Changeset 9b61265 for introduction/important/building-notes.xml

Timestamp:

07/02/2023 12:33:53 PM (12 months ago)

Author:

Xi Ruoyao <xry111@…>

Branches:

12.0, 12.1, kea, ken/TL2024, ken/tuningfonts, lazarus, plabs/newcss, python3.11, rahul/power-profiles-daemon, renodr/vulkan-addition, trunk, xry111/llvm18, xry111/xf86-video-removal

Children:

6feca4d

Parents:

6efdfbe

Message:

building-notes: Mention -Wl,{-O1,--as-needed} and -fno-semantic-interposition for optimization

File:

• introduction/important/building-notes.xml (modified) (3 diffs)

introduction/important/building-notes.xml

@@ -987 +987 @@
         <envar>CFLAGS</envar> or <envar>CXXFLAGS</envar>. For an
         introduction to the options available with gcc and g++ see <ulink
-        url="https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html"/> and <ulink
-        url="https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html"/>
-        and <command>info gcc</command>.
-
+        url="https://gcc.gnu.org/onlinedocs/gcc-&gcc-version;/gcc/Optimize-Options.html"/>.
+        The same content can be also found in <command>info gcc</command>.
       </para>
 
@@ -1036 +1034 @@
 
       <para>
+        When a shared library is built by GCC, a feature named
+        <quote>semantic interposition</quote> is enabled by default.  When
+        the shared library refers to a symbol name with external linkage
+        and default visbility, if the symbol exists in both the shared
+        library and the main executable, semantic interposition guarantees
+        the symbol in the main executable is always used.  This feature
+        was invented in an attempt to make the behavior of linking a shared
+        library and linking a static library as similar as possible.  Today
+        only a small number of packages still depend on semantic
+        interposition, but the feature is still on by the default of GCC,
+        causing many optimizations disabled for shared libraries because
+        they conflict with semantic interposition.  The
+        <option>-fno-semantic-interposition</option> option can be passed
+        to <command>gcc</command> or <command>g++</command> to disable
+        semantic interposition and enable more optimizations for shared
+        libraries.  This option is used as the default of some packages
+        (for example <xref linkend='python3'/>), and it's also the default
+        of Clang.
+      </para>
+
+      <para>
         There are also various other options which some people claim are
         beneficial. At worst, you get to recompile and test, and then
@@ -1046 +1065 @@
         used are those which were used by those <quote>parent</quote>
         packages.
+      </para>
+
+      <para>
+        For <envar>LDFLAGS</envar>, there are two options can be used
+        for optimization.  With <option>-Wl,-O1</option>, the linker will
+        optimize the hash table to speed up the dynamic linking.  And with
+        <option>-Wl,--as-needed</option>, the linker will disregard
+        unnecessary <option>-l<replaceable>foo</replaceable></option> options
+        from the command line, i. e. the shared library <systemitem
+        class='library'>lib<replaceable>foo</replaceable></systemitem>
+        will only be linked if a symbol in <systemitem
+        class='library'>lib<replaceable>foo</replaceable></systemitem> is
+        really referred from the executable or shared library being linked.
+        This can sometimes mitigate the <quote>excessive dependencies to
+        shared librarues</quote> issues caused by
+        <application>libtool</application>.  Note that the
+        <option>-Wl,</option> prefix is necessary because despite the
+        variable is named <envar>LDFLAGS</envar>, its content is actually
+        passed to <command>gcc</command> (or <command>g++</command>,
+        <command>clang</command>, etc.) during the link stage, not directly
+        passed to <command>ld</command>.
+      </para>
+
+      <para>
+        Note that <option>-Wl,-O1</option> is completely unrelated to the
+        compiler optimization flag <option>-O1</option>.  Some packages are
+        using <option>-Wl,-O1</option> and <option>-Wl,--as-needed</option>
+        as the default in their building systems.
       </para>