diff --git a/.github/workflows/shellcheck.yml b/.github/workflows/shellcheck.yml
index 16f20f5..6945985 100644
--- a/.github/workflows/shellcheck.yml
+++ b/.github/workflows/shellcheck.yml
@@ -11,6 +11,6 @@ jobs:
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
       - name: Run shellcheck
-        run: shellcheck cc.sh test/run.sh
+        run: shellcheck cc.sh test/*.sh benchmark/*.sh
       - name: Run tests
         run: sh test/run.sh
diff --git a/benchmark/benchmark.sh b/benchmark/benchmark.sh
new file mode 100755
index 0000000..7079590
--- /dev/null
+++ b/benchmark/benchmark.sh
@@ -0,0 +1,109 @@
+#!/bin/sh
+
+SCRIPT_DIR=$(CDPATH='' cd -- "$(dirname -- "$0")" && pwd)
+REPO_DIR=$(CDPATH='' cd -- "$SCRIPT_DIR/.." && pwd)
+CC_SH="$REPO_DIR/cc.sh"
+
+if [ ! -f "$CC_SH" ]; then
+    echo "Cannot find cc.sh at $CC_SH" >&2
+    exit 1
+fi
+
+export SPACK_COMPILER_WRAPPER_PATH="$SCRIPT_DIR"
+export SPACK_DEBUG_LOG_DIR="/tmp"
+export SPACK_DEBUG_LOG_ID="bench"
+export SPACK_SHORT_SPEC="py-torch@2.11.0%gcc@14.2.0 arch=linux-ubuntu24.04-aarch64"
+export SPACK_SYSTEM_DIRS="/usr/*|/lib/*"
+export SPACK_CXX="true"
+export SPACK_CC="true"
+export SPACK_FC="true"
+export SPACK_F77="true"
+export SPACK_CC_LINKER_ARG="-Wl,"
+export SPACK_CC_RPATH_ARG="-Wl,-rpath,"
+export SPACK_CXX_LINKER_ARG="-Wl,"
+export SPACK_CXX_RPATH_ARG="-Wl,-rpath,"
+export SPACK_FC_LINKER_ARG="-Wl,"
+export SPACK_FC_RPATH_ARG="-Wl,-rpath,"
+export SPACK_F77_LINKER_ARG="-Wl,"
+export SPACK_F77_RPATH_ARG="-Wl,-rpath,"
+export SPACK_CXX_HAS_FRANDOM_SEED=true
+export SPACK_DTAGS_TO_ADD="--enable-new-dtags"
+export SPACK_DTAGS_TO_STRIP="--disable-new-dtags"
+
+_P="/home/software/spack/__spack_path_placeholder__/__spack_path_placeholder__/__spack_path_placeholder__/__spack_path_placeholder__/__spack_path_placeholder__/__spack_path_placeholder__/__spack_path_placeholder__/__spack_path_placeholder__/__spack_path_placeh/linux-aarch64"
+
+export SPACK_MANAGED_DIRS="$_P/*"
+
+# Hash generator: produces a fake 32-char hex from an index.
+_h() {
+    printf '%032x' "$1"
+}
+
+# Build large dependency lists to stress the wrapper.
+_store_pkgs="python-3.13.2 py-numpy-2.4.1 py-setuptools-80.7.1 py-pyyaml-6.0.2 openblas-0.3.33 cuda-12.9.1 py-pip-24.2 py-wheel-0.43.0 py-cython-3.0.11 py-pybind11-2.13.6 py-typing-extensions-4.12.2 py-sympy-1.13.3 py-mpmath-1.3.0 py-filelock-3.16.1 py-jinja2-3.1.4 py-markupsafe-2.1.5 py-networkx-3.4.2 py-fsspec-2024.10.0 py-packaging-24.1 py-six-1.16.0 zlib-ng-2.2.2 bzip2-1.0.8 xz-5.4.6 zstd-1.5.6 openssl-3.3.2 sqlite-3.46.1 ncurses-6.5 readline-8.2 libffi-3.4.6 expat-2.6.4 gdbm-1.24 util-linux-uuid-2.40.2 tar-1.35 gettext-0.22.5 libiconv-1.17 pcre2-10.44 ca-certificates-2024 cmake-3.31.0 ninja-1.12.1"
+
+_dep_pkgs="libxfixes-6.0.1 libxdamage-1.1.6 libxshmfence-1.3.2 libxxf86vm-1.1.5 libxcursor-1.2.3 libxcomposite-0.4.6 libxinerama-1.1.5 libxrandr-1.5.4 libxrender-0.9.11 libxext-1.3.6 libxi-1.8.2 libxtst-1.2.5 libxkbcommon-1.7.0 libxkbfile-1.1.3 libxmu-1.2.1 libxt-1.3.0 libxaw-1.0.16 libxpm-3.5.17 libxft-2.3.8 libxss-1.2.4 libx11-1.8.10 libxcb-1.17.0 libxau-1.0.11 libxdmcp-1.1.5 libxres-1.2.2 libxv-1.0.12 libxvmc-1.0.14 libpciaccess-0.18.1 libdrm-2.4.123 mesa-24.2.5 libglvnd-1.7.0 libglx-1.7.0 libegl-1.7.0 libgles-1.7.0 wayland-1.23.1 xorgproto-2024.1 xtrans-1.5.1 fontconfig-2.15.0 freetype-2.13.3 harfbuzz-10.0.1 libpng-1.6.44 libjpeg-turbo-3.0.4 libtiff-4.7.0 libwebp-1.4.0 giflib-5.2.2 librsvg-2.59.0 cairo-1.18.2 pixman-0.43.4 pango-1.54.0 fribidi-1.0.16 graphite2-1.3.14"
+
+_idx=0
+_store_include=""
+_store_link=""
+for _pkg in $_store_pkgs; do
+    _hash=$(_h $_idx)
+    _store_include="$_store_include$_P/$_pkg-$_hash/include:"
+    _store_link="$_store_link$_P/$_pkg-$_hash/lib:"
+    _idx=$((_idx + 1))
+done
+
+_dep_include=""
+_dep_link=""
+for _pkg in $_dep_pkgs; do
+    _hash=$(_h $_idx)
+    _dep_include="$_dep_include$_P/$_pkg-$_hash/include:"
+    _dep_link="$_dep_link$_P/$_pkg-$_hash/lib:"
+    _idx=$((_idx + 1))
+done
+
+export SPACK_STORE_INCLUDE_DIRS="${_store_include%:}"
+export SPACK_STORE_LINK_DIRS="${_store_link%:}"
+export SPACK_STORE_RPATH_DIRS="$SPACK_STORE_LINK_DIRS"
+
+export SPACK_INCLUDE_DIRS="${_dep_include%:}"
+export SPACK_LINK_DIRS="${_dep_link%:}"
+export SPACK_RPATH_DIRS="$SPACK_LINK_DIRS"
+
+export SPACK_COMPILER_IMPLICIT_RPATHS="\
+$_P/gcc-16.1.0-a30491defb3c2a8a14f43c8c93ec2ef2/lib"
+
+export SPACK_COMPILER_EXTRA_RPATHS="\
+$_P/gcc-runtime-stuff-16.1.0-6bce6552b41bbeed94e4faee72dcbe96/lib"
+
+export SPACK_CFLAGS=""
+export SPACK_CXXFLAGS=""
+export SPACK_FFLAGS=""
+export SPACK_CPPFLAGS=""
+export SPACK_LDFLAGS=""
+export SPACK_LDLIBS=""
+export SPACK_ALWAYS_CFLAGS=""
+export SPACK_ALWAYS_CXXFLAGS=""
+export SPACK_ALWAYS_CPPFLAGS=""
+export SPACK_ALWAYS_FFLAGS=""
+export SPACK_TARGET_ARGS_CC=""
+export SPACK_TARGET_ARGS_CXX=""
+export SPACK_TARGET_ARGS_FORTRAN=""
+export SPACK_COMPILER_FLAGS_KEEP=""
+export SPACK_COMPILER_FLAGS_REPLACE=""
+
+# Warmup
+"$SCRIPT_DIR/g++" -c foo.c -o foo.o
+
+# Do a 1000 runs
+N=1000
+_start=$(date +%s)
+_i=0
+while [ $_i -lt $N ]; do
+    "$SCRIPT_DIR/g++" -c foo.c -o foo.o
+    _i=$((_i + 1))
+done
+_end=$(date +%s)
+_elapsed=$((_end - _start))
+printf '%s runs in %ss (avg %sms)\n' "$N" "$_elapsed" "$((_elapsed * 1000 / N))"
diff --git a/benchmark/g++ b/benchmark/g++
new file mode 120000
index 0000000..ca40160
--- /dev/null
+++ b/benchmark/g++
@@ -0,0 +1 @@
+../cc.sh
\ No newline at end of file
diff --git a/cc.sh b/cc.sh
index 4156eed..1523379 100755
--- a/cc.sh
+++ b/cc.sh
@@ -24,6 +24,8 @@
 # other separators, we set and reset it.
 unset IFS
 
+# BEGIN list functions
+
 # Separator for lists whose names end with `_list`.
 # We pick the alarm bell character, which is highly unlikely to
 # conflict with anything. This is a literal bell character (which
@@ -94,23 +96,6 @@ setsep() {
     esac
 }
 
-# prepend LISTNAME ELEMENT
-#
-# Prepend ELEMENT to the list stored in the variable LISTNAME.
-# Handles empty lists and single-element lists.
-prepend() {
-    varname="$1"
-    elt="$2"
-
-    if empty "$varname"; then
-        eval "$varname=\"\${elt}\""
-    else
-        # Get the appropriate separator for the list we're appending to.
-        setsep "$varname"
-        eval "$varname=\"\${elt}${sep}\${$varname}\""
-    fi
-}
-
 # append LISTNAME ELEMENT [SEP]
 #
 # Append ELEMENT to the list stored in the variable LISTNAME,
@@ -129,43 +114,76 @@ append() {
     fi
 }
 
-# extend LISTNAME1 LISTNAME2 [PREFIX]
+# extend DST_LISTNAME SRC_LISTNAME [PREFIX]
 #
-# Append the elements stored in the variable LISTNAME2
-# to the list stored in LISTNAME1.
+# Append the elements stored in the variable SRC_LISTNAME
+# to the list stored in DST_LISTNAME.
 # If PREFIX is provided, prepend it to each element.
 extend() {
-    # Figure out the appropriate IFS for the list we're reading.
-    setsep "$2"
-    if [ "$sep" != " " ]; then
-        IFS="$sep"
-    fi
-    eval "for elt in \${$2}; do append $1 \"$3\${elt}\"; done"
+    _dst="$1"
+    _src="$2"
+    _prefix="$3"
+
+    # Turn source list into positional parameters
+    setsep "$_src"
+    [ "$sep" != " " ] && IFS="$sep"
+    eval "set -- \${$_src}"
     unset IFS
+
+    [ $# -eq 0 ] && return
+
+    setsep "$_dst"; _dst_sep="$sep"
+
+    if [ -z "$_prefix" ]; then
+        # Fast concatenation when no prefix is needed
+        IFS="$_dst_sep"; _ext_str="$*"; unset IFS
+    else
+        _ext_str="${_prefix}$1"
+        shift
+        for elt; do
+            _ext_str="${_ext_str}${_dst_sep}${_prefix}${elt}"
+        done
+    fi
+
+    eval "$_dst=\"\${$_dst:+\${$_dst}$_dst_sep}\${_ext_str}\""
 }
 
-# preextend LISTNAME1 LISTNAME2 [PREFIX]
+# preextend DST_LISTNAME SRC_LISTNAME [PREFIX]
 #
-# Prepend the elements stored in the list at LISTNAME2
-# to the list at LISTNAME1, preserving order.
+# Prepend the elements stored in the list at SRC_LISTNAME
+# to the list at DST_LISTNAME, preserving order.
 # If PREFIX is provided, prepend it to each element.
 preextend() {
-    # Figure out the appropriate IFS for the list we're reading.
-    setsep "$2"
-    if [ "$sep" != " " ]; then
-        IFS="$sep"
-    fi
+    _dst="$1"
+    _src="$2"
+    _prefix="$3"
+
+    # Turn source list into positional parameters
+    setsep "$_src"
+    [ "$sep" != " " ] && IFS="$sep"
+    eval "set -- \${$_src}"
+    unset IFS
 
-    # first, reverse the list to prepend
-    _reversed_list=""
-    eval "for elt in \${$2}; do prepend _reversed_list \"$3\${elt}\"; done"
+    [ $# -eq 0 ] && return
 
-    # prepend reversed list to preextend in order
-    IFS="${lsep}"
-    for elt in $_reversed_list; do prepend "$1" "$3${elt}"; done
-    unset IFS
+    setsep "$_dst"; _dst_sep="$sep"
+
+    if [ -z "$_prefix" ]; then
+        # Fast concatenation when no prefix is needed
+        IFS="$_dst_sep"; _ext_str="$*"; unset IFS
+    else
+        _ext_str="${_prefix}$1"
+        shift
+        for elt; do
+            _ext_str="${_ext_str}${_dst_sep}${_prefix}${elt}"
+        done
+    fi
+
+    eval "$_dst=\"\${_ext_str}\${$_dst:+$_dst_sep\${$_dst}}\""
 }
 
+# END list functions
+
 execute() {
     # dump the full command if the caller supplies SPACK_TEST_COMMAND=dump-args
     if [ -n "${SPACK_TEST_COMMAND=}" ]; then
@@ -977,7 +995,7 @@ esac
 if [ -n "$SPACK_CCACHE_BINARY" ]; then
     case "$lang_flags" in
         C|CXX)  # ccache only supports C languages
-            prepend full_command_list "${SPACK_CCACHE_BINARY}"
+            full_command_list="${SPACK_CCACHE_BINARY}${lsep}${full_command_list}"
             # workaround for stage being a temp folder
             # see #3761#issuecomment-294352232
             export CCACHE_NOHASHDIR=yes
diff --git a/test/run.sh b/test/run.sh
old mode 100644
new mode 100755
index a59b0a9..3edb659
--- a/test/run.sh
+++ b/test/run.sh
@@ -1,10 +1,13 @@
 #!/bin/sh
+# shellcheck disable=SC2034  # vars are passed by name to functions sourced from cc.sh
+# shellcheck disable=SC2154  # lsep/sep are defined by the sourced cc.sh block
 #
 # Copyright Spack Project Developers. See COPYRIGHT file for details.
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 #
 # Exercises ../cc.sh via SPACK_TEST_COMMAND=dump-args/dump-mode/dump-env-<VAR>.
+# Also unit-tests the list-manipulation primitives extracted from cc.sh.
 #
 # Run all tests:        sh test/run.sh
 # Run a single test:    sh test/run.sh test_modes
@@ -21,12 +24,21 @@ if [ ! -f "$CC_SH" ]; then
 fi
 
 WRAPPER_DIR=$(mktemp -d)
-trap 'rm -rf "$WRAPPER_DIR"' EXIT INT TERM
 
 for name in cc c++ cpp fc ld; do
     ln -s "$CC_SH" "$WRAPPER_DIR/$name"
 done
 
+# Extract list-manipulation functions from cc.sh for unit testing.
+FUNCS_SH=$(mktemp)
+trap 'rm -rf "$WRAPPER_DIR" "$FUNCS_SH"' EXIT INT TERM
+
+awk '/^# BEGIN list functions$/{flag=1; next} /^# END list functions$/{flag=0} flag' \
+    "$CC_SH" > "$FUNCS_SH"
+
+# shellcheck disable=SC1090
+. "$FUNCS_SH"
+
 REAL_CC=/bin/mycc
 
 # ---------------------------------------------------------------------------
@@ -132,6 +144,32 @@ expect_not_contains() {
     fi
 }
 
+# expect_eq LABEL ACTUAL EXPECTED
+expect_eq() {
+    if [ "$2" != "$3" ]; then
+        # Render the bell separator visibly in error output.
+        _exp=$(printf '%s' "$3" | tr "$lsep" '|')
+        _act=$(printf '%s' "$2" | tr "$lsep" '|')
+        fail "$1: expected '$_exp', got '$_act'"
+    fi
+}
+
+# expect_true LABEL CMD...
+expect_true() {
+    _label="$1"; shift
+    if ! "$@"; then
+        fail "$_label: expected success, got failure"
+    fi
+}
+
+# expect_false LABEL CMD...
+expect_false() {
+    _label="$1"; shift
+    if "$@"; then
+        fail "$_label: expected failure, got success"
+    fi
+}
+
 # -------------------
 # Wrapper environment
 # -------------------
@@ -1056,11 +1094,202 @@ hello.c')
     expect_contains frandom_seed_user_passthrough "$_out" '-frandom-seed=custom'
 }
 
+# ---------------------------------------------------------------------------
+# List-ops unit tests (set +u: the sourced list functions use optional $3)
+# ---------------------------------------------------------------------------
+
+test_empty() {
+    unset tvar || true
+    expect_true  empty_unset empty tvar
+    tvar=''
+    expect_true  empty_empty empty tvar
+    tvar='x'
+    expect_false empty_nonempty empty tvar
+    tvar=' '
+    expect_false empty_space empty tvar
+    unset tvar
+}
+
+test_setsep() {
+    setsep foo_dirs;  expect_eq setsep_dirs   "$sep" ':'
+    setsep FOO_DIRS;  expect_eq setsep_DIRS   "$sep" ':'
+    setsep MYPATH;    expect_eq setsep_PATH   "$sep" ':'
+    setsep MYPATHS;   expect_eq setsep_PATHS  "$sep" ':'
+    setsep foo_list;  expect_eq setsep_list   "$sep" "$lsep"
+    setsep whatever;  expect_eq setsep_other  "$sep" ' '
+}
+
+test_append() {
+    # _list (lsep separator)
+    tgt_list=''
+    append tgt_list a
+    expect_eq append_list_empty   "$tgt_list" "a"
+    append tgt_list b
+    expect_eq append_list_two     "$tgt_list" "a${lsep}b"
+    append tgt_list c
+    expect_eq append_list_three   "$tgt_list" "a${lsep}b${lsep}c"
+
+    # _dirs (colon separator)
+    tgt_dirs=''
+    append tgt_dirs /a
+    append tgt_dirs /b
+    expect_eq append_dirs         "$tgt_dirs" "/a:/b"
+
+    # default (space separator)
+    tgt_other=''
+    append tgt_other x
+    append tgt_other y
+    expect_eq append_default      "$tgt_other" "x y"
+}
+
+test_extend_empty_source() {
+    src_list=''
+    tgt_list='existing'
+    extend tgt_list src_list
+    expect_eq extend_empty_src "$tgt_list" 'existing'
+}
+
+test_extend_single_into_empty() {
+    src_list='a'
+    tgt_list=''
+    extend tgt_list src_list
+    expect_eq extend_single "$tgt_list" 'a'
+}
+
+test_extend_multi_into_empty() {
+    src_list="a${lsep}b${lsep}c"
+    tgt_list=''
+    extend tgt_list src_list
+    expect_eq extend_multi_empty "$tgt_list" "a${lsep}b${lsep}c"
+}
+
+test_extend_multi_into_nonempty() {
+    src_list="b${lsep}c"
+    tgt_list='a'
+    extend tgt_list src_list
+    expect_eq extend_multi_nonempty "$tgt_list" "a${lsep}b${lsep}c"
+}
+
+test_extend_prefix() {
+    src_list="b${lsep}c"
+    tgt_list='a'
+    extend tgt_list src_list '-I'
+    expect_eq extend_prefix "$tgt_list" "a${lsep}-Ib${lsep}-Ic"
+}
+
+test_extend_cross_separator() {
+    # Source uses ':' (dirs), target uses lsep (list).
+    src_dirs='a:b:c'
+    tgt_list=''
+    extend tgt_list src_dirs
+    expect_eq extend_dirs_to_list_empty "$tgt_list" "a${lsep}b${lsep}c"
+
+    tgt_list='x'
+    extend tgt_list src_dirs '-L'
+    expect_eq extend_dirs_to_list_nonempty "$tgt_list" "x${lsep}-La${lsep}-Lb${lsep}-Lc"
+}
+
+test_extend_default_separator() {
+    src_other='a b c'
+    tgt_other='x'
+    extend tgt_other src_other
+    expect_eq extend_default_sep "$tgt_other" 'x a b c'
+}
+
+test_preextend_empty_source() {
+    src_list=''
+    tgt_list='existing'
+    preextend tgt_list src_list
+    expect_eq preextend_empty_src "$tgt_list" 'existing'
+}
+
+test_preextend_single_into_empty() {
+    src_list='a'
+    tgt_list=''
+    preextend tgt_list src_list
+    expect_eq preextend_single "$tgt_list" 'a'
+}
+
+test_preextend_multi_into_empty() {
+    # The original reversed-prepend logic existed to preserve source order.
+    src_list="a${lsep}b${lsep}c"
+    tgt_list=''
+    preextend tgt_list src_list
+    expect_eq preextend_multi_empty "$tgt_list" "a${lsep}b${lsep}c"
+}
+
+test_preextend_multi_into_nonempty() {
+    src_list="a${lsep}b"
+    tgt_list="c${lsep}d"
+    preextend tgt_list src_list
+    expect_eq preextend_multi_nonempty "$tgt_list" "a${lsep}b${lsep}c${lsep}d"
+}
+
+test_preextend_prefix() {
+    src_list="a${lsep}b"
+    tgt_list='c'
+    preextend tgt_list src_list '-I'
+    expect_eq preextend_prefix "$tgt_list" "-Ia${lsep}-Ib${lsep}c"
+}
+
+test_preextend_cross_separator() {
+    src_dirs='a:b:c'
+    tgt_list='x'
+    preextend tgt_list src_dirs
+    expect_eq preextend_dirs_to_list "$tgt_list" "a${lsep}b${lsep}c${lsep}x"
+}
+
+test_lsep_prepend_pattern() {
+    # The inline replacement for the old prepend helper, as used at
+    # 'full_command_list="${SPACK_CCACHE_BINARY}${lsep}${full_command_list}"'.
+    tgt_list=''
+    append tgt_list compiler
+    append tgt_list -O2
+    tgt_list="ccache${lsep}${tgt_list}"
+
+    IFS="$lsep"
+    # shellcheck disable=SC2086
+    set -- $tgt_list
+    unset IFS
+    expect_eq prepend_pattern_count "$#" 3
+    expect_eq prepend_pattern_first "$1" 'ccache'
+    expect_eq prepend_pattern_mid   "$2" 'compiler'
+    expect_eq prepend_pattern_last  "$3" '-O2'
+}
+
 # ---------------------------------------------------------------------------
 # Runner
 # ---------------------------------------------------------------------------
 
-all_tests='
+# List-ops tests need set +u because the sourced functions use optional $3.
+list_ops_tests='
+test_empty
+test_setsep
+test_append
+test_extend_empty_source
+test_extend_single_into_empty
+test_extend_multi_into_empty
+test_extend_multi_into_nonempty
+test_extend_prefix
+test_extend_cross_separator
+test_extend_default_separator
+test_preextend_empty_source
+test_preextend_single_into_empty
+test_preextend_multi_into_empty
+test_preextend_multi_into_nonempty
+test_preextend_prefix
+test_preextend_cross_separator
+test_lsep_prepend_pattern
+'
+
+is_list_ops_test() {
+    case "$list_ops_tests" in
+        *"$1"*) return 0 ;;
+        *) return 1 ;;
+    esac
+}
+
+wrapper_tests='
 test_no_wrapper_environment
 test_modes
 test_expected_args
@@ -1078,6 +1307,8 @@ test_frandom_seed_not_added_without_env
 test_frandom_seed_filters_args
 '
 
+all_tests="$wrapper_tests $list_ops_tests"
+
 if [ $# -gt 0 ]; then
     tests_to_run="$*"
 else
@@ -1086,7 +1317,11 @@ fi
 
 for t in $tests_to_run; do
     start_test "$t"
-    "$t"
+    if is_list_ops_test "$t"; then
+        set +u; "$t"; set -u
+    else
+        "$t"
+    fi
     end_test
 done