lto=thin is clang-only, even in Ubuntu 24.x, and we only really care about LTO for the binary bits that end up in the app package itself anyway. gradefully degrade to generic LTO when clang isn't detected on the system. when clang is detected, make sure we use it for generating the ICU host-run codegen

Author: matthargett

scripts/start.sh

@@ -48,12 +48,30 @@ patchAndMakeICU() {
   mkdir -p $TARGETDIR/icu/host
   cd $TARGETDIR/icu/host
 
+  local HAS_CLANG=0
+  if command -v clang >/dev/null 2>&1 && command -v clang++ >/dev/null 2>&1; then
+    HAS_CLANG=1
+  fi
+
   if [[ "$BUILD_TYPE" = "Release" ]]
   then
-    local OPT_FLAGS="-O2 -flto=thin -Wno-pass-failed=loop-vectorize"
+    local LTO_FLAG
+    local EXTRA_FLAGS=""
+    if [[ $HAS_CLANG -eq 1 ]]; then
+      LTO_FLAG="-flto=thin"
+      EXTRA_FLAGS="-Wno-pass-failed=loop-vectorize"
+    else
+      LTO_FLAG="-flto"
+    fi
+
+    local OPT_FLAGS="-O2 $LTO_FLAG"
+    if [[ -n "$EXTRA_FLAGS" ]]; then
+      OPT_FLAGS="$OPT_FLAGS $EXTRA_FLAGS"
+    fi
+
     CFLAGS="$OPT_FLAGS"
     CXXFLAGS="-std=c++20 $OPT_FLAGS"
-    LDFLAGS="-flto=thin"
+    LDFLAGS="$LTO_FLAG"
   else
     CFLAGS="-g2"
     CXXFLAGS="-std=c++20"
@@ -65,6 +83,9 @@ patchAndMakeICU() {
   if [[ -n "$LDFLAGS" ]]; then
     CONFIG_ENV+=("LDFLAGS=$LDFLAGS")
   fi
+  if [[ $HAS_CLANG -eq 1 ]]; then
+    CONFIG_ENV+=("CC=clang" "CXX=clang++")
+  fi
 
   if [[ -f "$ICU_FILTER_FILE" ]]; then
     printf "Using ICU data filter: %s\n" "$ICU_FILTER_FILE"