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

[Thomas Huth]

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 <dbu...@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?

 Thomas