Fix `llvm-config` to emit the linker flag for the combined shared object built by autoconfig/make instead of the individual components.

[DiamondLovesYou]

Summary

This fixes llvm-config instead of fixing the makefiles because, AIUI, LLVM's autoconfig/make build system is on the way out anyway.

This change only affects builds that use autoconfig/make.

[jfb]

IIUC this is a problem when a toolchain is built with autoconf with dynamic libraries, and only ships the dynamic libraries (not the intermediate .a files). A regular build of LLVM has these .a files and aggregates all of them into the .so, so a toolchain would be silly to distribute the .a files because they're redundant.

This change seems useful, but I'd defer to folks such as @echristo who know better than I what LLVM's build system expects.

[DiamondLovesYou]

Fix a comment.

[jfb]

lgtm after this change.

tools/llvm-config/llvm-config.cpp

402

Taking a second look at this, could you restructure to:

if (Lib.startswith("lib")) {
  size_t FromEnd = 0;
  if (Lib.endswith(".a")) {
    FromEnd = 2;
  } else if (Lib.endswith(".so")) {
    FromEnd = 3;
  } else if (Lib.endswith(".dylib")) {
    FromEnd = 6;
  } else {
    FromEnd = 0;
    break;
  }
  OS << "-l" << Lib.slice(3, Lib.size() - FromEnd);
  continue;

[DiamondLovesYou]

Restructure in response to JF's comments.

[jfb]

Reverted in r242152.

tools/llvm-config/BuildVariables.inc.in

 #define LLVM_CPPFLAGS "@LLVM_CPPFLAGS@"
 #define LLVM_CFLAGS "@LLVM_CFLAGS@"
 #define LLVM_LDFLAGS "@LLVM_LDFLAGS@"
 #define LLVM_CXXFLAGS "@LLVM_CXXFLAGS@"
 #define LLVM_BUILDMODE "@LLVM_BUILDMODE@"
 #define LLVM_LIBDIR_SUFFIX "@LLVM_LIBDIR_SUFFIX@"
 #define LLVM_TARGETS_BUILT "@LLVM_TARGETS_BUILT@"
 #define LLVM_SYSTEM_LIBS "@LLVM_SYSTEM_LIBS@"
+#define BUILD_SHARED_LIBS "@BUILD_SHARED_LIBS@"
+#define WAS_BUILT_WITH_CMAKE "@WAS_BUILT_WITH_CMAKE@"

tools/llvm-config/CMakeLists.txt

 set(LLVM_OBJ_ROOT ${LLVM_BINARY_DIR})
 set(LLVM_CPPFLAGS "${CMAKE_CPP_FLAGS} ${CMAKE_CPP_FLAGS_${uppercase_CMAKE_BUILD_TYPE}} ${LLVM_DEFINITIONS}")
 set(LLVM_CFLAGS "${CMAKE_C_FLAGS} ${CMAKE_C_FLAGS_${uppercase_CMAKE_BUILD_TYPE}} ${LLVM_DEFINITIONS}")
 set(LLVM_CXXFLAGS "${CMAKE_CXX_FLAGS} ${CMAKE_CXX_FLAGS_${uppercase_CMAKE_BUILD_TYPE}} ${LLVM_DEFINITIONS}")
 # Use the C++ link flags, since they should be a superset of C link flags.
 set(LLVM_LDFLAGS "${CMAKE_CXX_LINK_FLAGS}")
 set(LLVM_BUILDMODE ${CMAKE_BUILD_TYPE})
 set(LLVM_SYSTEM_LIBS ${SYSTEM_LIBS})
+set(WAS_BUILT_WITH_CMAKE "ON")
 string(REPLACE ";" " " LLVM_TARGETS_BUILT "${LLVM_TARGETS_TO_BUILD}")
 configure_file(${BUILDVARIABLES_SRCPATH} ${BUILDVARIABLES_OBJPATH} @ONLY)
 
 # Set build-time environment(s).
 add_definitions(-DCMAKE_CFG_INTDIR="${CMAKE_CFG_INTDIR}")
 
 # Add the llvm-config tool.
 add_llvm_tool(llvm-config

tools/llvm-config/Makefile

$(ObjDir)/BuildVariables.inc: $(BUILDVARIABLES_SRCPATH) Makefile $(ObjDir)/.dir
   $(Verb) $(ECHO) 's/@LLVM_BUILDMODE@/$(subst /,\/,$(BuildMode))/' \
     >> temp.sed
   $(Verb) $(ECHO) 's/@LLVM_LIBDIR_SUFFIX@//' \
     >> temp.sed
   $(Verb) $(ECHO) 's/@LLVM_SYSTEM_LIBS@/$(subst /,\/,$(LLVM_SYSTEM_LIBS))/' \
     >> temp.sed
   $(Verb) $(ECHO) 's/@LLVM_TARGETS_BUILT@/$(subst /,\/,$(TARGETS_TO_BUILD))/' \
     >> temp.sed
+  $(if $(filter-out $(ENABLE_SHARED),0),\
+    $(Verb) $(ECHO) 's/@BUILD_SHARED_LIBS@/ON/',\
+    $(Verb) $(ECHO) 's/@BUILD_SHARED_LIBS@/OFF/') \
+    >> temp.sed
+  $(Verb) $(ECHO) 's/@WAS_BUILT_WITH_CMAKE@/OFF/' \
+    >> temp.sed
   $(Verb) $(SED) -f temp.sed < $< > $@
   $(Verb) $(RM) temp.sed
 
 # When cross-compiling, install a version of llvm-config that runs on the host.
 ifeq ($(LLVM_CROSS_COMPILING),1)
 install:: $(DESTDIR)$(PROJ_bindir)
   $(Echo) Installing llvm-config-host
   $(Verb) $(ProgInstall) $(BuildLLVMToolDir)/llvm-config \
     $(DESTDIR)$(PROJ_bindir)/$(program_prefix)llvm-config-host
 endif

tools/llvm-config/llvm-config.cpp

if (Components.empty())
       Components.push_back("all");
 
     // Construct the list of all the required libraries.
     std::vector<StringRef> RequiredLibs;
     ComputeLibsForComponents(Components, RequiredLibs,
                              /*IncludeNonInstalled=*/IsInDevelopmentTree);
 
     if (PrintLibs || PrintLibNames || PrintLibFiles) {
+      // If LLVM was built as a shared library, there will be only one thing
+      // that users should link against.
+      const bool IsSharedLib = (std::strcmp(BUILD_SHARED_LIBS, "ON") == 0);
+      const bool WasBuiltWithCMake = (std::strcmp(WAS_BUILT_WITH_CMAKE, "ON") == 0);
+      // CMake correctly builds components as separate shared libraries, however
+      // autoconfig/make builds components a static libraries and then links
+      // them all together to form a single shared library. Thus, only when
+      // `WAS_BUILT_WITH_CMAKE` and `BUILD_SHARED_LIBS` are `ON` do we override
+      // `RequiredLibs` with the single library name.
+      if (IsSharedLib && !WasBuiltWithCMake) {
+        RequiredLibs.clear();
+        std::string Name = "libLLVM-" PACKAGE_VERSION;
+        const Triple HostTriple(LLVM_DEFAULT_TARGET_TRIPLE);
+        if (HostTriple.isOSWindows()) {
+          Name += ".dll";
+        } else if (HostTriple.isOSDarwin()) {
+          Name += ".dylib";
+        } else {
+          // default to linux' ext:
+          Name += ".so";
+        }
+        RequiredLibs.push_back(Name);
+      }
+
       for (unsigned i = 0, e = RequiredLibs.size(); i != e; ++i) {
         StringRef Lib = RequiredLibs[i];
         if (i)
           OS << ' ';
 
         if (PrintLibNames) {
           OS << Lib;
         } else if (PrintLibFiles) {
           OS << ActiveLibDir << '/' << Lib;
         } else if (PrintLibs) {
           // If this is a typical library name, include it using -l.
-          if (Lib.startswith("lib") && Lib.endswith(".a")) {
-            OS << "-l" << Lib.slice(3, Lib.size()-2);
-            continue;
+          if (Lib.startswith("lib")) {
+            size_t FromEnd = 0;
+            do {
+              if (Lib.endswith(".a")) {
+                FromEnd = 2;
+              } else if (Lib.endswith(".so")) {
+                FromEnd = 3;
+              } else if (Lib.endswith(".dylib")) {
+                FromEnd = 6;
+              } else {
+                FromEnd = 0;
+                break;
+              }
+              OS << "-l" << Lib.slice(3, Lib.size() - FromEnd);
+            } while(false);
+            if (FromEnd != 0) { continue; }
           }
 
           // Otherwise, print the full path.
           OS << ActiveLibDir << '/' << Lib;
         }
       }
       OS << '\n';
     }