Bug 949171 - SpiderMonkey: Remove fudge from the RawLengthForBits calculation, and fix Safepoint reading to be ok with this. r=nbp

js/src/jit/BitSet.h

@@ -21,8 +21,10 @@ namespace jit {
 class BitSet : private TempObject
 {
   public:
+    static const size_t BitsPerWord = 8 * sizeof(uint32_t);
+
     static size_t RawLengthForBits(size_t bits) {
-        return 1 + bits / (8 * sizeof(uint32_t));
+        return (bits + BitsPerWord - 1) / BitsPerWord;
     }
 
   private:
@@ -34,11 +36,11 @@ class BitSet : private TempObject
     uint32_t *bits_;
 
     static inline uint32_t bitForValue(unsigned int value) {
-        return 1l << (uint32_t)(value % (8 * sizeof(uint32_t)));
+        return 1l << uint32_t(value % BitsPerWord);
     }
 
     static inline unsigned int wordForValue(unsigned int value) {
-        return value / (8 * sizeof(uint32_t));
+        return value / BitsPerWord;
     }
 
     inline unsigned int numWords() const {

js/src/jit/Safepoints.cpp

@@ -374,22 +374,21 @@ SafepointReader::InvalidationPatchPoint(IonScript *script, const SafepointIndex
 void
 SafepointReader::advanceFromGcRegs()
 {
-    currentSlotChunkNumber_ = 0;
-    currentSlotChunk_ = stream_.readUnsigned();
+    currentSlotChunk_ = 0;
+    nextSlotChunkNumber_ = 0;
 }
 
 bool
 SafepointReader::getSlotFromBitmap(uint32_t *slot)
 {
     while (currentSlotChunk_ == 0) {
-        currentSlotChunkNumber_++;
-
         // Are there any more chunks to read?
-        if (currentSlotChunkNumber_ == BitSet::RawLengthForBits(frameSlots_))
+        if (nextSlotChunkNumber_ == BitSet::RawLengthForBits(frameSlots_))
             return false;
 
         // Yes, read the next chunk.
         currentSlotChunk_ = stream_.readUnsigned();
+        nextSlotChunkNumber_++;
     }
 
     // The current chunk still has bits in it, so get the next bit, then mask
@@ -399,7 +398,7 @@ SafepointReader::getSlotFromBitmap(uint32_t *slot)
 
     // Return the slot, taking care to add 1 back in since it was subtracted
     // when added in the original bitset.
-    *slot = (currentSlotChunkNumber_ * sizeof(uint32_t) * 8) + bit + 1;
+    *slot = ((nextSlotChunkNumber_ - 1) * BitSet::BitsPerWord) + bit + 1;
     return true;
 }
 
@@ -416,8 +415,8 @@ void
 SafepointReader::advanceFromGcSlots()
 {
     // No, reset the counter.
-    currentSlotChunkNumber_ = 0;
-    currentSlotChunk_ = stream_.readUnsigned();
+    currentSlotChunk_ = 0;
+    nextSlotChunkNumber_ = 0;
 }
 
 bool

js/src/jit/Safepoints.h

@@ -61,7 +61,7 @@ class SafepointReader
     CompactBufferReader stream_;
     uint32_t frameSlots_;
     uint32_t currentSlotChunk_;
-    uint32_t currentSlotChunkNumber_;
+    uint32_t nextSlotChunkNumber_;
     uint32_t osiCallPointOffset_;
     GeneralRegisterSet gcSpills_;
     GeneralRegisterSet valueSpills_;