Re: [Qemu-devel] [PATCH v2 00/13] tcg/sparc v8plus code generation

[Richard Henderson <rth@twiddle.net>]

On 07/17/2015 02:42 PM, Aurelien Jarno wrote:
> On 2015-07-17 12:23, Aurelien Jarno wrote:
>> On 2015-07-16 22:29, Richard Henderson wrote:
>>> On 07/15/2015 09:54 PM, Aurelien Jarno wrote:
>>>> While I understand why we need the new trunc_shr_i32 opcode for MIPS64
>>>> (the 32-bit values must be kept sign-extended), I currently fail to
>>>> see why it is needed for SPARC.
>>>
>>> As far as I recall, it improves code for extracting high parts of 64-bit
>>> quantities.  Without this, we wind up with a 64-bit shift, requiring a
>>> 64-bit temp register, followed by the "real" truncate which can copy the
>>> data to a 32-bit destination register.
>>
>> Ok, I understand the use case now. So it's not for correctness, but
>> rather to generate more optimized code.
>
> OTOH, it means that we always have to go through a 32-bit register first
> when truncating a 64-bit value.
>
> I mean we gain in the following case:
>    shr_i64 t64, t64, i
>    trunc_i64_i32 t32, t64
>    ...
>
> But we lose in the following case:
>    trunc_i64_i32 t32, t64
>    neg t32, t32
>    ...

Why do you beleive we're using an extra temp here?  Certainly you can't "neg 
t32, t64" in any circumstance.

Anyway, this comes up most often with interfacing with the sparcv8plus calling 
convention, in which 64-bit quantities must be passed in 2 registers.  Before, 
we'd emit code like

	shrx	%g2, 32, %g1
	mov	%g1, %o0
	mov	%g2, %o1

After, we're able to put the shift output directly to %o0.


r~