[liftoff] Add support for throwing exceptions

This is the first part of exception handling support in Liftoff. For
now, the only supported instruction is throw, and only with empty
payload or only i32 values.

R=thibaudm@chromium.org

Bug: v8:11453
Change-Id: I9fdf1328ef46655674a05186fb93216518886d03
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2704659Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72871}

src/wasm/baseline/liftoff-compiler.cc

@@ -3549,10 +3549,132 @@ class LiftoffCompiler {
     __ PushRegister(kWasmS128, dst);
   }
 
-  void Throw(FullDecoder* decoder, const ExceptionIndexImmediate<validate>&,
-             const Vector<Value>& args) {
-    unsupported(decoder, kExceptionHandling, "throw");
+  void ToSmi(Register reg) {
+    if (COMPRESS_POINTERS_BOOL || kSystemPointerSize == 4) {
+      __ emit_i32_shli(reg, reg, kSmiShiftSize + kSmiTagSize);
+    } else {
+      __ emit_i64_shli(LiftoffRegister{reg}, LiftoffRegister{reg},
+                       kSmiShiftSize + kSmiTagSize);
+    }
+  }
+
+  void Store32BitExceptionValue(Register values_array, int* index_in_array,
+                                Register value, LiftoffRegList pinned) {
+    LiftoffRegister tmp_reg = __ GetUnusedRegister(kGpReg, pinned);
+    // Get the lower half word into tmp_reg and extend to a Smi.
+    --*index_in_array;
+    __ emit_i32_andi(tmp_reg.gp(), value, 0xffff);
+    ToSmi(tmp_reg.gp());
+    __ StoreTaggedPointer(
+        values_array, no_reg,
+        wasm::ObjectAccess::ElementOffsetInTaggedFixedArray(*index_in_array),
+        tmp_reg, pinned);
+
+    // Get the upper half word into tmp_reg and extend to a Smi.
+    --*index_in_array;
+    __ emit_i32_shri(tmp_reg.gp(), value, 16);
+    ToSmi(tmp_reg.gp());
+    __ StoreTaggedPointer(
+        values_array, no_reg,
+        wasm::ObjectAccess::ElementOffsetInTaggedFixedArray(*index_in_array),
+        tmp_reg, pinned);
+  }
+
+  void StoreExceptionValue(ValueType type, Register values_array,
+                           int* index_in_array, LiftoffRegList pinned) {
+    // TODO(clemensb): Handle more types.
+    DCHECK_EQ(kWasmI32, type);
+    LiftoffRegister value = pinned.set(__ PopToRegister(pinned));
+    Store32BitExceptionValue(values_array, index_in_array, value.gp(), pinned);
+  }
+
+  void Throw(FullDecoder* decoder, const ExceptionIndexImmediate<validate>& imm,
+             const Vector<Value>& /* args */) {
+    LiftoffRegList pinned;
+
+    // Load the encoded size in a register for the builtin call.
+    int encoded_size = WasmExceptionPackage::GetEncodedSize(imm.exception);
+    LiftoffRegister encoded_size_reg =
+        pinned.set(__ GetUnusedRegister(kGpReg, pinned));
+    __ LoadConstant(encoded_size_reg, WasmValue(encoded_size));
+
+    // Call the WasmAllocateFixedArray builtin to create the values array.
+    DEBUG_CODE_COMMENT("call WasmAllocateFixedArray builtin");
+    compiler::CallDescriptor* create_values_descriptor =
+        GetBuiltinCallDescriptor<WasmAllocateFixedArrayDescriptor>(
+            compilation_zone_);
+
+    ValueType create_values_sig_reps[] = {kPointerValueType,
+                                          LiftoffAssembler::kWasmIntPtr};
+    FunctionSig create_values_sig(1, 1, create_values_sig_reps);
+
+    __ PrepareBuiltinCall(
+        &create_values_sig, create_values_descriptor,
+        {LiftoffAssembler::VarState{kSmiValueType,
+                                    LiftoffRegister{encoded_size_reg}, 0}});
+    __ CallRuntimeStub(WasmCode::kWasmAllocateFixedArray);
+    DefineSafepoint();
+
+    // The FixedArray for the exception values is now in the first gp return
+    // register.
+    DCHECK_EQ(kReturnRegister0.code(),
+              create_values_descriptor->GetReturnLocation(0).AsRegister());
+    LiftoffRegister values_array{kReturnRegister0};
+    pinned.set(values_array);
+
+    // Now store the exception values in the FixedArray. Do this from last to
+    // first value, such that we can just pop them from the value stack.
+    DEBUG_CODE_COMMENT("fill values array");
+    int index = encoded_size;
+    auto* sig = imm.exception->sig;
+    for (size_t param_idx = sig->parameter_count(); param_idx > 0;
+         --param_idx) {
+      ValueType type = sig->GetParam(param_idx - 1);
+      if (type != kWasmI32) {
+        unsupported(decoder, kExceptionHandling,
+                    "unsupported type in exception payload");
+        return;
       }
+      StoreExceptionValue(type, values_array.gp(), &index, pinned);
+    }
+    DCHECK_EQ(0, index);
+
+    // Since we need to call another builtin before the final use of the values
+    // array, put the reference on the stack, such that it survives the builtin
+    // call.
+    __ PushRegister(kPointerValueType, values_array);
+
+    // Load the exception tag.
+    DEBUG_CODE_COMMENT("load exception tag");
+    Register exception_tag =
+        pinned.set(__ GetUnusedRegister(kGpReg, pinned)).gp();
+    LOAD_TAGGED_PTR_INSTANCE_FIELD(exception_tag, ExceptionsTable, pinned);
+    __ LoadTaggedPointer(
+        exception_tag, exception_tag, no_reg,
+        wasm::ObjectAccess::ElementOffsetInTaggedFixedArray(imm.index), {});
+
+    // Pop the values array reference back into a register.
+    values_array = __ PopToRegister(pinned);
+
+    // Finally, call WasmThrow.
+    DEBUG_CODE_COMMENT("call WasmThrow builtin");
+    compiler::CallDescriptor* throw_descriptor =
+        GetBuiltinCallDescriptor<WasmThrowDescriptor>(compilation_zone_);
+
+    ValueType throw_sig_reps[] = {kPointerValueType, kPointerValueType};
+    FunctionSig throw_sig(0, 2, throw_sig_reps);
+
+    __ PrepareBuiltinCall(
+        &throw_sig, throw_descriptor,
+        {LiftoffAssembler::VarState{kPointerValueType,
+                                    LiftoffRegister{exception_tag}, 0},
+         LiftoffAssembler::VarState{kPointerValueType, values_array, 0}});
+    source_position_table_builder_.AddPosition(
+        __ pc_offset(), SourcePosition(decoder->position()), true);
+    __ CallRuntimeStub(WasmCode::kWasmThrow);
+    DefineSafepoint();
+  }
+
   void Rethrow(FullDecoder* decoder, const Value& exception) {
     unsupported(decoder, kExceptionHandling, "rethrow");
   }

test/mjsunit/regress/wasm/regress-8896.js

@@ -3,6 +3,8 @@
 // found in the LICENSE file.
 
 // Flags: --experimental-wasm-eh --allow-natives-syntax
+// Disable Liftoff so we can serialize the module.
+// Flags: --no-liftoff
 
 load("test/mjsunit/wasm/wasm-module-builder.js");