Re: [Xen-devel] [PATCH 2/2] tools: detect appropriate debug optimization level

[Doug Goldstein <cardoe@xxxxxxxxxx>]

On 3/16/16 2:14 PM, Doug Goldstein wrote:
> On 3/8/16 10:50 AM, Wei Liu wrote:
>> On Tue, Mar 08, 2016 at 10:34:42AM -0600, Doug Goldstein wrote:
>>> On 3/8/16 9:38 AM, Wei Liu wrote:
>>>> On Mon, Mar 07, 2016 at 08:23:40PM -0600, Doug Goldstein wrote:
>>>>> The build should not use -O0 as that results in miscompilations. There
>>>>
>>>> This needs some (concrete) references. Is that a known issue in gcc? If
>>>> so can you reference the bug number?
>>>
>>> So its not really a bug in GCC but just the complete lack of
>>> optimizations in play. inlines aren't inlined. dead code elimination
>>> isn't run so things are much bigger. structures aren't padded the same way.
>>>
>>
>> Urgh...
>>
>>> This came about from reading reports on the -devel and -user's ML that
>>> were solved by building Xen with debug=n. I was also striving to reduce
>>> the duplication of CFLAGS that are passed on the command line of builds.
>>>
>>
>> I agree this is a good idea.
>>
>>>>
>>>>> have been a few instances on the ML where users were told to switch
>>>>> from -O0 to -O1 or -O2 or to set debug=n and their issue went away. The
>>>>> preferred route should be to use -Og if its available, otherwise use
>>>>> -O1 which is the default. This change undoes the change from -O1 to -O0
>>>>
>>>> gcc manual says -O0 is the default.
>>>
>>> I wasn't clear about where the 'the default' came from. That's the
>>> default in the Xen tree (see: config/StdGNU.mk for example but every
>>> platform has -O1 set).
>>>
>>
>> OK. I thought you're talking about something in the manual.
>>
>>>>
>>>> Not that I disagree with this patch in general, but the commit message
>>>> seems a bit misleading.
>>>
>>> I can rewrite it. I'd also be willing to change the patch to prefer -Og
>>> if its available and use -O0 if its not.
>>>
>>
>> No need to do it now because ...
>>
>>>>
>>>>> in 1166ecf781b1016eaa61f8d5ba4fb1fde9d599b6.
>>>>>
>>>>
>>>> And I have no idea why -O1 confuses the debugger so I've CC'ed Euan for
>>>> more input.
>>>
>>> -O1 can optimize things out when you look at them with gdb but -Og is
>>> suppose to do the right thing.
>>>
>>
>> .. I don't know much about gcc so I would like to wait for Ian to give
>> some input.
>>
>> Wei.
>>
>>>>
>>>>> Signed-off-by: Doug Goldstein <cardoe@xxxxxxxxxx>
>>>>> ---
>>>>> CC: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
>>>>> CC: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
>>>>> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
>>>>> ---
>>>>>  tools/Rules.mk | 3 ++-
>>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/tools/Rules.mk b/tools/Rules.mk
>>>>> index 9ef0b47..ae6b01f 100644
>>>>> --- a/tools/Rules.mk
>>>>> +++ b/tools/Rules.mk
>>>>> @@ -137,7 +137,8 @@ SHLIB_libxenvchan  = $(SHDEPS_libxenvchan) 
>>>>> -Wl,-rpath-link=$(XEN_LIBVCHAN)
>>>>>  
>>>>>  ifeq ($(debug),y)
>>>>>  # Disable optimizations and enable debugging information for macros
>>>>> -CFLAGS += -O0 -g3
>>>>> +$(call cc-option-add,CFLAGS,CC,-Og)
>>>>> +CFLAGS += -g3
>>>>>  # But allow an override to -O0 in case Python enforces 
>>>>> -D_FORTIFY_SOURCE=<n>.
>>>>>  PY_CFLAGS += $(PY_NOOPT_CFLAGS)
>>>>>  endif
>>>>> -- 
>>>>> 2.4.10
>>>>>
>>>
>>>
>>> -- 
>>> Doug Goldstein
>>>
>>
>>
>>
> 
> ping?
> 
> 

ping the ping?

-- 
Doug Goldstein

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