Reland "[pkg/vm] Handle cross compilation in the precompiler2 script."

This is a reland of commit 11a3380

Adds "aarch64" as a `uname -m` possibility for ARM64.

TEST=manual testing

Original change's description:
> [pkg/vm] Handle cross compilation in the precompiler2 script.
>
> When cross compiling, gen_snapshot for the host architecture is in
> ${BUILD_DIR}/clang_<arch>/gen_snapshot.
>
> TEST=manual testing with native X64 and cross-compiling ARM64 on X64.
>
> Change-Id: I9f6af045675612651c659683081aca3e65acad1b
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/404421
> Reviewed-by: Slava Egorov <[email protected]>

Change-Id: I4fb4812dfa96ba582ca3bedbfd68d58ce2a4ab5b
Cq-Include-Trybots: luci.dart.try:benchmark-linux-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/405780
Reviewed-by: Slava Egorov <[email protected]>

Author: sstrickl

pkg/vm/tool/precompiler2

@@ -15,7 +15,7 @@
 set -e
 
 function follow_links() {
-  file="$1"
+  local file="$1"
   while [ -h "$file" ]; do
     # On Mac OS, readlink -f doesn't work.
     file="$(readlink "$file")"
@@ -31,6 +31,8 @@ CUR_DIR="$(cd "${PROG_NAME%/*}" ; pwd -P)"
 
 SDK_DIR="$CUR_DIR/../../.."
 
+. "$CUR_DIR/shared_functions.sh"
+
 OPTIONS=()
 GEN_KERNEL_OPTIONS=()
 PACKAGES=
@@ -98,19 +100,37 @@ else
   OUT_DIR="$SDK_DIR/out"
 fi
 
-HOST_ARCH="X64"
-if [[ `uname -m` == 'arm64' ]]; then
-  HOST_ARCH="ARM64"
-fi
+HOST_ARCH="$(host_arch)"
 
 export DART_CONFIGURATION=${DART_CONFIGURATION:-Release$HOST_ARCH}
+TARGET_ARCH="$(parse_target_arch "$DART_CONFIGURATION")"
 BUILD_DIR="$OUT_DIR/$DART_CONFIGURATION"
-
-DART="${SDK_DIR}/tools/sdks/dart-sdk/bin/dart"
-if [ ! -f "$DART" ]; then
-  DART="$BUILD_DIR/dart"
+if [ ! -d "$BUILD_DIR" ]; then
+  echo "$BUILD_DIR is not a directory" >&2
+  exit 1
 fi
 
+function find_dart {
+  local tools_dart="${SDK_DIR}/tools/sdks/dart-sdk/bin/dart"
+  if [ -f "$tools_dart" ]; then
+    echo "$tools_dart"
+  else
+    local host_dart=""
+    if is_cross_compiled "$HOST_ARCH" "$TARGET_ARCH"; then
+      host_dart="$OUT_DIR/Release$HOST_ARCH/dart"
+    else
+      host_dart="$BUILD_DIR/dart"
+    fi
+    if [ ! -f "$host_dart" ]; then
+      echo "Cannot find dart at either $tools_dart or $host_dart" >&2
+      exit 1
+    fi
+    echo "$host_dart"
+  fi
+}
+
+DART="$(find_dart)"
+
 function gen_kernel() {
   if [[ "$DART_GN_ARGS" == *"precompile_tools=true"* ]]; then
     # Precompile gen_kernel to an AOT app.
@@ -129,10 +149,29 @@ gen_kernel --platform "${BUILD_DIR}/vm_platform_strong.dill"                   \
      -o "$SNAPSHOT_FILE.dill"                                                  \
      "$SOURCE_FILE"
 
-
+GEN_SNAPSHOT="gen_snapshot"
+# If the target architecture differs from the host architecture, use
+# gen_snapshot for the host architecture for cross compilation, which is
+# denoted by an X between the build mode and the architecture name,
+# e.g., ReleaseXARM64 on X64.
+if [[ "$DART_CONFIGURATION" =~ (Debug|Release|Product)X[^6] ]] && \
+      is_cross_compiled "$HOST_ARCH" "$TARGET_ARCH"; then
+  case "$HOST_ARCH" in
+    "X64")
+    GEN_SNAPSHOT="clang_x64/${GEN_SNAPSHOT}"
+    ;;
+    "ARM64")
+    GEN_SNAPSHOT="clang_arm64/${GEN_SNAPSHOT}"
+    ;;
+    *)
+    echo "Unexpected host architecture $HOST_ARCH for cross compilation" >&2
+    exit 1
+    ;;
+  esac
+fi
 
 # Step 2: Generate snapshot from the Kernel binary.
-"$BUILD_DIR"/gen_snapshot                                                      \
+${BUILD_DIR}/${GEN_SNAPSHOT}                                                   \
      ${GEN_SNAPSHOT_FLAGS}                                                     \
      "$GEN_SNAPSHOT_OPTION"                                                    \
      "$GEN_SNAPSHOT_FILENAME"                                                  \

pkg/vm/tool/shared_functions.sh

@@ -0,0 +1,59 @@
+# Copyright (c) 2025, 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.
+
+# Shared functions for both precompiler2 and dart_precompiled_runtime2.
+
+function host_arch() {
+  # Use uname to determine the host architecture.
+  case `uname -m` in
+    x86_64)
+    echo "X64"
+    ;;
+    aarch64 | arm64 | armv8*)
+    echo "ARM64"
+    ;;
+    arm | arm7*)
+    echo "ARM"
+    ;;
+    *)
+    echo "Unknown host architecture" `uname -m` >&2
+    exit 1
+    ;;
+  esac
+}
+
+function parse_target_arch() {
+  case "$1" in
+  *X64)
+  echo "X64"
+  ;;
+  *ARM64 | \
+  *ARM64C)
+  echo "ARM64"
+  ;;
+  *ARM)
+  echo "ARM"
+  ;;
+  *IA32)
+  echo "IA32"
+  ;;
+  *RISCV32)
+  echo "RISCV32"
+  ;;
+  *RISCV64)
+  echo "RISCV64"
+  ;;
+  *)
+  echo "Cannot deduce target architecture from $1" >&2
+  exit 1
+  ;;
+  esac
+}
+
+function is_cross_compiled() {
+  local host_arch="$1"
+  local target_arch="$2"
+  [[ ("$host_arch" != "ARM64" || "$target_arch" != "ARM") && \
+    ("$host_arch" != "$target_arch") ]]
+}