Added constant propagation and better variable liveness tracking to

skslc. 

This allows skslc to track the values of variables with constant
values across multiple statements and replace variable references with
constant values where appropriate.

The improved liveness tracking allows skslc to realize that a
variable is no longer alive if all references to it have been
replaced. It is not yet doing much with this information; better
dead code elimination is coming in a followup change.

BUG=skia:

Change-Id: I6bf267d478b769caf0063ac3597dc16bbe618cb4
Reviewed-on: https://skia-review.googlesource.com/7033
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>

1 files changed, 8 insertions, 4 deletions

diff --git a/src/sksl/ir/SkSLVariable.h b/src/sksl/ir/SkSLVariable.h
index 39b8482a7b..2c3391dfa2 100644
--- a/src/sksl/ir/SkSLVariable.h
+++ b/src/sksl/ir/SkSLVariable.h
@@ -33,8 +33,8 @@ struct Variable : public Symbol {
     , fModifiers(modifiers)
     , fType(type)
     , fStorage(storage)
-    , fIsReadFrom(false)
-    , fIsWrittenTo(false) {}
+    , fReadCount(0)
+    , fWriteCount(0) {}
 
     virtual SkString description() const override {
         return fModifiers.description() + fType.fName + " " + fName;
@@ -44,8 +44,12 @@ struct Variable : public Symbol {
     const Type& fType;
     const Storage fStorage;
 
-    mutable bool fIsReadFrom;
-    mutable bool fIsWrittenTo;
+    // Tracks how many sites read from the variable. If this is zero for a non-out variable (or
+    // becomes zero during optimization), the variable is dead and may be eliminated.
+    mutable int fReadCount;
+    // Tracks how many sites write to the variable. If this is zero, the variable is dead and may be
+    // eliminated.
+    mutable int fWriteCount;
 
     typedef Symbol INHERITED;
 };