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

[Doug Goldstein <cardoe@xxxxxxxxxx>]

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?


-- 
Doug Goldstein

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