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Re: [Qemu-devel] [Qemu-ppc] [PATCH v4 06/15] target/ppc: remove xer spli

From: Nikunj A Dadhania
Subject: Re: [Qemu-devel] [Qemu-ppc] [PATCH v4 06/15] target/ppc: remove xer split-out flags(so, ov, ca)
Date: Fri, 24 Feb 2017 12:42:01 +0530
User-agent: Notmuch/0.21 (https://notmuchmail.org) Emacs/ (x86_64-redhat-linux-gnu)

Nikunj A Dadhania <address@hidden> writes:

> Richard Henderson <address@hidden> writes:
>> On 02/24/2017 01:58 PM, David Gibson wrote:
>>> On Fri, Feb 24, 2017 at 06:18:22AM +0530, Nikunj A Dadhania wrote:
>>>> Richard Henderson <address@hidden> writes:
>>>>> On 02/24/2017 06:56 AM, Nikunj A Dadhania wrote:
>>>>>> Now get rid all the split out variables so, ca, ov. After this patch,
>>>>>> all the bits are stored in CPUPPCState::xer at appropriate places.
>>>>>> Signed-off-by: Nikunj A Dadhania <address@hidden>
>>>>>> ---
>>>>>>  target/ppc/cpu.c        |   8 +---
>>>>>>  target/ppc/cpu.h        |  26 ++++++------
>>>>>>  target/ppc/int_helper.c |  12 +++---
>>>>>>  target/ppc/translate.c  | 106 
>>>>>> +++++++++++++++++++++++++-----------------------
>>>>>>  4 files changed, 78 insertions(+), 74 deletions(-)
>>>>> I do not think this is a good direction to take this.
>>>> Hmm, any particular reason?
>>> Right, I suggested this, but based only a suspicion that the split
>>> variables weren't worth the complexity.  I'm happy to be corrected by
>>> someone with better knowledge of TCG, but it'd be nice to know why.
>> Normally we're interested in minimizing the size of the generated code, 
>> delaying computation until we can show it being used.
>> Now, ppc is a bit different from other targets (which might compute overflow 
>> for any addition insn) in that it only computes overflow when someone asks 
>> for 
>> it.  Moreover, it's fairly rare for the addo/subo/nego instructions to
>> be used.
>> Therefore, I'm not 100% sure what the "best" solution is.
> Agreed, with that logic, wont it be more efficient to move the OV/CA
> updationg to respective callers, and when xer_read/write happens, its
> just one tcg_ops.

BTW, i haven't seen remarkable difference in the boot time after this


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