[Web] Replace string with TVMFFIByteArray* to avoid memory issues (#18467)

Passing in a string to `ArrayDecodeStorage` via the packed function
definition led to memory issues for larger models (such as
`gemma-2-9b-it-q4f32_1-MLC`). Replacing string with TVMFFIByteArray*
fixes this issue and also alleviates the stack pollution issue discussed
in an earlier PR (#18415). Note that
this does not completely fix generation for q0f32 models.

Author: akaashrp

web/emcc/wasm_runtime.cc

@@ -125,24 +125,25 @@ TVM_FFI_STATIC_INIT_BLOCK() {
       });
 }
 
-void ArrayDecodeStorage(Tensor cpu_arr, std::string bytes, std::string format, std::string dtype) {
+void ArrayDecodeStorage(Tensor cpu_arr, TVMFFIByteArray* bytes, const std::string& format,
+                        const std::string& dtype) {
+  ICHECK_NE(bytes, nullptr);
+  const char* byte_data = bytes->data;
+  const size_t byte_size = bytes->size;
   if (format == "f32-to-bf16" && dtype == "float32") {
-    std::vector<uint16_t> buffer(bytes.length() / 2);
-    std::memcpy(buffer.data(), bytes.data(), buffer.size() * 2);
-    // decode bf16 to f32
-    const uint16_t* bf16 = reinterpret_cast<const uint16_t*>(buffer.data());
+    const uint16_t* bf16 = reinterpret_cast<const uint16_t*>(byte_data);
     uint32_t* data = static_cast<uint32_t*>(cpu_arr->data);
     ICHECK(cpu_arr.IsContiguous());
     size_t size = 1;
     for (int i = 0; i < cpu_arr->ndim; ++i) {
       size *= cpu_arr->shape[i];
     }
-    ICHECK_EQ(size, bytes.length() / 2);
+    ICHECK_EQ(size, byte_size / 2);
     for (size_t i = 0; i < size; ++i) {
       data[i] = static_cast<uint32_t>(bf16[i]) << 16;
     }
   } else {
-    cpu_arr.CopyFromBytes(bytes.data(), bytes.length());
+    cpu_arr.CopyFromBytes(byte_data, byte_size);
   }
 }
 
@@ -151,16 +152,10 @@ TVM_FFI_STATIC_INIT_BLOCK() {
   refl::GlobalDef().def_packed(
       "tvmjs.array.decode_storage", [](ffi::PackedArgs args, ffi::Any* ret) {
         Tensor cpu_arr = args[0].cast<Tensor>();
-        auto bytes = args[1].cast<ffi::Bytes>();
+        TVMFFIByteArray* bytes = args[1].cast<TVMFFIByteArray*>();
         std::string format = args[2].cast<ffi::String>().operator std::string();
         std::string dtype = args[3].cast<ffi::String>().operator std::string();
         ArrayDecodeStorage(cpu_arr, bytes, format, dtype);
-        if (ret != nullptr) {
-          auto* ret_data = reinterpret_cast<TVMFFIAny*>(ret);
-          ret_data->type_index = TVMFFITypeIndex::kTVMFFINone;
-          ret_data->zero_padding = 0;
-          ret_data->v_int64 = 0;
-        }
       });
 }