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: I068c5d2e9a362e75299b1de1f4575339f5ddc3bb
Reviewed-on: https://skia-review.googlesource.com/7302
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>

1 files changed, 14 insertions, 5 deletions

diff --git a/src/sksl/SkSLCFGGenerator.h b/src/sksl/SkSLCFGGenerator.h
index c37850112c..337fdfac35 100644
--- a/src/sksl/SkSLCFGGenerator.h
+++ b/src/sksl/SkSLCFGGenerator.h
@@ -27,14 +27,23 @@ struct BasicBlock {
         };
 
         Kind fKind;
-        const IRNode* fNode;
+        // if false, this node should not be subject to constant propagation. This happens with
+        // compound assignment (i.e. x *= 2), in which the value x is used as an rvalue for
+        // multiplication by 2 and then as an lvalue for assignment purposes. Since there is only
+        // one "x" node, replacing it with a constant would break the assignment and we suppress
+        // it. Down the road, we should handle this more elegantly by substituting a regular
+        // assignment if the target is constant (i.e. x = 1; x *= 2; should become x = 1; x = 1 * 2;
+        // and then collapse down to a simple x = 2;).
+        bool fConstantPropagation;
+        std::unique_ptr<Expression>* fExpression;
+        const Statement* fStatement;
     };
-    
+
     std::vector<Node> fNodes;
     std::set<BlockId> fEntrances;
     std::set<BlockId> fExits;
     // variable definitions upon entering this basic block (null expression = undefined)
-    std::unordered_map<const Variable*, const Expression*> fBefore;
+    DefinitionMap fBefore;
 };
 
 struct CFG {
@@ -77,9 +86,9 @@ public:
 private:
     void addStatement(CFG& cfg, const Statement* s);
 
-    void addExpression(CFG& cfg, const Expression* e);
+    void addExpression(CFG& cfg, std::unique_ptr<Expression>* e, bool constantPropagate);
 
-    void addLValue(CFG& cfg, const Expression* e);
+    void addLValue(CFG& cfg, std::unique_ptr<Expression>* e);
 
     std::stack<BlockId> fLoopContinues;
     std::stack<BlockId> fLoopExits;