diff --git a/Makefile b/Makefile
index 68c56b6801..2c85d576ab 100644
--- a/Makefile
+++ b/Makefile
@@ -27,6 +27,13 @@ all: lib
 # User-specified CFLAGS.
 USER_CFLAGS=$(strip $(shell test -f perf-cppflags && cat perf-cppflags))
 
+# If the user doesn't specify an -O setting, we default to -O3, except
+# for upb_def which gets -Os.
+ifeq (, $(findstring -O, $(USER_CFLAGS)))
+  USER_CFLAGS += -O3
+  DEF_OPT = -Os
+endif
+
 # Basic compiler/flag setup.
 CC=gcc
 CXX=g++
@@ -146,11 +153,11 @@ $(LIBUPB_PIC): $(PICOBJ)
 # critical path but gets very large when -O3 is used.
 src/upb_def.o: src/upb_def.c
 	$(E) CC $<
-	$(Q) $(CC) $(CFLAGS) $(CPPFLAGS) -O0 -c -o $@ $<
+	$(Q) $(CC) $(CFLAGS) $(CPPFLAGS) $(DEF_OPT) -c -o $@ $<
 
 src/upb_def.lo: src/upb_def.c
 	$(E) 'CC -fPIC' $<
-	$(Q) $(CC) $(CFLAGS) $(CPPFLAGS) -O0 -c -o $@ $< -fPIC
+	$(Q) $(CC) $(CFLAGS) $(CPPFLAGS) $(DEF_OPT) -c -o $@ $< -fPIC
 
 src/upb_decoder_x64.o: src/upb_decoder_x64.asm
 	$(E) NASM $<
diff --git a/perf-regression-test.py b/perf-regression-test.py
index ae3259c5f0..09f4569bcc 100755
--- a/perf-regression-test.py
+++ b/perf-regression-test.py
@@ -10,7 +10,7 @@ set -v
 # Generate numbers for baseline.
 rm -rf perf-tmp
 git clone . perf-tmp
-(cd perf-tmp && ../perf-tests.sh upb)
+(cd perf-tmp && ./perf-tests.sh upb)
 cp perf-tmp/perf-tests.out perf-tests.baseline
 
 # Generate numbers for working directory.
diff --git a/perf-tests.sh b/perf-tests.sh
index bfa8bd69f1..003f866e61 100755
--- a/perf-tests.sh
+++ b/perf-tests.sh
@@ -13,22 +13,22 @@ rm -f perf-tests.out
 
 if [ x`uname -m` = xx86_64 ]; then
   make clean
-  echo "-O3 -DNDEBUG -msse3 -m32" > perf-cppflags
+  echo "-DNDEBUG -m32" > perf-cppflags
   make upb_benchmarks
   make benchmark | sed -e 's/^/plain32./g' | tee -a perf-tests.out
 
   make clean
-  echo "-O3 -DNDEBUG -fomit-frame-pointer -msse3 -m32" > perf-cppflags
+  echo "-DNDEBUG -fomit-frame-pointer -m32" > perf-cppflags
   make upb_benchmarks
   make benchmark | sed -e 's/^/omitfp32./g' | tee -a perf-tests.out
 fi
 
 make clean
-echo "-O3 -DNDEBUG -msse3" > perf-cppflags
+echo "-DNDEBUG" > perf-cppflags
 make $MAKETARGET
 make benchmark | sed -e 's/^/plain./g' | tee -a perf-tests.out
 
 make clean
-echo "-O3 -DNDEBUG -fomit-frame-pointer -msse3" > perf-cppflags
+echo "-DNDEBUG -fomit-frame-pointer" > perf-cppflags
 make $MAKETARGET
 make benchmark | sed -e 's/^/omitfp./g' | tee -a perf-tests.out
diff --git a/src/upb_def.c b/src/upb_def.c
index 3b2144367d..61d1a3e4fe 100644
--- a/src/upb_def.c
+++ b/src/upb_def.c
@@ -728,7 +728,7 @@ static bool upb_fielddef_setdefault(upb_string *dstr, upb_value *d, int type) {
       case UPB_TYPE(UINT32):
       case UPB_TYPE(FIXED32):
         if (strz) {
-          long val = strtoul(strz, &end, 0);
+          unsigned long val = strtoul(strz, &end, 0);
           if (val > UINT32_MAX || errno == ERANGE || *end)
             success = false;
           else