Re: [PATCH v4 1/5] configure,meson: add option to enable LTO

From: Thomas Huth <thuth@redhat.com>
To: Daniele Buono <dbuono@linux.vnet.ibm.com>, qemu-devel@nongnu.org
Cc: Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH v4 1/5] configure,meson: add option to enable LTO
Date: Thu, 15 Jul 2021 17:46:57 +0200	[thread overview]
Message-ID: <1b20b214-81f4-16a5-1e99-d12f6868f3ba@redhat.com> (raw)
In-Reply-To: <4c4542ca-a5cd-5229-6902-f3ea35c6e59f@redhat.com>

On 11/07/2021 12.22, Thomas Huth wrote:
> On 05/12/2020 00.06, Daniele Buono wrote:
>> This patch allows to compile QEMU with link-time optimization (LTO).
>> Compilation with LTO is handled directly by meson. This patch only
>> adds the option in configure and forwards the request to meson
>>
>> Tested with all major versions of clang from 6 to 12
>>
>> Signed-off-by: Daniele Buono <dbuono@linux.vnet.ibm.com>
>> ---
>>   configure   | 7 +++++++
>>   meson.build | 1 +
>>   2 files changed, 8 insertions(+)
>>
>> diff --git a/configure b/configure
>> index 18c26e0389..fee118518b 100755
>> --- a/configure
>> +++ b/configure
>> @@ -242,6 +242,7 @@ host_cc="cc"
>>   audio_win_int=""
>>   libs_qga=""
>>   debug_info="yes"
>> +lto="false"
>>   stack_protector=""
>>   safe_stack=""
>>   use_containers="yes"
>> @@ -1167,6 +1168,10 @@ for opt do
>>     ;;
>>     --disable-werror) werror="no"
>>     ;;
>> +  --enable-lto) lto="true"
>> +  ;;
>> +  --disable-lto) lto="false"
>> +  ;;
>>     --enable-stack-protector) stack_protector="yes"
>>     ;;
>>     --disable-stack-protector) stack_protector="no"
>> @@ -1751,6 +1756,7 @@ disabled with --disable-FEATURE, default is enabled 
>> if available:
>>     module-upgrades try to load modules from alternate paths for upgrades
>>     debug-tcg       TCG debugging (default is disabled)
>>     debug-info      debugging information
>> +  lto             Enable Link-Time Optimization.
>>     sparse          sparse checker
>>     safe-stack      SafeStack Stack Smash Protection. Depends on
>>                     clang/llvm >= 3.7 and requires coroutine backend 
>> ucontext.
>> @@ -7014,6 +7020,7 @@ NINJA=$ninja $meson setup \
>>           -Diconv=$iconv -Dcurses=$curses -Dlibudev=$libudev\
>>           -Ddocs=$docs -Dsphinx_build=$sphinx_build -Dinstall_blobs=$blobs \
>>           -Dvhost_user_blk_server=$vhost_user_blk_server \
>> +        -Db_lto=$lto \
>>           $cross_arg \
>>           "$PWD" "$source_path"
>> diff --git a/meson.build b/meson.build
>> index e3386196ba..ebd1c690e0 100644
>> --- a/meson.build
>> +++ b/meson.build
>> @@ -2044,6 +2044,7 @@ summary_info += {'gprof enabled':     
>> config_host.has_key('CONFIG_GPROF')}
>>   summary_info += {'sparse enabled':    sparse.found()}
>>   summary_info += {'strip binaries':    get_option('strip')}
>>   summary_info += {'profiler':          
>> config_host.has_key('CONFIG_PROFILER')}
>> +summary_info += {'link-time optimization (LTO)': get_option('b_lto')}
>>   summary_info += {'static build':      config_host.has_key('CONFIG_STATIC')}
>>   if targetos == 'darwin'
>>     summary_info += {'Cocoa support': config_host.has_key('CONFIG_COCOA')}
>>
> 
> I just came across this --enable-lto option ... but looking at the 
> implementation here, it seems only to emit a line in the summary_info, 
> without adding any compiler flags? Was this patch incomplete? Or do I miss 
> something?

Never mind, I now learnt that b_lto is apparently an option that is directly 
understood by meson already :-)

  Thomas