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

[Doug Goldstein <cardoe@xxxxxxxxxx>]

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.

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.

> 
>> 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).

> 
> 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.

> 
>> 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.

> 
>> 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

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