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

From: Nikunj A Dadhania
Subject: Re: [Qemu-devel] [PATCH v4 06/15] target/ppc: remove xer split-out flags(so, ov, ca)
Date: Fri, 24 Feb 2017 12:35:02 +0530
User-agent: Notmuch/0.21 (https://notmuchmail.org) Emacs/ (x86_64-redhat-linux-gnu)

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.

> However, I'd be surprised if the least amount of code places all of
> the bits into their canonical location within XER.
> Do note that when looking at this, the various methods by which the OV/SO 
> bits 
> are copied to CR flags ought to be taken into account.

I lost you in the last two para, can you explain in detail?


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