Title: [180688] trunk/Source/bmalloc
- Revision
- 180688
- Author
- [email protected]
- Date
- 2015-02-26 11:30:11 -0800 (Thu, 26 Feb 2015)
Log Message
bmalloc: free up a bit in BoundaryTag
https://bugs.webkit.org/show_bug.cgi?id=142048
Reviewed by Brady Eidson.
We were wasting a bit by accident, and I need one now.
* bmalloc/Algorithm.h:
(bmalloc::rightShift): Deleted. Not needed, now that I've simplified
the math.
* bmalloc/BoundaryTag.h: Since each boundary tag bucket is 1024 bytes
long, the maximum offset into a bucket is 1023.
You need 5 bits to count up to 1024, but only 4 to count up to 1023.
Math is hard.
(bmalloc::BoundaryTag::compactBegin): Switched to division because it
is simpler, and easier to match up with our ASSERT. The compiler will
turn division by constant power of two into a shift for us.
(bmalloc::BoundaryTag::setRange): Added an ASSERT for compactBegin
because we do encode it, so we should ASSERT that encoding did not
lose information.
* bmalloc/Sizes.h: Shifting is no longer used since we use division
instead.
Modified Paths
Diff
Modified: trunk/Source/bmalloc/ChangeLog (180687 => 180688)
--- trunk/Source/bmalloc/ChangeLog 2015-02-26 19:29:48 UTC (rev 180687)
+++ trunk/Source/bmalloc/ChangeLog 2015-02-26 19:30:11 UTC (rev 180688)
@@ -1,3 +1,34 @@
+2015-02-26 Geoffrey Garen <[email protected]>
+
+ bmalloc: free up a bit in BoundaryTag
+ https://bugs.webkit.org/show_bug.cgi?id=142048
+
+ Reviewed by Brady Eidson.
+
+ We were wasting a bit by accident, and I need one now.
+
+ * bmalloc/Algorithm.h:
+ (bmalloc::rightShift): Deleted. Not needed, now that I've simplified
+ the math.
+
+ * bmalloc/BoundaryTag.h: Since each boundary tag bucket is 1024 bytes
+ long, the maximum offset into a bucket is 1023.
+
+ You need 5 bits to count up to 1024, but only 4 to count up to 1023.
+
+ Math is hard.
+
+ (bmalloc::BoundaryTag::compactBegin): Switched to division because it
+ is simpler, and easier to match up with our ASSERT. The compiler will
+ turn division by constant power of two into a shift for us.
+
+ (bmalloc::BoundaryTag::setRange): Added an ASSERT for compactBegin
+ because we do encode it, so we should ASSERT that encoding did not
+ lose information.
+
+ * bmalloc/Sizes.h: Shifting is no longer used since we use division
+ instead.
+
2015-02-24 Stephanie Lewis <[email protected]>
Rolling out http://trac.webkit.org/changeset/180430 as it causes the PLT to crash.
Modified: trunk/Source/bmalloc/bmalloc/Algorithm.h (180687 => 180688)
--- trunk/Source/bmalloc/bmalloc/Algorithm.h 2015-02-26 19:29:48 UTC (rev 180687)
+++ trunk/Source/bmalloc/bmalloc/Algorithm.h 2015-02-26 19:30:11 UTC (rev 180688)
@@ -53,11 +53,6 @@
return reinterpret_cast<T>(reinterpret_cast<uintptr_t>(value) & mask);
}
-template<typename T> inline constexpr T rightShift(T value, uintptr_t shift)
-{
- return reinterpret_cast<T>(reinterpret_cast<uintptr_t>(value) >> shift);
-}
-
template<typename T> inline constexpr bool test(T value, uintptr_t mask)
{
return !!(reinterpret_cast<uintptr_t>(value) & mask);
Modified: trunk/Source/bmalloc/bmalloc/BoundaryTag.h (180687 => 180688)
--- trunk/Source/bmalloc/bmalloc/BoundaryTag.h 2015-02-26 19:29:48 UTC (rev 180687)
+++ trunk/Source/bmalloc/bmalloc/BoundaryTag.h 2015-02-26 19:30:11 UTC (rev 180688)
@@ -68,12 +68,17 @@
private:
static const size_t flagBits = 3;
- static const size_t compactBeginBits = 5;
+ static const size_t compactBeginBits = 4;
static const size_t sizeBits = bitCount<unsigned>() - flagBits - compactBeginBits;
- static_assert((1 << compactBeginBits) - 1 >= largeMin / largeAlignment, "compactBegin must be encodable in a BoundaryTag.");
- static_assert((1 << sizeBits) - 1 >= largeMax, "largeMax must be encodable in a BoundaryTag.");
+ static_assert(
+ (1 << compactBeginBits) - 1 >= (largeMin - 1) / largeAlignment,
+ "compactBegin must be encodable in a BoundaryTag.");
+ static_assert(
+ (1 << sizeBits) - 1 >= largeMax,
+ "largeMax must be encodable in a BoundaryTag.");
+
bool m_isFree: 1;
bool m_isEnd: 1;
bool m_hasPhysicalPages: 1;
@@ -84,14 +89,14 @@
inline unsigned BoundaryTag::compactBegin(void* object)
{
return static_cast<unsigned>(
- reinterpret_cast<uintptr_t>(
- rightShift(
- mask(object, largeMin - 1), largeAlignmentShift)));
+ reinterpret_cast<uintptr_t>(mask(object, largeMin - 1)) / largeAlignment);
}
inline void BoundaryTag::setRange(const Range& range)
{
m_compactBegin = compactBegin(range.begin());
+ BASSERT(this->compactBegin() == compactBegin(range.begin()));
+
m_size = static_cast<unsigned>(range.size());
BASSERT(this->size() == range.size());
}
Modified: trunk/Source/bmalloc/bmalloc/Sizes.h (180687 => 180688)
--- trunk/Source/bmalloc/bmalloc/Sizes.h 2015-02-26 19:29:48 UTC (rev 180687)
+++ trunk/Source/bmalloc/bmalloc/Sizes.h 2015-02-26 19:30:11 UTC (rev 180688)
@@ -76,8 +76,6 @@
static const size_t largeChunkMask = ~(largeChunkSize - 1ul);
static const size_t largeAlignment = 64;
- static const size_t largeAlignmentShift = 6;
- static_assert(1 << largeAlignmentShift == largeAlignment, "largeAlignmentShift be log2(largeAlignment).");
static const size_t largeMax = largeChunkSize * 99 / 100; // Plenty of room for metadata.
static const size_t largeMin = mediumMax;
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