improve performance by ~25% in heavy GC situations

Author: kavon

src/hash_table.c

@@ -7,29 +7,32 @@
  * The implementation is one round of the xorshift64* algorithm.
  * Code Source: Wikipedia
  */
-uint64_t hashFn(uint64_t x) {
+inline uint64_t hashFn(uint64_t x) {
     x ^= x >> 12; // a
     x ^= x << 25; // b
     x ^= x >> 27; // c
     return x * UINT64_C(2685821657736338717);
 }
 
-uint64_t computeBucketIndex(statepoint_table_t* table, uint64_t key) {
+inline uint64_t computeBucketIndex(statepoint_table_t* table, uint64_t key) {
     // Using modulo may introduce a little bias in the table.
     // If you care, use the unbiased version that's floating around the internet.
-    return hashFn(key) % table->size;
+
+    // NOTE: we use bitwise AND instead of modulo because the size
+    // is a power-of-two. This is very important for performance.
+    return hashFn(key) & (table->size - 1);
 }
 
-size_t size_of_frame(uint16_t numSlots) {
+inline size_t size_of_frame(uint16_t numSlots) {
     return sizeof(frame_info_t) + numSlots * sizeof(pointer_slot_t);
 }
 
-size_t frame_size(frame_info_t* frame) {
+inline size_t frame_size(frame_info_t* frame) {
     return size_of_frame(frame->numSlots);
 }
 
 // returns the next frame relative the current frame
-frame_info_t* next_frame(frame_info_t* cur) {
+inline frame_info_t* next_frame(frame_info_t* cur) {
     uint8_t* next = ((uint8_t*)cur) + frame_size(cur);
     return (frame_info_t*)next;
 }
@@ -41,6 +44,11 @@ statepoint_table_t* new_table(float loadFactor, uint64_t expectedElms) {
 
     uint64_t numBuckets = (expectedElms / loadFactor) + 1;
 
+    // round up to nearest power of two. this implementation requires
+    // it for the performance of lookup_return_address
+    uint64_t factor = ceil(log2((double) numBuckets));
+    numBuckets = 1ULL << factor;
+
     table_bucket_t* buckets = calloc(numBuckets, sizeof(table_bucket_t));
     assert(buckets && "bad alloc");
 

src/include/hash_table.h

@@ -9,6 +9,7 @@
 #include <assert.h>
 #include <stdlib.h>
 #include <string.h>
+#include <math.h>
 
 /** Functions **/
 

test/Makefile

@@ -1,11 +1,12 @@
 CC := cc
 OPT_CC := -O3 -fPIC
 OPT_LLC := -O3 -relocation-model=pic
+LD_FLAGS := -lm
 
 all: statepoints a.out
 
 a.out: fib.o driver.o shim.S ../dist/llvm-statepoint-tablegen.a
-	$(CC) $(OPT_CC) $^
+	$(CC) $(OPT_CC) $^ $(LD_FLAGS)
 
 fib.o: fib.ll
 	llc $(OPT_LLC) fib.ll -o fib.s
@@ -19,4 +20,4 @@ statepoints:
 	cd .. && make
 
 clean:
-	rm -f fib.s fib.o driver.o a.out
+	rm -f fib.s fib.o driver.o a.out output.txt