SCons: Refactor LTO options with `lto=<none|thin|full>`

Adds support for LTO on macOS and Android. We don't have much experience
with LTO on these platforms so for now we keep it disabled by default
even when `production=yes` is set.

Similarly for iOS where we ship object files for the user to link in
Xcode so LTO makes builds extremely slow to link.

`production=yes` defaults to full LTO.
ThinLTO is much faster for LLVM-based compilers but seems to produce
bigger binaries (at least for the Web platform).

1 files changed, 14 insertions, 10 deletions

diff --git a/platform/windows/detect.py b/platform/windows/detect.py
index 5607eab342..e6e1874fc0 100644
--- a/platform/windows/detect.py
+++ b/platform/windows/detect.py
@@ -191,7 +191,6 @@ def get_opts():
         ),
         BoolVariable("use_mingw", "Use the Mingw compiler, even if MSVC is installed.", False),
         BoolVariable("use_llvm", "Use the LLVM compiler", False),
-        BoolVariable("use_thinlto", "Use ThinLTO", False),
         BoolVariable("use_static_cpp", "Link MinGW/MSVC C++ runtime libraries statically", True),
         BoolVariable("use_asan", "Use address sanitizer (ASAN)", False),
     ]
@@ -449,7 +448,10 @@ def configure_msvc(env, vcvars_msvc_config):
 
     ## LTO
 
-    if env["use_lto"]:
+    if env["lto"] != "none":
+        if env["lto"] == "thin":
+            print("ThinLTO is only compatible with LLVM, use `use_llvm=yes` or `lto=full`.")
+            sys.exit(255)
         env.AppendUnique(CCFLAGS=["/GL"])
         env.AppendUnique(ARFLAGS=["/LTCG"])
         if env["progress"]:
@@ -562,17 +564,19 @@ def configure_mingw(env):
         if try_cmd("gcc-ranlib --version", env["mingw_prefix"], env["arch"]):
             env["RANLIB"] = mingw_bin_prefix + "gcc-ranlib"
 
-    if env["use_lto"]:
-        if not env["use_llvm"] and env.GetOption("num_jobs") > 1:
+    if env["lto"] != "none":
+        if env["lto"] == "thin":
+            if not env["use_llvm"]:
+                print("ThinLTO is only compatible with LLVM, use `use_llvm=yes` or `lto=full`.")
+                sys.exit(255)
+            env.Append(CCFLAGS=["-flto=thin"])
+            env.Append(LINKFLAGS=["-flto=thin"])
+        elif not env["use_llvm"] and env.GetOption("num_jobs") > 1:
             env.Append(CCFLAGS=["-flto"])
             env.Append(LINKFLAGS=["-flto=" + str(env.GetOption("num_jobs"))])
         else:
-            if env["use_thinlto"]:
-                env.Append(CCFLAGS=["-flto=thin"])
-                env.Append(LINKFLAGS=["-flto=thin"])
-            else:
-                env.Append(CCFLAGS=["-flto"])
-                env.Append(LINKFLAGS=["-flto"])
+            env.Append(CCFLAGS=["-flto"])
+            env.Append(LINKFLAGS=["-flto"])
 
     env.Append(LINKFLAGS=["-Wl,--stack," + str(STACK_SIZE)])