[LLVM][Cygwin] Fix llvm shared library name

Treat Cygwin like WIN32 in llvm-shlib for naming the library.

Add a new variable to BuildVariables to have CMake's shared library
prefix, to avoid having to patch between "cyg" and "msys-" on MSYS2.

Author: jeremyd2019

llvm/tools/llvm-config/BuildVariables.inc.in

@@ -35,3 +35,4 @@
 #define LLVM_DYLIB_COMPONENTS "@LLVM_DYLIB_COMPONENTS_expanded@"
 #define LLVM_DYLIB_VERSION "@LLVM_DYLIB_VERSION@"
 #define LLVM_TOOLS_INSTALL_DIR "@LLVM_TOOLS_INSTALL_DIR@"
+#define LLVM_SHARED_LIBRARY_PREFIX "@LLVM_SHARED_LIBRARY_PREFIX@"

llvm/tools/llvm-config/CMakeLists.txt

@@ -65,6 +65,7 @@ set(LLVM_CXXFLAGS "${CMAKE_CXX${CMAKE_CXX_STANDARD}_STANDARD_COMPILE_OPTION} ${L
 set(LLVM_BUILD_SYSTEM cmake)
 set(LLVM_HAS_RTTI ${LLVM_CONFIG_HAS_RTTI})
 set(LLVM_DYLIB_VERSION "${LLVM_VERSION_MAJOR}${LLVM_VERSION_SUFFIX}")
+set(LLVM_SHARED_LIBRARY_PREFIX "${CMAKE_SHARED_LIBRARY_PREFIX}")
 
 # Use the C++ link flags, since they should be a superset of C link flags.
 set(LLVM_LDFLAGS "${CMAKE_CXX_LINK_FLAGS}")

llvm/tools/llvm-config/llvm-config.cpp

@@ -390,7 +390,7 @@ int main(int argc, char **argv) {
     SharedExt = "dll";
     SharedVersionedExt = LLVM_DYLIB_VERSION ".dll";
     if (HostTriple.isOSCygMing()) {
-      SharedPrefix = "lib";
+      SharedPrefix = LLVM_SHARED_LIBRARY_PREFIX;
       StaticExt = "a";
       StaticPrefix = "lib";
     } else {
@@ -454,7 +454,7 @@ int main(int argc, char **argv) {
   /// extension. Returns true if Lib is in a recognized format.
   auto GetComponentLibraryNameSlice = [&](const StringRef &Lib,
                                           StringRef &Out) {
-    if (Lib.starts_with("lib")) {
+    if (Lib.starts_with(StaticPrefix) || Lib.starts_with(SharedPrefix)) {
       unsigned FromEnd;
       if (Lib.ends_with(StaticExt)) {
         FromEnd = StaticExt.size() + 1;
@@ -465,7 +465,10 @@ int main(int argc, char **argv) {
       }
 
       if (FromEnd != 0) {
-        Out = Lib.slice(3, Lib.size() - FromEnd);
+        unsigned FromStart = Lib.starts_with(SharedPrefix)
+                                 ? SharedPrefix.size()
+                                 : StaticPrefix.size();
+        Out = Lib.slice(FromStart, Lib.size() - FromEnd);
         return true;
       }
     }

llvm/tools/llvm-shlib/CMakeLists.txt

@@ -33,7 +33,7 @@ if(LLVM_BUILD_LLVM_DYLIB)
   if (LLVM_LINK_LLVM_DYLIB)
     set(INSTALL_WITH_TOOLCHAIN INSTALL_WITH_TOOLCHAIN)
   endif()
-  if (WIN32)
+  if (WIN32 OR CYGWIN)
     add_llvm_library(LLVM SHARED DISABLE_LLVM_LINK_LLVM_DYLIB SONAME ${INSTALL_WITH_TOOLCHAIN} ${SOURCES})
   else()
     add_llvm_library(LLVM SHARED DISABLE_LLVM_LINK_LLVM_DYLIB OUTPUT_NAME LLVM ${INSTALL_WITH_TOOLCHAIN} ${SOURCES})