But certainly all 4 of those are, in the gcc sense, TRULY_NOOP_TRUNCATION
machines, meaning we can truncate to 32 bits merely by ignoring the garbage
in the high bits.  In practice it means that they have different 32-bit and
64-bit comparison instructions.


r~