diff --git a/.github/workflows/testing-macos.yml b/.github/workflows/testing-macos.yml
index 2db562535cf0..7fea62c89cf3 100644
--- a/.github/workflows/testing-macos.yml
+++ b/.github/workflows/testing-macos.yml
@@ -28,6 +28,27 @@ permissions:
   contents: read
 
 jobs:
+  coverage:
+    if: "!contains(github.event.pull_request.labels.*.name, 'skip_buildbots')"
+    name: arm-64 / coverage
+    runs-on: macos-26
+
+    steps:
+      - uses: actions/checkout@v6
+
+      - name: Install coverage dependencies
+        run: brew install llvm@21 lld@21 emscripten flatbuffers wabt
+
+      - name: Run coverage suite
+        run: tools/run-coverage.sh
+
+      - name: Upload coverage to Codecov
+        uses: codecov/codecov-action@v5
+        with:
+          files: build/macOS-coverage/coverage.info
+          fail_ci_if_error: true
+          token: ${{ secrets.CODECOV_TOKEN }}
+
   macos:
     if: "!contains(github.event.pull_request.labels.*.name, 'skip_buildbots')"
     name: ${{ matrix.arch }} / ${{ matrix.uv_group }}
diff --git a/CMakePresets.json b/CMakePresets.json
index b0a50f2f72fe..6692d18b300c 100644
--- a/CMakePresets.json
+++ b/CMakePresets.json
@@ -202,6 +202,33 @@
         "CMAKE_SHARED_LINKER_FLAGS": "-fsanitize=fuzzer"
       }
     },
+    {
+      "name": "macOS-coverage",
+      "displayName": "macOS (Coverage)",
+      "description": "macOS build with source-based LLVM coverage instrumentation",
+      "inherits": "macOS",
+      "toolchainFile": "${sourceDir}/cmake/toolchain.macos-homebrew.cmake",
+      "cacheVariables": {
+        "CMAKE_BUILD_TYPE": "Debug",
+        "CMAKE_C_FLAGS": "-fprofile-instr-generate -fcoverage-mapping",
+        "CMAKE_CXX_FLAGS": "-fprofile-instr-generate -fcoverage-mapping",
+        "CMAKE_EXE_LINKER_FLAGS": "-fprofile-instr-generate",
+        "CMAKE_MODULE_LINKER_FLAGS": "-fprofile-instr-generate",
+        "CMAKE_SHARED_LINKER_FLAGS": "-fprofile-instr-generate",
+        "WITH_TUTORIALS": "NO",
+        "WITH_PYTHON_BINDINGS": "NO",
+        "WITH_UTILS": "NO",
+        "WITH_TEST_AUTO_SCHEDULE": "YES",
+        "WITH_TEST_ERROR": "YES",
+        "WITH_TEST_WARNING": "NO",
+        "WITH_TEST_PERFORMANCE": "NO",
+        "WITH_TEST_GENERATOR": "YES",
+        "WITH_TEST_RUNTIME": "NO",
+        "WITH_TEST_FUZZ": "NO",
+        "WITH_TEST_CORRECTNESS": "YES",
+        "WITH_SERIALIZATION_JIT_ROUNDTRIP_TESTING": "YES"
+      }
+    },
     {
       "name": "macOS-vcpkg",
       "inherits": [
@@ -242,5 +269,30 @@
         "BUILD_SHARED_LIBS": "NO"
       }
     }
+  ],
+  "buildPresets": [
+    {
+      "name": "macOS-coverage",
+      "configurePreset": "macOS-coverage",
+      "displayName": "macOS (Coverage)",
+      "description": "Build with LLVM_PROFILE_FILE set so generator binaries write profiles to the profiles/ dir",
+      "environment": {
+        "LLVM_PROFILE_FILE": "${sourceDir}/build/macOS-coverage/profiles/%p-%m.profraw"
+      }
+    }
+  ],
+  "testPresets": [
+    {
+      "name": "macOS-coverage",
+      "configurePreset": "macOS-coverage",
+      "displayName": "macOS (Coverage)",
+      "description": "Run correctness tests with LLVM source-based coverage profiling",
+      "environment": {
+        "LLVM_PROFILE_FILE": "${sourceDir}/build/macOS-coverage/profiles/%p-%m.profraw"
+      },
+      "output": {
+        "outputOnFailure": true
+      }
+    }
   ]
 }
diff --git a/cmake/HalideTestHelpers.cmake b/cmake/HalideTestHelpers.cmake
index c46a4672d3ce..26fa0d377e8f 100644
--- a/cmake/HalideTestHelpers.cmake
+++ b/cmake/HalideTestHelpers.cmake
@@ -99,8 +99,8 @@ function(add_halide_test TARGET)
                           VISIBILITY_INLINES_HIDDEN TRUE)
 
 
-    if (WITH_SERIALIZATION_JIT_ROUNDTRIP_TESTING)
-        if (WITH_SERIALIZATION)
+    if (WITH_SERIALIZATION AND WITH_SERIALIZATION_JIT_ROUNDTRIP_TESTING)
+        if (NOT Halide_TARGET MATCHES "wasm")
             target_compile_definitions(${TARGET} PRIVATE WITH_SERIALIZATION_JIT_ROUNDTRIP_TESTING)
         endif ()
     endif ()
diff --git a/test/correctness/simd_op_check.h b/test/correctness/simd_op_check.h
index 0fe23f1c1ef9..2dfa6e466c25 100644
--- a/test/correctness/simd_op_check.h
+++ b/test/correctness/simd_op_check.h
@@ -503,7 +503,14 @@ class SimdOpCheckTest {
             if (!sharder.should_run(t)) continue;
             const auto &task = tasks.at(t);
             futures.push_back(pool.async([&]() {
-                return check_one(task.op, task.name, task.vector_width, task.expr);
+                // Run check_one on a large-stack thread to avoid overflowing the
+                // default 512 KB worker stack during deeply recursive LLVM codegen
+                // (especially under coverage instrumentation).
+                TestResult result;
+                Internal::run_with_large_stack([&] {
+                    result = check_one(task.op, task.name, task.vector_width, task.expr);
+                });
+                return result;
             }));
         }
 
diff --git a/tools/run-coverage.sh b/tools/run-coverage.sh
new file mode 100755
index 000000000000..1fbd20b8aa31
--- /dev/null
+++ b/tools/run-coverage.sh
@@ -0,0 +1,100 @@
+#!/usr/bin/env bash
+# Collect source-based LLVM coverage for src/ by running the test suite four times:
+#   1. host                        — all correctness, generator, error, and autoscheduler tests
+#   2. host-metal                  — only tests whose name contains "gpu" or "metal"
+#   3. host-opencl                 — only tests whose name contains "gpu" or "opencl"
+#   4. wasm-32-wasmrt-wasm_simd128 — all tests (exercises WasmExecutor for JIT)
+#
+# Prerequisites: Homebrew LLVM 21 (for llvm-profdata and llvm-cov).
+#
+# Usage: tools/run-coverage.sh [--report-only]
+#   --report-only  Skip build and test steps; re-merge existing profraw files
+#                  and regenerate all reports. Useful after re-running a single
+#                  test manually.
+
+set -euo pipefail
+
+REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+BUILD_DIR="$REPO_ROOT/build/macOS-coverage"
+LLVM_DIR="/opt/homebrew/opt/llvm@21/bin"
+NPROC=$(sysctl -n hw.logicalcpu)
+
+REPORT_ONLY=0
+for arg in "$@"; do
+    case "$arg" in
+        --report-only) REPORT_ONLY=1 ;;
+        *)
+            echo "Unknown argument: $arg" >&2
+            exit 1
+            ;;
+    esac
+done
+
+cd "$REPO_ROOT"
+
+if [[ $REPORT_ONLY -eq 0 ]]; then
+
+    mkdir -p "$BUILD_DIR/profiles"
+
+    # ── Configure + Build ─────────────────────────────────────────────────────────
+    cmake --preset macOS-coverage
+    cmake --build --preset macOS-coverage
+
+    # ── Run 1: host — all tests ──────────────────────────────────────────────────
+    ctest --preset macOS-coverage -j"$NPROC" || echo "Warning: some tests failed (see above)"
+
+    # ── Reconfigure + rebuild for host-metal ─────────────────────────────────────
+    cmake --preset macOS-coverage -DHalide_TARGET=host-metal
+    cmake --build --preset macOS-coverage
+
+    # ── Run 2: host-metal — GPU tests only ───────────────────────────────────────
+    ctest --preset macOS-coverage -j"$NPROC" -R "gpu|metal" || echo "Warning: some tests failed (see above)"
+
+    # ── Reconfigure + rebuild for host-opencl ────────────────────────────────────
+    cmake --preset macOS-coverage -DHalide_TARGET=host-opencl
+    cmake --build --preset macOS-coverage
+
+    # ── Run 3: host-opencl — GPU and OpenCL-specific tests ───────────────────────
+    ctest --preset macOS-coverage -j"$NPROC" -R "gpu|opencl" || echo "Warning: some tests failed (see above)"
+
+    # ── Reconfigure + rebuild for wasm-32-wasmrt-wasm_simd128 ────────────────────
+    cmake --preset macOS-coverage -DHalide_TARGET=wasm-32-wasmrt-wasm_simd128
+    cmake --build --preset macOS-coverage
+
+    # ── Run 4: wasm — all tests (exercises WasmExecutor for JIT compilation) ─────
+    ctest --preset macOS-coverage -j"$NPROC" || echo "Warning: some tests failed (see above)"
+
+fi # REPORT_ONLY
+
+# ── Merge profiles ────────────────────────────────────────────────────────────
+"$LLVM_DIR/llvm-profdata" merge \
+    "$BUILD_DIR/profiles"/*.profraw \
+    -o "$BUILD_DIR/merged.profdata"
+
+# ── Generate coverage reports ─────────────────────────────────────────────────
+HALIDE_LIB=$(find "$BUILD_DIR/src" -maxdepth 1 -name "libHalide.*.*.*.dylib" | head -1)
+OBJECTS=(-object "$HALIDE_LIB")
+
+while IFS= read -r bin; do
+    OBJECTS+=(-object "$bin")
+done < <(
+    # Exclude generator_aot_* and generator_aotcpp_*: they run pre-compiled
+    # Halide pipelines at test time and add no unique src/ coverage.
+    find "$BUILD_DIR/test" -maxdepth 2 -type f -perm +0111 |
+        grep -v '/generator_aot_\|/generator_aotcpp_' | sort
+)
+
+COV_ARGS=(
+    --instr-profile="$BUILD_DIR/merged.profdata"
+    "${OBJECTS[@]}"
+    --ignore-filename-regex='build/|test/|tutorial/|python_bindings/|apps/|packaging/|include/'
+    --sources "$REPO_ROOT/src"
+)
+
+"$LLVM_DIR/llvm-cov" report "${COV_ARGS[@]}" 2>/dev/null >"$BUILD_DIR/coverage-report.txt"
+"$LLVM_DIR/llvm-cov" show "${COV_ARGS[@]}" --format=html --output-dir="$BUILD_DIR/coverage-html" 2>/dev/null
+"$LLVM_DIR/llvm-cov" export "${COV_ARGS[@]}" --format=lcov 2>/dev/null >"$BUILD_DIR/coverage.info"
+
+echo "Coverage report: $BUILD_DIR/coverage-report.txt"
+echo "LCOV data:       $BUILD_DIR/coverage.info"
+echo "HTML report:     $BUILD_DIR/coverage-html/index.html"