Implement interprocedural optimization for GNU compilers

Honor the `INTERPROCEDURAL_OPTIMIZATION` target property for GNU
compilers by activating their link-time-optimization (LTO) flags.

Help/release/dev/gcc-ipo.rst

@@ -0,0 +1,7 @@
+GCC IPO
+-------
+
+* Interprocedural optimization (IPO) is now supported for GNU
+  compilers using link time optimization (LTO) flags.  See the
+  :prop_tgt:`INTERPROCEDURAL_OPTIMIZATION` target property and
+  :module:`CheckIPOSupported` module.

Modules/Compiler/Clang.cmake

@@ -27,5 +27,13 @@ else()
       set(CMAKE_${lang}_COMPILE_OPTIONS_TARGET "--target=")
       set(CMAKE_${lang}_COMPILE_OPTIONS_EXTERNAL_TOOLCHAIN "--gcc-toolchain=")
     endif()
+
+    set(_CMAKE_IPO_SUPPORTED_BY_CMAKE NO)
+    set(_CMAKE_IPO_MAY_BE_SUPPORTED_BY_COMPILER NO)
+
+    unset(CMAKE_${lang}_COMPILE_OPTIONS_IPO)
+    unset(CMAKE_${lang}_ARCHIVE_CREATE_IPO)
+    unset(CMAKE_${lang}_ARCHIVE_APPEND_IPO)
+    unset(CMAKE_${lang}_ARCHIVE_FINISH_IPO)
   endmacro()
 endif()

Modules/Compiler/GNU.cmake

@@ -45,4 +45,43 @@ macro(__compiler_gnu lang)
   if(NOT APPLE OR NOT CMAKE_${lang}_COMPILER_VERSION VERSION_LESS 4) # work around #4462
     set(CMAKE_INCLUDE_SYSTEM_FLAG_${lang} "-isystem ")
   endif()
+
+  set(_CMAKE_IPO_SUPPORTED_BY_CMAKE YES)
+  set(_CMAKE_IPO_MAY_BE_SUPPORTED_BY_COMPILER NO)
+
+  # '-flto' introduced since GCC 4.5:
+  # * https://gcc.gnu.org/onlinedocs/gcc-4.4.7/gcc/Option-Summary.html (no)
+  # * https://gcc.gnu.org/onlinedocs/gcc-4.5.4/gcc/Option-Summary.html (yes)
+  if(NOT CMAKE_${lang}_COMPILER_VERSION VERSION_LESS 4.5)
+    set(_CMAKE_IPO_MAY_BE_SUPPORTED_BY_COMPILER YES)
+    set(__lto_flags -flto)
+
+    if(NOT CMAKE_${lang}_COMPILER_VERSION VERSION_LESS 4.7)
+      # '-ffat-lto-objects' introduced since GCC 4.7:
+      # * https://gcc.gnu.org/onlinedocs/gcc-4.6.4/gcc/Option-Summary.html (no)
+      # * https://gcc.gnu.org/onlinedocs/gcc-4.7.4/gcc/Option-Summary.html (yes)
+      list(APPEND __lto_flags -fno-fat-lto-objects)
+    endif()
+
+    set(CMAKE_${lang}_COMPILE_OPTIONS_IPO ${__lto_flags})
+
+    # Need to use version of 'ar'/'ranlib' with plugin support.
+    # Quote from [documentation][1]:
+    #
+    #   To create static libraries suitable for LTO,
+    #   use gcc-ar and gcc-ranlib instead of ar and ranlib
+    #
+    # [1]: https://gcc.gnu.org/onlinedocs/gcc-4.9.4/gcc/Optimize-Options.html
+    set(CMAKE_${lang}_ARCHIVE_CREATE_IPO
+      "${CMAKE_GCC_AR} cr <TARGET> <LINK_FLAGS> <OBJECTS>"
+    )
+
+    set(CMAKE_${lang}_ARCHIVE_APPEND_IPO
+      "${CMAKE_GCC_AR} r <TARGET> <LINK_FLAGS> <OBJECTS>"
+    )
+
+    set(CMAKE_${lang}_ARCHIVE_FINISH_IPO
+      "${CMAKE_GCC_RANLIB} <TARGET>"
+    )
+  endif()
 endmacro()

Modules/Compiler/QCC.cmake

@@ -12,4 +12,12 @@ macro(__compiler_qcc lang)
 
   set(CMAKE_INCLUDE_SYSTEM_FLAG_${lang} "-Wp,-isystem,")
   set(CMAKE_DEPFILE_FLAGS_${lang} "-Wc,-MD,<DEPFILE>,-MT,<OBJECT>,-MF,<DEPFILE>")
+
+  set(_CMAKE_IPO_SUPPORTED_BY_CMAKE NO)
+  set(_CMAKE_IPO_MAY_BE_SUPPORTED_BY_COMPILER NO)
+
+  unset(CMAKE_${lang}_COMPILE_OPTIONS_IPO)
+  unset(CMAKE_${lang}_ARCHIVE_CREATE_IPO)
+  unset(CMAKE_${lang}_ARCHIVE_APPEND_IPO)
+  unset(CMAKE_${lang}_ARCHIVE_FINISH_IPO)
 endmacro()

Source/cmGeneratorTarget.cxx

@@ -2466,19 +2466,28 @@ void cmGeneratorTarget::GetAppleArchs(const std::string& config,
   }
 }
 
+//----------------------------------------------------------------------------
+std::string cmGeneratorTarget::GetFeatureSpecificLinkRuleVariable(
+  std::string const& var, std::string const& config) const
+{
+  if (this->IsIPOEnabled(config)) {
+    std::string varIPO = var + "_IPO";
+    if (this->Makefile->IsDefinitionSet(varIPO)) {
+      return varIPO;
+    }
+  }
+
+  return var;
+}
+
+//----------------------------------------------------------------------------
 std::string cmGeneratorTarget::GetCreateRuleVariable(
   std::string const& lang, std::string const& config) const
 {
   switch (this->GetType()) {
     case cmStateEnums::STATIC_LIBRARY: {
       std::string var = "CMAKE_" + lang + "_CREATE_STATIC_LIBRARY";
-      if (this->IsIPOEnabled(config)) {
-        std::string varIPO = var + "_IPO";
-        if (this->Makefile->GetDefinition(varIPO)) {
-          return varIPO;
-        }
-      }
-      return var;
+      return this->GetFeatureSpecificLinkRuleVariable(var, config);
     }
     case cmStateEnums::SHARED_LIBRARY:
       return "CMAKE_" + lang + "_CREATE_SHARED_LIBRARY";

Source/cmGeneratorTarget.h

@@ -309,6 +309,9 @@ public:
   void GetAppleArchs(const std::string& config,
                      std::vector<std::string>& archVec) const;
 
+  std::string GetFeatureSpecificLinkRuleVariable(
+    std::string const& var, std::string const& config) const;
+
   /** Return the rule variable used to create this type of target.  */
   std::string GetCreateRuleVariable(std::string const& lang,
                                     std::string const& config) const;

Source/cmMakefileLibraryTargetGenerator.cxx

@@ -129,14 +129,9 @@ void cmMakefileLibraryTargetGenerator::WriteStaticLibraryRules()
 {
   std::string linkLanguage =
     this->GeneratorTarget->GetLinkerLanguage(this->ConfigName);
-  std::string linkRuleVar = "CMAKE_";
-  linkRuleVar += linkLanguage;
-  linkRuleVar += "_CREATE_STATIC_LIBRARY";
 
-  if (this->GeneratorTarget->IsIPOEnabled(this->ConfigName) &&
-      this->Makefile->GetDefinition(linkRuleVar + "_IPO")) {
-    linkRuleVar += "_IPO";
-  }
+  std::string linkRuleVar = this->GeneratorTarget->GetCreateRuleVariable(
+    linkLanguage, this->ConfigName);
 
   std::string extraFlags;
   this->LocalGenerator->GetStaticLibraryFlags(
@@ -676,18 +671,30 @@ void cmMakefileLibraryTargetGenerator::WriteLibraryRules(
     std::string arCreateVar = "CMAKE_";
     arCreateVar += linkLanguage;
     arCreateVar += "_ARCHIVE_CREATE";
+
+    arCreateVar = this->GeneratorTarget->GetFeatureSpecificLinkRuleVariable(
+      arCreateVar, this->ConfigName);
+
     if (const char* rule = this->Makefile->GetDefinition(arCreateVar)) {
       cmSystemTools::ExpandListArgument(rule, archiveCreateCommands);
     }
     std::string arAppendVar = "CMAKE_";
     arAppendVar += linkLanguage;
     arAppendVar += "_ARCHIVE_APPEND";
+
+    arAppendVar = this->GeneratorTarget->GetFeatureSpecificLinkRuleVariable(
+      arAppendVar, this->ConfigName);
+
     if (const char* rule = this->Makefile->GetDefinition(arAppendVar)) {
       cmSystemTools::ExpandListArgument(rule, archiveAppendCommands);
     }
     std::string arFinishVar = "CMAKE_";
     arFinishVar += linkLanguage;
     arFinishVar += "_ARCHIVE_FINISH";
+
+    arFinishVar = this->GeneratorTarget->GetFeatureSpecificLinkRuleVariable(
+      arFinishVar, this->ConfigName);
+
     if (const char* rule = this->Makefile->GetDefinition(arFinishVar)) {
       cmSystemTools::ExpandListArgument(rule, archiveFinishCommands);
     }

Source/cmNinjaNormalTargetGenerator.cxx

@@ -516,6 +516,10 @@ std::vector<std::string> cmNinjaNormalTargetGenerator::ComputeLinkCmd()
         std::string linkCmdVar = "CMAKE_";
         linkCmdVar += this->TargetLinkLanguage;
         linkCmdVar += "_ARCHIVE_CREATE";
+
+        linkCmdVar = this->GeneratorTarget->GetFeatureSpecificLinkRuleVariable(
+          linkCmdVar, this->GetConfigName());
+
         const char* linkCmd = mf->GetRequiredDefinition(linkCmdVar);
         cmSystemTools::ExpandListArgument(linkCmd, linkCmds);
       }
@@ -523,6 +527,10 @@ std::vector<std::string> cmNinjaNormalTargetGenerator::ComputeLinkCmd()
         std::string linkCmdVar = "CMAKE_";
         linkCmdVar += this->TargetLinkLanguage;
         linkCmdVar += "_ARCHIVE_FINISH";
+
+        linkCmdVar = this->GeneratorTarget->GetFeatureSpecificLinkRuleVariable(
+          linkCmdVar, this->GetConfigName());
+
         const char* linkCmd = mf->GetRequiredDefinition(linkCmdVar);
         cmSystemTools::ExpandListArgument(linkCmd, linkCmds);
       }