diff --git a/py/makeqstrdefs.py b/py/makeqstrdefs.py
index 1db00d9633..f514ae0c10 100644
--- a/py/makeqstrdefs.py
+++ b/py/makeqstrdefs.py
@@ -8,6 +8,7 @@ This script works with Python 2.6, 2.7, 3.3 and 3.4.
 from __future__ import print_function
 
 import re
+import subprocess
 import sys
 import io
 import os
@@ -20,6 +21,31 @@ _MODE_QSTR = "qstr"
 _MODE_COMPRESS = "compress"
 
 
+def preprocess():
+    if any(src in args.dependencies for src in args.changed_sources):
+        sources = args.sources
+    elif any(args.changed_sources):
+        sources = args.changed_sources
+    else:
+        sources = args.sources
+    csources = []
+    cxxsources = []
+    for source in sources:
+        if source.endswith(".cpp"):
+            cxxsources.append(source)
+        else:
+            csources.append(source)
+    try:
+        os.makedirs(os.path.dirname(args.output[0]))
+    except OSError:
+        pass
+    with open(args.output[0], "w") as out_file:
+        if csources:
+            subprocess.check_call(args.pp + args.cflags + csources, stdout=out_file)
+        if cxxsources:
+            subprocess.check_call(args.pp + args.cxxflags + cxxsources, stdout=out_file)
+
+
 def write_out(fname, output):
     if output:
         for m, r in [("/", "__"), ("\\", "__"), (":", "@"), ("..", "@@")]:
@@ -105,7 +131,7 @@ def cat_together():
 
 
 if __name__ == "__main__":
-    if len(sys.argv) != 6:
+    if len(sys.argv) < 6:
         print("usage: %s command mode input_filename output_dir output_file" % sys.argv[0])
         sys.exit(2)
 
@@ -114,6 +140,37 @@ if __name__ == "__main__":
 
     args = Args()
     args.command = sys.argv[1]
+
+    if args.command == "pp":
+        named_args = {
+            s: []
+            for s in [
+                "pp",
+                "output",
+                "cflags",
+                "cxxflags",
+                "sources",
+                "changed_sources",
+                "dependencies",
+            ]
+        }
+
+        for arg in sys.argv[1:]:
+            if arg in named_args:
+                current_tok = arg
+            else:
+                named_args[current_tok].append(arg)
+
+        if not named_args["pp"] or len(named_args["output"]) != 1:
+            print("usage: %s %s ..." % (sys.argv[0], " ... ".join(named_args)))
+            sys.exit(2)
+
+        for k, v in named_args.items():
+            setattr(args, k, v)
+
+        preprocess()
+        sys.exit(0)
+
     args.mode = sys.argv[2]
     args.input_filename = sys.argv[3]  # Unused for command=cat
     args.output_dir = sys.argv[4]
diff --git a/py/mkrules.mk b/py/mkrules.mk
index f003174ea5..3bfe64d753 100644
--- a/py/mkrules.mk
+++ b/py/mkrules.mk
@@ -15,10 +15,12 @@ CFLAGS += -DMICROPY_ROM_TEXT_COMPRESSION=1
 endif
 
 # QSTR generation uses the same CFLAGS, with these modifications.
+QSTR_GEN_FLAGS = -DNO_QSTR -I$(BUILD)/tmp
 # Note: := to force evalulation immediately.
 QSTR_GEN_CFLAGS := $(CFLAGS)
-QSTR_GEN_CFLAGS += -DNO_QSTR
-QSTR_GEN_CFLAGS += -I$(BUILD)/tmp
+QSTR_GEN_CFLAGS += $(QSTR_GEN_FLAGS)
+QSTR_GEN_CXXFLAGS := $(CXXFLAGS)
+QSTR_GEN_CXXFLAGS += $(QSTR_GEN_FLAGS)
 
 # This file expects that OBJ contains a list of all of the object files.
 # The directory portion of each object file is used to locate the source
@@ -95,14 +97,14 @@ $(BUILD)/%.pp: %.c
 # to get built before we try to compile any of them.
 $(OBJ): | $(HEADER_BUILD)/qstrdefs.generated.h $(HEADER_BUILD)/mpversion.h $(OBJ_EXTRA_ORDER_DEPS)
 
-# The logic for qstr regeneration is:
+# The logic for qstr regeneration (applied by makeqstrdefs.py) is:
 # - if anything in QSTR_GLOBAL_DEPENDENCIES is newer, then process all source files ($^)
 # - else, if list of newer prerequisites ($?) is not empty, then process just these ($?)
 # - else, process all source files ($^) [this covers "make -B" which can set $? to empty]
 # See more information about this process in docs/develop/qstr.rst.
 $(HEADER_BUILD)/qstr.i.last: $(SRC_QSTR) $(QSTR_GLOBAL_DEPENDENCIES) | $(QSTR_GLOBAL_REQUIREMENTS)
 	$(ECHO) "GEN $@"
-	$(Q)$(CPP) $(QSTR_GEN_CFLAGS) $(if $(filter $?,$(QSTR_GLOBAL_DEPENDENCIES)),$^,$(if $?,$?,$^)) >$(HEADER_BUILD)/qstr.i.last
+	$(Q)$(PYTHON) $(PY_SRC)/makeqstrdefs.py pp $(CPP) output $(HEADER_BUILD)/qstr.i.last cflags $(QSTR_GEN_CFLAGS) cxxflags $(QSTR_GEN_CXXFLAGS) sources $^ dependencies $(QSTR_GLOBAL_DEPENDENCIES) changed_sources $?
 
 $(HEADER_BUILD)/qstr.split: $(HEADER_BUILD)/qstr.i.last
 	$(ECHO) "GEN $@"