Couple of fixes to allow SIMD on Emscripten

- Add `-msimd128` to flags to actually enable WebAssembly SIMD
   when performing SIMD detection. It's currently required in
   addition to `-msse*` / `-mfpu=neon` flags which only perform
   translation of corresponding intrinsics to Wasm SIMD ones.
   See a discussion at emscripten-core/emscripten#12714 for
   automating this and making easier in the future.
 - Remove compilation branch that prevented definitions of
   `WEBP_USE_SSE` and `WEBP_USE_NEON` on Emscripten even when
   SIMD support was detected at compile-time.
 - Add an implementation of `VP8GetCPUInfo` for Emscripten which
   uses static `WEBP_USE_*` flags to determine if a corresponding
   SIMD instruction is supported. This is because Wasm doesn't
   have proper feature detection (yet) and requires making separate
   build for SIMD version anyway.

Change-Id: I77592081b91fd0e4cbc9242f5600ce905184f506

CMakeLists.txt

@@ -3,7 +3,13 @@ cmake_minimum_required(VERSION 3.5)
 project(WebP C)
 
 # Options for coder / decoder executables.
-option(WEBP_ENABLE_SIMD "Enable any SIMD optimization." ON)
+if(NOT EMSCRIPTEN)
+  # Disable SIMD on Emscripten by default, as it's a new unstable Wasm feature.
+  # Users can still explicitly opt-in to make a SIMD-enabled build.
+  set(WEBP_ENABLE_SIMD_DEFAULT ON)
+endif()
+option(WEBP_ENABLE_SIMD "Enable any SIMD optimization."
+       ${WEBP_ENABLE_SIMD_DEFAULT})
 option(WEBP_BUILD_ANIM_UTILS "Build animation utilities." ON)
 option(WEBP_BUILD_CWEBP "Build the cwebp command line tool." ON)
 option(WEBP_BUILD_DWEBP "Build the dwebp command line tool." ON)
@@ -26,7 +32,6 @@ if(WIN32)
 endif()
 
 if(WEBP_BUILD_WEBP_JS)
-  set(WEBP_ENABLE_SIMD OFF)
   set(WEBP_BUILD_ANIM_UTILS OFF)
   set(WEBP_BUILD_CWEBP OFF)
   set(WEBP_BUILD_DWEBP OFF)

cmake/cpu.cmake

@@ -70,6 +70,9 @@ foreach(I_SIMD RANGE ${WEBP_SIMD_FLAGS_RANGE})
   webp_check_compiler_flag(${WEBP_SIMD_FLAG} ${WEBP_ENABLE_SIMD})
   if(NOT WEBP_HAVE_${WEBP_SIMD_FLAG})
     list(GET SIMD_ENABLE_FLAGS ${I_SIMD} SIMD_COMPILE_FLAG)
+    if(EMSCRIPTEN)
+      set(SIMD_COMPILE_FLAG "-msimd128 ${SIMD_COMPILE_FLAG}")
+    endif()
     set(CMAKE_REQUIRED_FLAGS ${SIMD_COMPILE_FLAG})
     webp_check_compiler_flag(${WEBP_SIMD_FLAG} ${WEBP_ENABLE_SIMD})
   else()

src/dsp/cpu.c

@@ -179,6 +179,30 @@ static int AndroidCPUInfo(CPUFeature feature) {
   return 0;
 }
 VP8CPUInfo VP8GetCPUInfo = AndroidCPUInfo;
+#elif defined(EMSCRIPTEN) // also needs to be before generic NEON test
+// Use compile flags as an indicator of SIMD support instead of a runtime check.
+static int wasmCPUInfo(CPUFeature feature) {
+  switch (feature) {
+#ifdef WEBP_USE_SSE2
+    case kSSE2:
+      return 1;
+#endif
+#ifdef WEBP_USE_SSE41
+    case kSSE3:
+    case kSlowSSSE3:
+    case kSSE4_1:
+      return 1;
+#endif
+#ifdef WEBP_USE_NEON
+    case kNEON:
+      return 1;
+#endif
+    default:
+      break;
+  }
+  return 0;
+}
+VP8CPUInfo VP8GetCPUInfo = wasmCPUInfo;
 #elif defined(WEBP_USE_NEON)
 // define a dummy function to enable turning off NEON at runtime by setting
 // VP8DecGetCPUInfo = NULL

src/dsp/dsp.h

@@ -51,9 +51,6 @@ extern "C" {
 # define __has_builtin(x) 0
 #endif
 
-// for now, none of the optimizations below are available in emscripten
-#if !defined(EMSCRIPTEN)
-
 #if defined(_MSC_VER) && _MSC_VER > 1310 && \
     (defined(_M_X64) || defined(_M_IX86))
 #define WEBP_MSC_SSE2  // Visual C++ SSE2 targets
@@ -110,8 +107,6 @@ extern "C" {
 #define WEBP_USE_MSA
 #endif
 
-#endif  /* EMSCRIPTEN */
-
 #ifndef WEBP_DSP_OMIT_C_CODE
 #define WEBP_DSP_OMIT_C_CODE 1
 #endif