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fix(aot): reserve x18 register on macOS ARM64 (#4775)
* fix(aot): reserve x18 register on macOS ARM64 Apple reserves CPU register x18 for TLS on ARM64. When generating AOT code for aarch64 on macOS, LLVM may use x18, causing crashes when the AOT code runs on macOS ARM64 (M1/M2/M3). This patch: 1. Detects darwin/macho ABI and sets correct vendor string 2. Detects darwin/apple in default triple for platform detection 3. Adds +reserve-x18 to LLVM target features for aarch64 on macOS The fix only applies when compiling on macOS ARM64 hosts, ensuring generated AOT code is compatible with Apple's platform requirements. * test(aot): add x18 register reservation test for macOS ARM64 Add a standalone test to verify that the +reserve-x18 LLVM flag is correctly applied when compiling AOT for macOS ARM64. On macOS ARM64, x18 is reserved by Apple for TLS (Thread Local Storage). Without the +reserve-x18 flag, LLVM may generate code that uses x18, causing random SIGSEGV crashes (~80% crash rate in testing). The test: - Creates a WASM module with 24 local variables to stress register allocation - Compiles to AOT with -O3 optimization (which would use x18 without the fix) - Runs 1000 iterations to verify no crashes occur - Only runs on macOS ARM64 (skipped on other platforms) Test results: - Without fix: 82/100 crash rate - With fix: 0/100 crash rate (1000 iterations verified)
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‎.github/workflows/compilation_on_macos.yml‎

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@@ -440,3 +440,13 @@ jobs:
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cmake ..
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cmake --build . --config Release --parallel 4
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./import-func-callback
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- name: Test x18 register reservation (macOS ARM64 only)
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if: matrix.os == 'macos-15'
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run: |
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cd product-mini/platforms/darwin
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mkdir -p build && cd build
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cmake .. -DWAMR_BUILD_AOT=1
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cmake --build . --config Release --parallel 4
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cd ../../../../tests/standalone/test-aot-x18-reserve
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./run.sh --aot

‎core/iwasm/compilation/aot_llvm.c‎

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@@ -3005,6 +3005,10 @@ aot_create_comp_context(const AOTCompData *comp_data, aot_comp_option_t option)
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else
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vendor_sys = "-pc-windows-";
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}
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else if (!strcmp(abi, "darwin") || !strcmp(abi, "macho")) {
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/* macOS/Darwin: x18 is reserved by Apple */
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vendor_sys = "-apple-";
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}
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else {
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if (is_baremetal_target(arch, cpu, abi))
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vendor_sys = "-unknown-none-";
@@ -3050,6 +3054,14 @@ aot_create_comp_context(const AOTCompData *comp_data, aot_comp_option_t option)
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if (!abi)
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abi = "gnu";
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}
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else if (strstr(default_triple, "darwin")
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|| strstr(default_triple, "apple")) {
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/* macOS/Darwin: x18 is reserved by Apple, must use correct
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* triple to prevent LLVM from using it */
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vendor_sys = "-apple-darwin";
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if (!abi)
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abi = "";
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}
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else {
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vendor_sys = "-pc-linux-";
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if (!abi)
@@ -3139,6 +3151,30 @@ aot_create_comp_context(const AOTCompData *comp_data, aot_comp_option_t option)
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if (!features)
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features = "";
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#if (defined(__APPLE__) || defined(__MACH__)) && defined(BUILD_TARGET_AARCH64)
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/* On macOS ARM64, x18 is reserved by Apple for TLS. Even though we're
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* generating ELF (Linux-style) AOT files, we must tell LLVM to not
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* use x18, otherwise the AOT code will crash when running on macOS. */
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{
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bool is_aarch64 = false;
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if (arch && !strncmp(arch, "aarch64", 7))
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is_aarch64 = true;
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else if (triple_norm && strstr(triple_norm, "aarch64"))
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is_aarch64 = true;
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if (is_aarch64) {
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if (features[0] != '\0') {
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snprintf(features_buf, sizeof(features_buf),
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"%s,+reserve-x18", features);
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features = features_buf;
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}
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else {
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features = "+reserve-x18";
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}
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}
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}
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#endif
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/* Get target with triple, note that LLVMGetTargetFromTriple()
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return 0 when success, but not true. */
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if (LLVMGetTargetFromTriple(triple_norm, &target, &err) != 0) {

‎tests/standalone/test-aot-x18-reserve/run.sh‎

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#!/bin/bash
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#
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# Copyright (C) 2019 Intel Corporation. All rights reserved.
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# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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#
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# Test for x18 register reservation on macOS ARM64 (aarch64).
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#
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# On macOS ARM64, x18 is reserved by Apple for TLS (Thread Local Storage).
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# Without the +reserve-x18 LLVM flag, the AOT compiler may generate code
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# that uses x18, causing random SIGSEGV crashes when run on macOS.
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#
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# This test compiles a WASM module that stresses register allocation
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# (forcing x18 usage without the fix) and runs it 1000 times to verify
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# no crashes occur.
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#
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SCRIPT_DIR=$(cd "$(dirname "$0")" && pwd)
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WAMR_DIR="${SCRIPT_DIR}/../../.."
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# Detect platform
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UNAME_S=$(uname -s)
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UNAME_M=$(uname -m)
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# Only run this test on macOS ARM64
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if [[ "${UNAME_S}" != "Darwin" ]] || [[ "${UNAME_M}" != "arm64" ]]; then
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echo "Skipping x18 reserve test: only applicable on macOS ARM64"
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echo "Current platform: ${UNAME_S} ${UNAME_M}"
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exit 0
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fi
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# Determine iwasm path based on platform
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if [[ "${UNAME_S}" == "Darwin" ]]; then
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IWASM_CMD="${WAMR_DIR}/product-mini/platforms/darwin/build/iwasm"
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else
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IWASM_CMD="${WAMR_DIR}/product-mini/platforms/linux/build/iwasm"
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fi
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WAMRC_CMD="${WAMR_DIR}/wamr-compiler/build/wamrc"
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# Check if required binaries exist
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if [[ ! -x "${IWASM_CMD}" ]]; then
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echo "Error: iwasm not found at ${IWASM_CMD}"
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echo "Please build iwasm first"
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exit 1
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fi
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if [[ ! -x "${WAMRC_CMD}" ]]; then
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echo "Error: wamrc not found at ${WAMRC_CMD}"
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echo "Please build wamrc first"
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exit 1
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fi
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cd "${SCRIPT_DIR}"
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# Find wat2wasm (check CI path first, then system PATH)
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if [[ -x "/opt/wabt/bin/wat2wasm" ]]; then
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WAT2WASM="/opt/wabt/bin/wat2wasm"
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elif command -v wat2wasm &> /dev/null; then
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WAT2WASM="wat2wasm"
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else
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echo "Error: wat2wasm not found"
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echo "Please install wabt tools"
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exit 1
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fi
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# Compile WAT to WASM if needed
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if [[ ! -f stress_registers.wasm ]] || [[ stress_registers.wat -nt stress_registers.wasm ]]; then
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echo "Compiling stress_registers.wat to WASM..."
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if ! ${WAT2WASM} stress_registers.wat -o stress_registers.wasm; then
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echo "Error: Failed to compile WAT to WASM"
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exit 1
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fi
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fi
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if [[ $1 != "--aot" ]]; then
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echo "============> run stress_registers.wasm (interpreter mode)"
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echo "Running 1000 iterations in interpreter mode..."
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for i in $(seq 1 1000); do
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if ! ${IWASM_CMD} stress_registers.wasm 2>&1; then
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echo "FAILED: Crash at iteration $i"
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exit 1
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fi
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done
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echo "PASSED: 1000 iterations completed without crash"
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else
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echo "============> compile stress_registers.wasm to AOT"
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# Compile to AOT - the fix should add +reserve-x18 automatically on macOS ARM64
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if ! ${WAMRC_CMD} --opt-level=3 -o stress_registers.aot stress_registers.wasm; then
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echo "Error: Failed to compile WASM to AOT"
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exit 1
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fi
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echo "============> run stress_registers.aot"
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echo "Running 1000 iterations to verify x18 is properly reserved..."
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echo "(Without the fix, this would crash ~80% of the time)"
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failed=0
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for i in $(seq 1 1000); do
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if ! ${IWASM_CMD} stress_registers.aot 2>&1 > /dev/null; then
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echo "FAILED: Crash at iteration $i"
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failed=1
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break
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fi
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# Progress indicator every 100 iterations
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if [[ $((i % 100)) -eq 0 ]]; then
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echo " Progress: $i/1000 iterations completed"
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fi
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done
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if [[ ${failed} -eq 0 ]]; then
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echo "PASSED: 1000 iterations completed without crash"
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echo "The +reserve-x18 fix is working correctly"
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exit 0
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else
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echo "FAILED: x18 register corruption detected"
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echo "The +reserve-x18 fix may not be applied correctly"
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exit 1
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fi
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fi

‎tests/standalone/test-aot-x18-reserve/stress_registers.wat‎

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;; Copyright (C) 2019 Intel Corporation. All rights reserved.
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;; SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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;; Test module that uses many local variables to stress register allocation.
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;; On ARM64, this will force LLVM to use x18 register if +reserve-x18 is not set.
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;; x18 is reserved by Apple on macOS for TLS, so using it causes crashes.
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(module
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(memory (export "memory") 1)
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(func $stress_registers (export "stress_registers") (param $input i64) (result i64)
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(local $a i64) (local $b i64) (local $c i64) (local $d i64)
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(local $e i64) (local $f i64) (local $g i64) (local $h i64)
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(local $i i64) (local $j i64) (local $k i64) (local $l i64)
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(local $m i64) (local $n i64) (local $o i64) (local $p i64)
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(local $q i64) (local $r i64) (local $s i64) (local $t i64)
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(local $u i64) (local $v i64) (local $w i64) (local $x i64)
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;; Initialize all locals with different values based on input
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(local.set $a (i64.add (local.get $input) (i64.const 1)))
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(local.set $b (i64.mul (local.get $a) (i64.const 2)))
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(local.set $c (i64.add (local.get $b) (i64.const 3)))
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(local.set $d (i64.mul (local.get $c) (i64.const 4)))
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(local.set $e (i64.add (local.get $d) (i64.const 5)))
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(local.set $f (i64.mul (local.get $e) (i64.const 6)))
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(local.set $g (i64.add (local.get $f) (i64.const 7)))
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(local.set $h (i64.mul (local.get $g) (i64.const 8)))
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(local.set $i (i64.add (local.get $h) (i64.const 9)))
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(local.set $j (i64.mul (local.get $i) (i64.const 10)))
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(local.set $k (i64.add (local.get $j) (i64.const 11)))
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(local.set $l (i64.mul (local.get $k) (i64.const 12)))
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(local.set $m (i64.add (local.get $l) (i64.const 13)))
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(local.set $n (i64.mul (local.get $m) (i64.const 14)))
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(local.set $o (i64.add (local.get $n) (i64.const 15)))
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(local.set $p (i64.mul (local.get $o) (i64.const 16)))
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(local.set $q (i64.add (local.get $p) (i64.const 17)))
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(local.set $r (i64.mul (local.get $q) (i64.const 18)))
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(local.set $s (i64.add (local.get $r) (i64.const 19)))
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(local.set $t (i64.mul (local.get $s) (i64.const 20)))
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(local.set $u (i64.add (local.get $t) (i64.const 21)))
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(local.set $v (i64.mul (local.get $u) (i64.const 22)))
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(local.set $w (i64.add (local.get $v) (i64.const 23)))
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(local.set $x (i64.mul (local.get $w) (i64.const 24)))
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;; Now use all of them together to prevent optimization
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(i64.add
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(i64.add
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(i64.add
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(i64.add
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(i64.add
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(i64.add
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(i64.add
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(i64.add
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(i64.add
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(i64.add
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(i64.add
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(i64.add
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(local.get $a)
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(local.get $b))
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(local.get $c))
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(local.get $d))
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(local.get $e))
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(local.get $f))
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(local.get $g))
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(local.get $h))
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(local.get $i))
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(local.get $j))
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(local.get $k))
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(local.get $l))
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(i64.add
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(i64.add
72+
(i64.add
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(i64.add
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(i64.add
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(i64.add
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(i64.add
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(i64.add
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(i64.add
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(i64.add
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(i64.add
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(local.get $m)
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(local.get $n))
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(local.get $o))
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(local.get $p))
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(local.get $q))
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(local.get $r))
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(local.get $s))
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(local.get $t))
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(local.get $u))
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(local.get $v))
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(local.get $w))
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(local.get $x))))
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(func $_start (export "_start")
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(drop (call $stress_registers (i64.const 42)))
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)
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)

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