Do not cache the result of RenderStyle::computedLineHeight()

We currently use 26-bits in RenderBlock to cache the result of
RenderStyle::computedLineHeight(). However when you look at the
computations behind that function the main components are already cached
so at most we are saving a handful of computations per call.

In https://codereview.chromium.org/256593009/ we added a test that calls
RenderBlock::lineHeight() ~425,000 times on each run. This CL results in
no measurable degradation in performance against that test.

So since we cannot get a measurable improvement in performance as a result
of the caching when testing locally, remove it and free up space for more useful
bits in RenderBlock. We will let it cycle through the perf bots for a while
before allowing people to start using the spare bits though.

The cached result originally sat in RenderText and was inherited from the
fork from KHTML. http://trac.webkit.org/changeset/6010 moved it to RenderBlock
on the basis that "calls to fontMetrics().lineSpacing() are expensive." This
is no longer the case as far as I can tell, since both lineSpacing() and
fontMetrics() are now cached themselves.

Review URL: https://codereview.chromium.org/260073005

git-svn-id: svn://svn.chromium.org/blink/trunk@176233 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/core/rendering/FastTextAutosizer.cpp

@@ -970,8 +970,6 @@ void FastTextAutosizer::applyMultiplier(RenderObject* renderer, float multiplier
 
         renderer->setStyleInternal(style.release());
         renderer->setNeedsLayoutAndFullPaintInvalidation();
-        if (renderer->isRenderBlock())
-            toRenderBlock(renderer)->invalidateLineHeight();
         break;
 
     case LayoutNeeded:

third_party/WebKit/Source/core/rendering/RenderBR.cpp

@@ -35,7 +35,6 @@ static PassRefPtr<StringImpl> newlineString()
 
 RenderBR::RenderBR(Node* node)
     : RenderText(node, newlineString())
-    , m_lineHeight(-1)
 {
 }
 
@@ -45,22 +44,13 @@ RenderBR::~RenderBR()
 
 int RenderBR::lineHeight(bool firstLine) const
 {
-    if (firstLine && document().styleEngine()->usesFirstLineRules()) {
-        RenderStyle* s = style(firstLine);
-        if (s != style())
-            return s->computedLineHeight();
-    }
-
-    if (m_lineHeight == -1)
-        m_lineHeight = style()->computedLineHeight();
-
-    return m_lineHeight;
+    RenderStyle* s = style(firstLine && document().styleEngine()->usesFirstLineRules());
+    return s->computedLineHeight();
 }
 
 void RenderBR::styleDidChange(StyleDifference diff, const RenderStyle* oldStyle)
 {
     RenderText::styleDidChange(diff, oldStyle);
-    m_lineHeight = -1;
 }
 
 int RenderBR::caretMinOffset() const

third_party/WebKit/Source/core/rendering/RenderBR.h

@@ -55,9 +55,6 @@ public:
 
 protected:
     virtual void styleDidChange(StyleDifference, const RenderStyle* oldStyle) OVERRIDE;
-
-private:
-    mutable int m_lineHeight;
 };
 
 DEFINE_RENDER_OBJECT_TYPE_CASTS(RenderBR, isBR());

third_party/WebKit/Source/core/rendering/RenderBlock.cpp

@@ -155,7 +155,6 @@ private:
 
 RenderBlock::RenderBlock(ContainerNode* node)
     : RenderBox(node)
-    , m_lineHeight(-1)
     , m_hasMarginBeforeQuirk(false)
     , m_hasMarginAfterQuirk(false)
     , m_beingDestroyed(false)
@@ -351,7 +350,6 @@ void RenderBlock::styleDidChange(StyleDifference diff, const RenderStyle* oldSty
         textAutosizer->record(this);
 
     propagateStyleToAnonymousChildren(true);
-    invalidateLineHeight();
 
     // It's possible for our border/padding to change, but for the overall logical width of the block to
     // end up being the same. We keep track of this change so in layoutBlock, we can know to set relayoutChildren=true.
@@ -3784,16 +3782,8 @@ LayoutUnit RenderBlock::lineHeight(bool firstLine, LineDirectionMode direction,
     if (isReplaced() && linePositionMode == PositionOnContainingLine)
         return RenderBox::lineHeight(firstLine, direction, linePositionMode);
 
-    if (firstLine && document().styleEngine()->usesFirstLineRules()) {
-        RenderStyle* s = style(firstLine);
-        if (s != style())
-            return s->computedLineHeight();
-    }
-
-    if (m_lineHeight == -1)
-        m_lineHeight = style()->computedLineHeight();
-
-    return m_lineHeight;
+    RenderStyle* s = style(firstLine && document().styleEngine()->usesFirstLineRules());
+    return s->computedLineHeight();
 }
 
 int RenderBlock::beforeMarginInLineDirection(LineDirectionMode direction) const

third_party/WebKit/Source/core/rendering/RenderBlock.h

@@ -505,8 +505,6 @@ protected:
 public:
     virtual LayoutUnit offsetFromLogicalTopOfFirstPage() const OVERRIDE FINAL;
 
-    void invalidateLineHeight() { m_lineHeight = -1; }
-
 public:
 
     // Allocated only when some of these fields have non-default values
@@ -534,7 +532,7 @@ protected:
     RenderObjectChildList m_children;
     RenderLineBoxList m_lineBoxes;   // All of the root line boxes created for this block flow.  For example, <div>Hello<br>world.</div> will have two total lines for the <div>.
 
-    mutable signed m_lineHeight : 26;
+    // WARNING: Don't add any bits here until we are comfortable that removing m_lineHeight has not regressed performance. See http://crrev.com/260073005 for more information.
     unsigned m_hasMarginBeforeQuirk : 1; // Note these quirk values can't be put in RenderBlockRareData since they are set too frequently.
     unsigned m_hasMarginAfterQuirk : 1;
     unsigned m_beingDestroyed : 1;