[vm] Adjust MSAN compile flags to match google3 to reproduce MSAN error in the regexp interpreter.

TEST=msan
Bug: b/489355669
Change-Id: I3ade526a67e5e7d95c8d8862eb92924337057d94
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/485240
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Slava Egorov <vegorov@google.com>

build/config/compiler/BUILD.gn

@@ -156,7 +156,16 @@ config("compiler") {
       ldflags += [ "-fsanitize=leak" ]
     }
     if (is_msan) {
-      cflags += [ "-fsanitize=memory" ]
+      cflags += [
+        "-fsanitize=memory",
+
+        # Matching the google3 settings. Whether MSAN reports a use of an
+        # uninitialized value is sensitive to inlining.
+        "-mllvm",
+        "-inline-instr-cost=20",
+        "-mllvm",
+        "-inline-memaccess-cost=20",
+      ]
       ldflags += [ "-fsanitize=memory" ]
     }
     if (is_tsan) {

runtime/tests/vm/dart/msan/regexp_b489355669_test.dart

@@ -0,0 +1,9 @@
+// Copyright (c) 2026, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Regression test for b/489355669
+
+main() {
+  print(new RegExp(r"[^a-df-z\s]+", caseSensitive: false).firstMatch("900s"));
+}

runtime/vm/regexp/regexp-interpreter.cc

@@ -175,7 +175,7 @@ class InterpreterRegisters {
   InterpreterRegisters(int total_register_count,
                        RegisterT* output_registers,
                        int output_register_count)
-      : registers_(total_register_count, 0),
+      : registers_(total_register_count, kNoMatchValue),
         output_registers_(output_registers),
         total_register_count_(total_register_count),
         output_register_count_(output_register_count) {
@@ -186,10 +186,6 @@ class InterpreterRegisters {
     SBXCHECK_GE(total_register_count, output_register_count);
     SBXCHECK_LE(total_register_count, RegExpMacroAssembler::kMaxRegisterCount);
     DCHECK_NOT_NULL(output_registers);
-
-    // Initialize the output register region to -1 signifying 'no match'.
-    std::memset(registers_.data(), kNoMatchValue,
-                output_register_count * sizeof(RegisterT));
     USE(total_register_count_);
   }