1 files changed, 27 insertions, 12 deletions

diff --git a/layout/style/nsCSSParser.cpp b/layout/style/nsCSSParser.cpp
index b409bfed7e..720a123d51 100644
--- a/layout/style/nsCSSParser.cpp
+++ b/layout/style/nsCSSParser.cpp
@@ -9843,7 +9843,22 @@ CSSParserImpl::ParseGridLine(nsCSSValue& aValue)
   // Make the contained value be defined even though we really want a
   // Nothing here.  This works around an otherwise difficult to avoid
   // Memcheck false positive when this is compiled by gcc-5.3 -O2.
-  // See bug 1301856.
+  // The problem is that gcc 5.4 and later, when using high
+  // optimization, inline Maybe::{isSome,ref} here
+  // 
+  //     if (integer.isSome() && integer.ref() < 0) {
+  // 
+  // and then proceed to evaluate the expression right-to-left, as if it
+  // had been written
+  // 
+  //     integer.ref() < 0 && integer.isSome()
+  // 
+  // This is a legitimate transformation because gcc presumably can prove
+  // via dataflow analysis that integer.isSome() is false whenever integer.ref()
+  // is undefined, so the overall expression result is still defined.  
+  // Valgrind/Memcheck assumes that all conditional branches in the program
+  // are important, and that assumption is deeply wired in, so there is no
+  // easy way to avoid the warning apart from to force-initialise |integer|.
   integer.emplace(0);
   integer.reset();
 #endif
@@ -15314,17 +15329,17 @@ CSSParserImpl::ParseFontFeatureSettings(nsCSSValue& aValue)
   return true;
 }
 
-bool

-CSSParserImpl::ParseFontVariationSettings(nsCSSValue& aValue)

-{

-  // TODO: Actually implement this.

-  

-  // This stub is here because websites insist on considering this

-  // very hardware-dependent and O.S.-variable low-level font-control

-  // as a "critical feature" which it isn't as there is 0 guarantee

-  // that font variation settings are supported or honored by any

-  // operating system used by the client.

-  return true;

+bool
+CSSParserImpl::ParseFontVariationSettings(nsCSSValue& aValue)
+{
+  // TODO: Actually implement this.
+  
+  // This stub is here because websites insist on considering this
+  // very hardware-dependent and O.S.-variable low-level font-control
+  // as a "critical feature" which it isn't as there is 0 guarantee
+  // that font variation settings are supported or honored by any
+  // operating system used by the client.
+  return true;
 }
 
 bool