[CMake] use findzlib util to include zlib

[Holman]

Summary

See PR19403:
https://bugs.llvm.org/show_bug.cgi?id=19403

This change makes it much easier to use zlib when it isn't on the system include path (e.g. on Windows).
Instead of modifying compiler and linker flags, the user just has to set the CMAKE variable ZLIB_ROOT or ZLIB_INCLUDE_DIRS/ZLIB_LIBRARIES.

[labath]

With this patch, what is the output of llvm-config --system-libs ? I remember that as being the main reason for why this code is written the way it is... If it stays as -lz then I think this should be fine (and better than the previous version).

[Holman]

Ah thanks for the context! Unfortunately, it does change that to -l/usr/lib/x86_64-linux-gnu/libz.so. What do you think if I call FindZLIB if (ZLIB_ROOT OR ZLIB_INCLUDE_DIR OR ZLIB_LIBRARY), and otherwise use the existing loop?

[Holman]

Only use FindZLIB if one of the relevant variables is set.

[labath]

In D70519#1755566, @Holman wrote:

Ah thanks for the context! Unfortunately, it does change that to -l/usr/lib/x86_64-linux-gnu/libz.so. What do you think if I call FindZLIB if (ZLIB_ROOT OR ZLIB_INCLUDE_DIR OR ZLIB_LIBRARY), and otherwise use the existing loop?

I think that's... suboptimal? Let me pull in @beanz here as he has a lot of ideas about cmake (though some of his ideas include deleting llvm-config :P).

Overall, I very much like using standard cmake solution for finding dependencies, but don't like duplicating logic even more. Maybe we could pry apart (either in cmake or llvm-config) that /usr/lib/x86_64-linux-gnu/libz.so into appropriate -L -l switches? Or just get llvm-config to drop the -l from -l/usr/lib/x86_64-linux-gnu/libz.so, so it produces a valid (though not completely equivalent) compile line?

[hans]

Looks like a different version of this landed in abb00753069554c538f3d850897373d093389945 ?

[Holman]

Yup, it looks like that supersedes this change. Closing this one out.

llvm/cmake/config-ix.cmake

 check_include_file(sys/param.h HAVE_SYS_PARAM_H)
 check_include_file(sys/resource.h HAVE_SYS_RESOURCE_H)
 check_include_file(sys/stat.h HAVE_SYS_STAT_H)
 check_include_file(sys/time.h HAVE_SYS_TIME_H)
 check_include_file(sys/types.h HAVE_SYS_TYPES_H)
 check_include_file(termios.h HAVE_TERMIOS_H)
 check_include_file(unistd.h HAVE_UNISTD_H)
 check_include_file(valgrind/valgrind.h HAVE_VALGRIND_VALGRIND_H)
-check_include_file(zlib.h HAVE_ZLIB_H)
 check_include_file(fenv.h HAVE_FENV_H)
 check_symbol_exists(FE_ALL_EXCEPT "fenv.h" HAVE_DECL_FE_ALL_EXCEPT)
 check_symbol_exists(FE_INEXACT "fenv.h" HAVE_DECL_FE_INEXACT)
 
 check_include_file(mach/mach.h HAVE_MACH_MACH_H)
 check_include_file(histedit.h HAVE_HISTEDIT_H)
 check_include_file(CrashReporterClient.h HAVE_CRASHREPORTERCLIENT_H)
 if(APPLE)
if(HAVE_LIBPTHREAD)
   set(LLVM_PTHREAD_LIB ${CMAKE_THREAD_LIBS_INIT})
 endif()
 
 # Don't look for these libraries if we're using MSan, since uninstrumented third
 # party code may call MSan interceptors like strlen, leading to false positives.
 if(NOT LLVM_USE_SANITIZER MATCHES "Memory.*")
   set(HAVE_LIBZ 0)
   if(LLVM_ENABLE_ZLIB)
+    if(ZLIB_ROOT OR ZLIB_LIBRARY OR ZLIB_INCLUDE_DIR)
+      include(FindZLIB)
+
+      if (ZLIB_FOUND)
+        include_directories(${ZLIB_INCLUDE_DIRS})
+        set(HAVE_ZLIB_H 1)
+        string(TOUPPER ${ZLIB_LIBRARIES} library_suffix)
+        check_library_exists(${ZLIB_LIBRARIES} compress2 "" HAVE_LIBZ_${library_suffix})
+        if(HAVE_LIBZ_${library_suffix})
+          set(HAVE_LIBZ 1)
+        endif()
+      endif()
+    else()
+      check_include_file(zlib.h HAVE_ZLIB_H)
       foreach(library z zlib_static zlib)
         string(TOUPPER ${library} library_suffix)
         check_library_exists(${library} compress2 "" HAVE_LIBZ_${library_suffix})
         if(HAVE_LIBZ_${library_suffix})
           set(HAVE_LIBZ 1)
           set(ZLIB_LIBRARIES "${library}")
           break()
         endif()
       endforeach()
     endif()
+  endif()
 
   # Don't look for these libraries on Windows.
   if (NOT PURE_WINDOWS)
     # Skip libedit if using ASan as it contains memory leaks.
     if (LLVM_ENABLE_LIBEDIT AND HAVE_HISTEDIT_H AND NOT LLVM_USE_SANITIZER MATCHES ".*Address.*")
       check_library_exists(edit el_init "" HAVE_LIBEDIT)
     else()
       set(HAVE_LIBEDIT 0)