diff --git a/src/tools/miri/ci/ci.sh b/src/tools/miri/ci/ci.sh
index cccf10a7d703..8827cb2b60e9 100755
--- a/src/tools/miri/ci/ci.sh
+++ b/src/tools/miri/ci/ci.sh
@@ -44,7 +44,12 @@ export CARGO_EXTRA_FLAGS="$CARGO_EXTRA_FLAGS --all-features"
 
 endgroup
 
-# Test
+# Run tests. Recognizes these variables:
+# - MIRI_TEST_TARGET: the target to test. Empty for host target.
+# - GC_STRESS: if non-empty, run the GC stress test for the main test suite.
+# - MIR_OPT: if non-empty, re-run test `pass` tests with mir-opt-level=4
+# - MANY_SEEDS: if set to N, run the "many-seeds" tests N times
+# - TEST_BENCH: if non-empty, check that the benchmarks all build
 function run_tests {
   if [ -n "${MIRI_TEST_TARGET:-}" ]; then
     begingroup "Testing foreign architecture $MIRI_TEST_TARGET"
@@ -53,18 +58,14 @@ function run_tests {
   fi
 
   ## ui test suite
-  # On the host, also stress-test the GC.
-  if [ -z "${MIRI_TEST_TARGET:-}" ]; then
+  if [ -n "${GC_STRESS:-}" ]; then
     MIRIFLAGS="${MIRIFLAGS:-} -Zmiri-provenance-gc=1" ./miri test
   else
     ./miri test
   fi
 
-  # Host-only tests
-  if [ -z "${MIRI_TEST_TARGET:-}" ]; then
-    # Running these on all targets is unlikely to catch more problems and would
-    # cost a lot of CI time.
-
+  ## advanced tests
+  if [ -n "${MIR_OPT:-}" ]; then
     # Tests with optimizations (`-O` is what cargo passes, but crank MIR optimizations up all the
     # way, too).
     # Optimizations change diagnostics (mostly backtraces), so we don't check
@@ -72,13 +73,15 @@ function run_tests {
     # We explicitly enable debug-assertions here, they are disabled by -O but we have tests
     # which exist to check that we panic on debug assertion failures.
     MIRIFLAGS="${MIRIFLAGS:-} -O -Zmir-opt-level=4 -Cdebug-assertions=yes" MIRI_SKIP_UI_CHECKS=1 ./miri test -- tests/{pass,panic}
-
+  fi
+  if [ -n "${MANY_SEEDS:-}" ]; then
     # Also run some many-seeds tests. 64 seeds means this takes around a minute per test.
     # (Need to invoke via explicit `bash -c` for Windows.)
     for FILE in tests/many-seeds/*.rs; do
-      MIRI_SEEDS=64 ./miri many-seeds "$BASH" -c "./miri run '$FILE'"
+      MIRI_SEEDS=$MANY_SEEDS ./miri many-seeds "$BASH" -c "./miri run '$FILE'"
     done
-
+  fi
+  if [ -n "${TEST_BENCH:-}" ]; then
     # Check that the benchmarks build and run, but without actually benchmarking.
     HYPERFINE="'$BASH' -c" ./miri bench
   fi
@@ -126,34 +129,43 @@ function run_tests_minimal {
 
 ## Main Testing Logic ##
 
-# Host target.
-run_tests
-
-# Extra targets.
 # In particular, fully cover all tier 1 targets.
 case $HOST_TARGET in
   x86_64-unknown-linux-gnu)
+    # Host
+    GC_STRESS=1 MIR_OPT=1 MANY_SEEDS=64 TEST_BENCH=1 run_tests
+    # Extra tier 1
     MIRI_TEST_TARGET=i686-unknown-linux-gnu run_tests
     MIRI_TEST_TARGET=aarch64-unknown-linux-gnu run_tests
-    MIRI_TEST_TARGET=aarch64-apple-darwin run_tests
     MIRI_TEST_TARGET=i686-pc-windows-gnu run_tests
     MIRI_TEST_TARGET=x86_64-pc-windows-gnu run_tests
+    # Extra tier 2
+    MIRI_TEST_TARGET=aarch64-apple-darwin run_tests
     MIRI_TEST_TARGET=arm-unknown-linux-gnueabi run_tests
-    # Some targets are only partially supported.
+    # Partially supported targets (tier 2)
     MIRI_TEST_TARGET=x86_64-unknown-freebsd run_tests_minimal hello integer vec panic/panic concurrency/simple pthread-threadname libc-getentropy libc-getrandom libc-misc libc-fs atomic env align num_cpus
     MIRI_TEST_TARGET=i686-unknown-freebsd run_tests_minimal hello integer vec panic/panic concurrency/simple pthread-threadname libc-getentropy libc-getrandom libc-misc libc-fs atomic env align num_cpus
-
     MIRI_TEST_TARGET=aarch64-linux-android run_tests_minimal hello integer vec panic/panic
     MIRI_TEST_TARGET=wasm32-wasi run_tests_minimal no_std integer strings wasm
     MIRI_TEST_TARGET=wasm32-unknown-unknown run_tests_minimal no_std integer strings wasm
-    MIRI_TEST_TARGET=thumbv7em-none-eabihf run_tests_minimal no_std # no_std embedded architecture
-    MIRI_TEST_TARGET=tests/avr.json MIRI_NO_STD=1 run_tests_minimal no_std # JSON target file
+    MIRI_TEST_TARGET=thumbv7em-none-eabihf run_tests_minimal no_std
+    # Custom target JSON file
+    MIRI_TEST_TARGET=tests/avr.json MIRI_NO_STD=1 run_tests_minimal no_std
     ;;
   x86_64-apple-darwin)
-    MIRI_TEST_TARGET=s390x-unknown-linux-gnu run_tests # big-endian architecture
+    # Host
+    GC_STRESS=1 MIR_OPT=1 MANY_SEEDS=64 TEST_BENCH=1 run_tests
+    # Extra tier 1
     MIRI_TEST_TARGET=x86_64-pc-windows-msvc run_tests
+    # Extra tier 2
+    MIRI_TEST_TARGET=s390x-unknown-linux-gnu run_tests # big-endian architecture
     ;;
   i686-pc-windows-msvc)
+    # Host
+    GC_STRESS=1 MIR_OPT=1 MANY_SEEDS=64 TEST_BENCH=1 run_tests
+    # Extra tier 1
+    # We really want to ensure a Linux target works on a Windows host,
+    # and a 64bit target works on a 32bit host.
     MIRI_TEST_TARGET=x86_64-unknown-linux-gnu run_tests
     ;;
   *)