diff --git a/core/upb_def.c b/core/upb_def.c
index 8abb8bc101..651afc1038 100644
--- a/core/upb_def.c
+++ b/core/upb_def.c
@@ -727,6 +727,8 @@ static upb_flow_t upb_msgdef_endmsg(void *_b) {
     // want to use a different ordering that puts all the required bits
     // together.
     f->field_index = i;
+    f->set_bit_mask = 1 << (i % 8);
+    f->set_bit_offset = i / 8;
 
     size_t size, align;
     if (upb_isarray(f)) {
diff --git a/core/upb_def.h b/core/upb_def.h
index 34f31ecae2..28cc2586a0 100644
--- a/core/upb_def.h
+++ b/core/upb_def.h
@@ -101,14 +101,16 @@ typedef struct _upb_fielddef {
   uint32_t byte_offset;           // Where in a upb_msg to find the data.
 
   // These are set only when this fielddef is part of a msgdef.
+  upb_field_number_t number;
   upb_field_count_t field_index;  // Indicates set bit.
 
-  upb_field_number_t number;
   upb_fieldtype_t type;
   upb_label_t label;
   // True if we own a ref on "def" (above).  This is true unless this edge is
   // part of a cycle.
   bool owned;
+  uint8_t set_bit_mask;
+  uint16_t set_bit_offset;
 } upb_fielddef;
 
 // A variety of tests about the type of a field.
diff --git a/core/upb_msg.c b/core/upb_msg.c
index 05ee1e952c..9dfbea4c90 100644
--- a/core/upb_msg.c
+++ b/core/upb_msg.c
@@ -142,7 +142,7 @@ void upb_msg_recycle(upb_msg **_msg, upb_msgdef *msgdef) {
 }
 
 INLINE void upb_msg_sethas(upb_msg *msg, upb_fielddef *f) {
-  msg->data[f->field_index/8] |= (1 << (f->field_index % 8));
+  msg->data[f->set_bit_offset] |= f->set_bit_mask;
 }
 
 static upb_valueptr upb_msg_getappendptr(upb_msg *msg, upb_fielddef *f) {
diff --git a/core/upb_msg.h b/core/upb_msg.h
index 1318e08fa6..8a3c63f296 100644
--- a/core/upb_msg.h
+++ b/core/upb_msg.h
@@ -198,7 +198,7 @@ void upb_msg_recycle(upb_msg **msg, upb_msgdef *msgdef);
 // Tests whether the given field is explicitly set, or whether it will return a
 // default.
 INLINE bool upb_msg_has(upb_msg *msg, upb_fielddef *f) {
-  return (msg->data[f->field_index/8] & (1 << (f->field_index % 8))) != 0;
+  return (msg->data[f->set_bit_offset] & f->set_bit_mask) != 0;
 }
 
 INLINE upb_value upb_msg_get(upb_msg *msg, upb_fielddef *f) {
diff --git a/perf-regression-test.py b/perf-regression-test.py
index acd3ebcd84..ae3259c5f0 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.
@@ -22,6 +22,7 @@ for line in baseline_file:
   test, speed = line.split(":")
   baseline[test] = int(speed)
 
+print("\n\n=== PERFORMANCE REGRESSION TEST RESULTS:\n")
 wd_file = open("perf-tests.out")
 for line in wd_file:
   test, speed = line.split(":")
diff --git a/perf-tests.sh b/perf-tests.sh
index 1d8f4a8d2e..3833572be6 100755
--- a/perf-tests.sh
+++ b/perf-tests.sh
@@ -3,6 +3,7 @@
 # multiple times with a few different compiler flag combinations.
 # The output will be dumped to stdout and to perf-tests.out.
 
+set -e
 MAKETARGET=benchmarks
 if [ x$1 == xupb ]; then
   MAKETARGET=upb_benchmarks
@@ -11,23 +12,23 @@ fi
 rm -f perf-tests.out
 
 make clean
-echo "-O3 -DNDEBUG -msse3" > perf-cppflags
+echo "-O3 -DNDEBUG -msse3 -DUPB_THREAD_UNSAFE" > 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 "-O3 -DNDEBUG -fomit-frame-pointer -msse3 -DUPB_THREAD_UNSAFE" > perf-cppflags
 make $MAKETARGET
 make benchmark | sed -e 's/^/omitfp./g' | tee -a perf-tests.out
 
 if [ x`uname -m` == xx86_64 ]; then
   make clean
-  echo "-O3 -DNDEBUG -msse3 -m32" > perf-cppflags
+  echo "-O3 -DNDEBUG -msse3 -m32 -DUPB_THREAD_UNSAFE" > 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 "-O3 -DNDEBUG -fomit-frame-pointer -msse3 -m32 -DUPB_THREAD_UNSAFE" > perf-cppflags
   make upb_benchmarks
   make benchmark | sed -e 's/^/omitfp32./g' | tee -a perf-tests.out
 fi