Again roll back wasm-simd assumption, more fixes required

PiperOrigin-RevId: 907700209

Author: jan-wassenberg

g3doc/quick_reference.md

@@ -391,13 +391,6 @@ You can detect and influence the set of supported targets:
     order to test each supported target. Calling with `b == 0` restores the
     normal `SupportedTargets` behavior.
 
-**Note on `HWY_ARCH_WASM`**. The target is automatically detected and enabled;
-compiler flags such as -msimd128 are not required. This auto-detection
-behavior is consistent with other architectures (e.g., AVX2, NEON).
-Developers wishing to disable the `HWY_ARCH_WASM` target (e.g., to increase
-browser compatibility, or workaround compiler bugs) should use the
-`HWY_DISABLED_TARGETS` macro.
-
 ## Operations
 
 In the following, the argument or return type `V` denotes a vector with `N`
@@ -2988,13 +2981,6 @@ binary will likely crash. This can only happen if:
     also not supported by the current CPU, and baseline targets (in particular
     `HWY_SCALAR`) were explicitly disabled.
 
-**Note on WebAssembly:**: The browser validates the WebAssembly module during
-instantiation when using `WebAssembly.instantiate` APIs. If SIMD features are
-unsupported, the entire module is rejected. Therefore, dynamic dispatch between
-targets (such as `HWY_SCALAR` and `HWY_WASM`) based on runtime feature detection
-is not feasible, unlike with native targets. Developers should configure the
-`HWY_ARCH_WASM` target at compile time using the `HWY_DISABLED_TARGETS` macro.
-
 ## Advanced configuration macros
 
 The following macros govern which targets to generate. Unless specified

hwy/detect_compiler_arch.h

@@ -356,10 +356,6 @@
 #define HWY_ARCH_ARM_OLD 0
 #endif
 
-// Note: The WASM target is auto-detected and enabled; compiler flags like
-// "-msimd128" are not required. This behavior is consistent with other
-// architectures (e.g., AVX2, NEON). To disable HWY_ARCH_WASM, use the
-// HWY_DISABLED_TARGETS macro. For details, see the quick reference manual.
 #if defined(__EMSCRIPTEN__) || defined(__wasm__) || defined(__WASM__)
 #define HWY_ARCH_WASM 1
 #else

hwy/detect_targets.h

@@ -432,7 +432,7 @@
 // Also check HWY_ARCH to ensure that simulating unknown platforms ends up with
 // HWY_TARGET == HWY_BASELINE_SCALAR.
 
-#if HWY_ARCH_WASM
+#if HWY_ARCH_WASM && defined(__wasm_simd128__)
 #if defined(HWY_WANT_WASM2)
 #define HWY_BASELINE_WASM HWY_WASM_EMU256
 #else