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Re: [PATCH] powerpc: Fix POWER7/PPC64 performance regression on LE
- From: Richard Henderson <rth at twiddle dot net>
- To: Adhemerval Zanella <azanella at linux dot vnet dot ibm dot com>, libc-alpha at sourceware dot org
- Date: Tue, 13 Jan 2015 21:15:31 -0800
- Subject: Re: [PATCH] powerpc: Fix POWER7/PPC64 performance regression on LE
- Authentication-results: sourceware.org; auth=none
- References: <54B3BD8A dot 1010608 at linux dot vnet dot ibm dot com> <54B57559 dot 2080108 at linux dot vnet dot ibm dot com> <54B57C0E dot 9090703 at twiddle dot net> <54B5C3F9 dot 9080801 at linux dot vnet dot ibm dot com>
On 01/13/2015 05:18 PM, Adhemerval Zanella wrote:
> On 13-01-2015 18:11, Richard Henderson wrote:
>> On 01/13/2015 11:43 AM, Adhemerval Zanella wrote:
>>> + std rWORD8, rWORD8SAVE(r1)
>>> + cfi_offset(rWORD8, rWORD8SAVE)
>>> + std rWORD7, rWORD7SAVE(r1)
>>> + cfi_offset(rWORD7, rWORD7SAVE)
>>> + std rOFF8, rOFF8SAVE(r1)
>>> + cfi_offset(rWORD7, rOFF8SAVE)
>>> + std rOFF16, rOFF16SAVE(r1)
>>> + cfi_offset(rWORD7, rOFF16SAVE)
>>> + std rOFF24, rOFF24SAVE(r1)
>>> + cfi_offset(rWORD7, rOFF24SAVE)
>>> + std rOFF32, rOFF32SAVE(r1)
>>> + cfi_offset(rWORD7, rOFF32SAVE)
>> Given a sequence of stores like this, it's better to
>> put all of the cfi_offset annotations after the last store.
>>
>> By interspersing them like this, you're just adding unnecessary
>> DW_CFA_advance opcodes to the unwind info.
>>
>>
>> r~
>>
> I was not aware of this, but I think the performance implications for such
> organization is just to unwind and debuggers, right?
>
True, and that's why this is interesting as a size optimization,
because it'll "almost never" be used.
r~