[stringrefs] Implement string.concat

Bug: v8:12868
Change-Id: I7f5487dc01b006caef582f792e007d3a3cabb324
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3702435
Commit-Queue: Andy Wingo <wingo@igalia.com>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81151}

src/builtins/wasm.tq

@@ -829,6 +829,10 @@ builtin WasmStringEncodeWtf16(
       LoadContextFromInstance(instance), instance, memory, string,
       WasmUint32ToNumber(offset), SmiConstant(0), SmiFromInt32(string.length));
 }
+builtin WasmStringConcat(a: String, b: String): String {
+  const context = LoadContextFromFrame();
+  return a + b;
+}
 transitioning builtin WasmStringViewWtf16GetCodeUnit(
     string: String, offset: uint32): uint32 {
   try {

src/compiler/wasm-compiler.cc

@@ -5809,6 +5809,15 @@ Node* WasmGraphBuilder::StringEncodeWtf16(uint32_t memory, Node* string,
                             string, offset, gasm_->SmiConstant(memory));
 }
 
+Node* WasmGraphBuilder::StringConcat(Node* head, CheckForNull head_null_check,
+                                     Node* tail, CheckForNull tail_null_check,
+                                     wasm::WasmCodePosition position) {
+  if (head_null_check == kWithNullCheck) head = AssertNotNull(head, position);
+  if (tail_null_check == kWithNullCheck) tail = AssertNotNull(tail, position);
+  return gasm_->CallBuiltin(Builtin::kWasmStringConcat, Operator::kNoDeopt,
+                            head, tail);
+}
+
 Node* WasmGraphBuilder::StringViewWtf16GetCodeUnit(
     Node* string, CheckForNull null_check, Node* offset,
     wasm::WasmCodePosition position) {

src/compiler/wasm-compiler.h

@@ -553,6 +553,9 @@ class WasmGraphBuilder {
   Node* StringEncodeWtf16(uint32_t memory, Node* string,
                           CheckForNull null_check, Node* offset,
                           wasm::WasmCodePosition position);
+  Node* StringConcat(Node* head, CheckForNull head_null_check, Node* tail,
+                     CheckForNull tail_null_check,
+                     wasm::WasmCodePosition position);
   Node* StringViewWtf16GetCodeUnit(Node* string, CheckForNull null_check,
                                    Node* offset,
                                    wasm::WasmCodePosition position);

src/wasm/baseline/liftoff-compiler.cc

@@ -6198,7 +6198,27 @@ class LiftoffCompiler {
 
   void StringConcat(FullDecoder* decoder, const Value& head, const Value& tail,
                     Value* result) {
-    UNIMPLEMENTED();
+    LiftoffRegList pinned;
+
+    LiftoffRegister tail_reg = pinned.set(__ PopToRegister(pinned));
+    MaybeEmitNullCheck(decoder, tail_reg.gp(), pinned, tail.type);
+    LiftoffAssembler::VarState tail_var(kRef, tail_reg, 0);
+
+    LiftoffRegister head_reg = pinned.set(__ PopToRegister(pinned));
+    MaybeEmitNullCheck(decoder, head_reg.gp(), pinned, head.type);
+    LiftoffAssembler::VarState head_var(kRef, head_reg, 0);
+
+    CallRuntimeStub(WasmCode::kWasmStringConcat,
+                    MakeSig::Returns(kRef).Params(kRef, kRef),
+                    {
+                        head_var,
+                        tail_var,
+                    },
+                    decoder->position());
+    RegisterDebugSideTableEntry(decoder, DebugSideTableBuilder::kDidSpill);
+
+    LiftoffRegister result_reg(kReturnRegister0);
+    __ PushRegister(kRef, result_reg);
   }
 
   void StringEq(FullDecoder* decoder, const Value& a, const Value& b,

src/wasm/graph-builder-interface.cc

@@ -1448,7 +1448,9 @@ class WasmGraphBuildingInterface {
 
   void StringConcat(FullDecoder* decoder, const Value& head, const Value& tail,
                     Value* result) {
-    UNIMPLEMENTED();
+    result->node =
+        builder_->StringConcat(head.node, NullCheckFor(head.type), tail.node,
+                               NullCheckFor(tail.type), decoder->position());
   }
 
   void StringEq(FullDecoder* decoder, const Value& a, const Value& b,

src/wasm/wasm-code-manager.h

@@ -130,6 +130,7 @@ struct WasmModule;
   V(WasmStringMeasureWtf8)               \
   V(WasmStringEncodeWtf8)                \
   V(WasmStringEncodeWtf16)               \
+  V(WasmStringConcat)                    \
   V(WasmStringViewWtf16GetCodeUnit)      \
   V(WasmStringViewWtf16Encode)           \
   V(WasmStringViewWtf16Slice)

test/mjsunit/wasm/stringrefs-exec.js

@@ -15,6 +15,8 @@ let kSig_w_wii = makeSig([kWasmStringRef, kWasmI32, kWasmI32],
 let kSig_v_wi = makeSig([kWasmStringRef, kWasmI32], []);
 let kSig_v_wiii = makeSig([kWasmStringRef, kWasmI32, kWasmI32, kWasmI32],
                           []);
+let kSig_w_ww = makeSig([kWasmStringRef, kWasmStringRef], [kWasmStringRef]);
+let kSig_w_w = makeSig([kWasmStringRef], [kWasmStringRef]);
 
 function encodeWtf8(str) {
   // String iterator coalesces surrogate pairs.
@@ -433,6 +435,46 @@ function HasIsolatedSurrogate(str) {
   }
 })();
 
+(function TestStringConcat() {
+  let builder = new WasmModuleBuilder();
+
+  builder.addFunction("concat", kSig_w_ww)
+    .exportFunc()
+    .addBody([
+      kExprLocalGet, 0,
+      kExprLocalGet, 1,
+      kGCPrefix, kExprStringConcat
+    ]);
+
+  builder.addFunction("concat_null_head", kSig_w_w)
+    .exportFunc()
+    .addBody([
+      kExprRefNull, kStringRefCode,
+      kExprLocalGet, 0,
+      kGCPrefix, kExprStringConcat
+    ]);
+  builder.addFunction("concat_null_tail", kSig_w_w)
+    .exportFunc()
+    .addBody([
+      kExprLocalGet, 0,
+      kExprRefNull, kStringRefCode,
+      kGCPrefix, kExprStringConcat
+    ]);
+
+  let instance = builder.instantiate();
+
+  for (let head of interestingStrings) {
+    for (let tail of interestingStrings) {
+      assertEquals(head + tail, instance.exports.concat(head, tail));
+    }
+  }
+
+  assertThrows(() => instance.exports.concat_null_head("hey"),
+               WebAssembly.RuntimeError, "dereferencing a null pointer");
+  assertThrows(() => instance.exports.concat_null_tail("hey"),
+               WebAssembly.RuntimeError, "dereferencing a null pointer");
+})();
+
 (function TestStringViewWtf16() {
   let builder = new WasmModuleBuilder();