wasm: Add WASM_OBJECT_FILES=1 build mode

This mode significantly improves build (link) time.
Opt-in by passing “-device-option WASM_OBJECT_FILES=1”
to configure.

This requires a custom emsdk build which uses upstream
llvm.

Task-number: QTBUG-72537
Change-Id: I47bab2b58ae7e49ca104233ec14fff7b93516d36
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>

configure.pri

@@ -291,6 +291,8 @@ defineTest(qtConfTest_architecture) {
         content = $$cat($$test_out_dir/libarch.so, blob)
     else: wasm:exists($$test_out_dir/arch.wasm): \
         content = $$cat($$test_out_dir/arch.wasm, blob)
+    else: wasm:exists($$test_out_dir/arch.o): \
+        content = $$cat($$test_out_dir/arch.o, blob)
     else: \
         error("$$eval($${1}.label) detection binary not found.")
 

mkspecs/wasm-emscripten/qmake.conf

@@ -5,6 +5,17 @@ QMAKE_PLATFORM          = wasm unix
 include(../common/gcc-base.conf)
 include(../common/clang.conf)
 
+load(device_config)
+
+# Support setting WASM_OBJECT_FILES with -device-option WASM_OBJECT_FILES=1
+!isEmpty(WASM_OBJECT_FILES): {
+    !equals(WASM_OBJECT_FILES, 1):!equals(WASM_OBJECT_FILES, 0): \
+        message(Error: The value for WASM_OBJECT_FILES must be 0 or 1)
+    QMAKE_CFLAGS += -s WASM_OBJECT_FILES=$$WASM_OBJECT_FILES
+    QMAKE_CXXFLAGS += -s WASM_OBJECT_FILES=$$WASM_OBJECT_FILES
+    QMAKE_LFLAGS += -s WASM_OBJECT_FILES=$$WASM_OBJECT_FILES
+}
+
 EMTERP_FLAGS = \
     -s EMTERPRETIFY=1  \
     -s EMTERPRETIFY_ASYNC=1 \
@@ -22,10 +33,14 @@ EMCC_COMMON_LFLAGS = \
     -s \"BINARYEN_TRAP_MODE=\'clamp\'\"
 
 EMCC_USE_PORTS_FLAGS = \
-    -s USE_LIBPNG=1 \
     -s USE_FREETYPE=1 \
     -s USE_ZLIB=1
 
+# libpng does not build for WASM_OBJECT_FILES=1, see
+# https://github.com/emscripten-core/emscripten/issues/8143
+equals(WASM_OBJECT_FILES, 0):\
+    EMCC_USE_PORTS_FLAGS += -s USE_LIBPNG=1
+
 # The -s arguments can also be used with release builds,
 # but are here in debug for clarity.
 EMCC_COMMON_LFLAGS_DEBUG = \
@@ -38,6 +53,35 @@ EMCC_COMMON_LFLAGS_DEBUG = \
     # -s SOCKET_DEBUG \ #print out socket,network data transfer
     -s GL_DEBUG=1
 
+# Set up debug/optimization flags
+QMAKE_CXXFLAGS_RELEASE -= -O2
+QMAKE_CFLAGS_RELEASE   -= -O2
+equals(WASM_OBJECT_FILES, 1) {
+    QMAKE_LFLAGS_DEBUG     += -g
+
+    QMAKE_CXXFLAGS_RELEASE += -O3
+    QMAKE_CFLAGS_RELEASE   += -O3
+    QMAKE_LFLAGS_RELEASE   += -O3
+    QMAKE_CFLAGS_OPTIMIZE  += -O3
+    QMAKE_CFLAGS_OPTIMIZE_FULL += -Oz
+} else {
+    # Practical debugging setup:
+    #    "-g4" preserves function names for stack traces
+    #    "-Os" produces reasonably sized binaries
+    QMAKE_CFLAGS_DEBUG     -= -g
+    QMAKE_CXXFLAGS_DEBUG   -= -g
+    QMAKE_CFLAGS_DEBUG     += -Os -g4
+    QMAKE_CXXFLAGS_DEBUG   += -Os -g4
+    QMAKE_LFLAGS_DEBUG     += -Os -g4
+
+    QMAKE_CXXFLAGS_RELEASE += -O3
+    QMAKE_CFLAGS_RELEASE   += -O3
+    QMAKE_LFLAGS_RELEASE   += -O3
+    QMAKE_CFLAGS_OPTIMIZE  += -O3
+    QMAKE_CFLAGS_OPTIMIZE_FULL += -Oz
+}
+
+
 QMAKE_COMPILER         += emscripten
 
 QMAKE_CC                = emcc
@@ -46,23 +90,6 @@ QMAKE_CXX               = em++
 QMAKE_CFLAGS           += $$EMCC_USE_PORTS_FLAGS
 QMAKE_CXXFLAGS         += $$EMCC_USE_PORTS_FLAGS
 
-# Practical debugging setup:
-#    "-g4" preserves function names for stack traces
-#    "-Os" produces reasonably sized binaries
-QMAKE_CFLAGS_DEBUG     -= -g
-QMAKE_CXXFLAGS_DEBUG   -= -g
-QMAKE_CFLAGS_DEBUG     += -Os -g4
-QMAKE_CXXFLAGS_DEBUG   += -Os -g4
-QMAKE_LFLAGS_DEBUG     += -Os -g4
-
-QMAKE_CXXFLAGS_RELEASE -= -O2
-QMAKE_CXXFLAGS_RELEASE += -O3
-QMAKE_CFLAGS_RELEASE   -= -O2
-QMAKE_CFLAGS_RELEASE   += -O3
-QMAKE_LFLAGS_RELEASE   += -O3
-QMAKE_CFLAGS_OPTIMIZE  += -O3
-QMAKE_CFLAGS_OPTIMIZE_FULL += -Oz
-
 QMAKE_LINK              = $$QMAKE_CXX
 QMAKE_LINK_SHLIB        = $$QMAKE_CXX
 QMAKE_LINK_C            = $$QMAKE_CC