[emscripten] Simply LTO flag setup. NFC (#7997)

We have a setting (BYN_ENABLE_LTO) which controls if LTO is enabled, and
this is enabled for emscripten by default.

With emscripten we also want to add `-flto=thin` to the linker flags (on
other targets its not needed but in emscripten we need this so the
linker knows to use specific LTO system libraries)

I'm not sure if was intentional to use `-flto` and some places and
`-flto=thin` in others, but this change consistency does thing LTO. If
we want to add option for full LTO we could do that later.

Also, don't enable LTO by default in debug build.

Author: sbc100

CMakeLists.txt

@@ -31,7 +31,14 @@ endif()
 # more useful error reports from users.
 option(BYN_ENABLE_ASSERTIONS "Enable assertions" ON)
 
-option(BYN_ENABLE_LTO "Build with LTO" ${EMSCRIPTEN})
+if(EMSCRIPTEN AND NOT "${CMAKE_BUILD_TYPE}" MATCHES "Debug")
+  # in opt builds, LTO helps so much (>20%) it's worth slow compile times,
+  # so we enable LTO by default in non-debug emscripten builds.
+  set(DEFAULT_LTO ON)
+else()
+  set(DEFAULT_LTO OFF)
+endif()
+option(BYN_ENABLE_LTO "Build with LTO" ${DEFAULT_LTO})
 
 # Turn this off to avoid the dependency on gtest.
 option(BUILD_TESTS "Build GTest-based tests" ON)
@@ -222,6 +229,11 @@ if(BYN_ENABLE_LTO)
   set_property(GLOBAL APPEND PROPERTY JOB_POOLS link_job_pool=2)
   set(CMAKE_JOB_POOL_LINK link_job_pool)
   add_compile_flag("-flto=thin")
+  if(EMSCRIPTEN)
+    # Under emscripten we also add `-flto` to the linker flags to tell the
+    # compiler driver to choose LTO versions of the system libraries.
+    add_link_flag("-flto=thin")
+  endif()
 endif()
 
 if(MSVC)
@@ -353,10 +365,6 @@ if(EMSCRIPTEN)
     # On Node.js, make the tools immediately usable.
     add_link_flag("-sNODERAWFS")
   endif()
-  if (BYN_ENABLE_LTO)
-    # in opt builds, LTO helps so much (>20%) it's worth slow compile times
-    add_nondebug_compile_flag("-flto")
-  endif()
   if(EMSCRIPTEN_ENABLE_WASM64)
     add_compile_flag("-sMEMORY64")
     add_link_flag("-sMEMORY64")
@@ -560,9 +568,6 @@ if(EMSCRIPTEN)
   target_link_libraries(binaryen_wasm PRIVATE optimized "--closure=1")
   # TODO: Fix closure warnings! (#5062)
   target_link_libraries(binaryen_wasm PRIVATE optimized "-Wno-error=closure")
-  if (BYN_ENABLE_LTO)
-    target_link_libraries(binaryen_wasm PRIVATE optimized "-flto")
-  endif()
   target_link_libraries(binaryen_wasm PRIVATE debug "--profiling")
   # Avoid catching exit as that can confuse error reporting in Node,
   # see https://github.com/emscripten-core/emscripten/issues/17228
@@ -614,9 +619,6 @@ if(EMSCRIPTEN)
   endif()
   # TODO: Fix closure warnings! (#5062)
   target_link_libraries(binaryen_js PRIVATE optimized "-Wno-error=closure")
-  if(BYN_ENABLE_LTO)
-    target_link_libraries(binaryen_js PRIVATE optimized "-flto")
-  endif()
   target_link_libraries(binaryen_js PRIVATE debug "--profiling")
   target_link_libraries(binaryen_js PRIVATE debug "-sASSERTIONS")
   # Avoid catching exit as that can confuse error reporting in Node,