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, 69 insertions, 2 deletions

diff --git a/src/sksl/ir/SkSLVariableReference.h b/src/sksl/ir/SkSLVariableReference.h
index c6a2ea0511..fecb04e2e5 100644
--- a/src/sksl/ir/SkSLVariableReference.h
+++ b/src/sksl/ir/SkSLVariableReference.h
@@ -20,16 +20,83 @@ namespace SkSL {
  * there is only one Variable 'x', but two VariableReferences to it.
  */
 struct VariableReference : public Expression {
-    VariableReference(Position position, const Variable& variable)
+    enum RefKind {
+        kRead_RefKind,
+        kWrite_RefKind,
+        kReadWrite_RefKind
+    };
+
+    VariableReference(Position position, const Variable& variable, RefKind refKind = kRead_RefKind)
     : INHERITED(position, kVariableReference_Kind, variable.fType)
-    , fVariable(variable) {}
+    , fVariable(variable)
+    , fRefKind(refKind) {
+        if (refKind != kRead_RefKind) {
+            fVariable.fWriteCount++;
+        }
+        if (refKind != kWrite_RefKind) {
+            fVariable.fReadCount++;
+        }
+    }
+
+    virtual ~VariableReference() override {
+        if (fRefKind != kWrite_RefKind) {
+            fVariable.fReadCount--;
+        }
+    }
+
+    RefKind refKind() {
+        return fRefKind;
+    }
+
+    void setRefKind(RefKind refKind) {
+        if (fRefKind != kRead_RefKind) {
+            fVariable.fWriteCount--;
+        }
+        if (fRefKind != kWrite_RefKind) {
+            fVariable.fReadCount--;
+        }
+        if (refKind != kRead_RefKind) {
+            fVariable.fWriteCount++;
+        }
+        if (refKind != kWrite_RefKind) {
+            fVariable.fReadCount++;
+        }
+        fRefKind = refKind;
+    }
 
     SkString description() const override {
         return fVariable.fName;
     }
 
+    virtual std::unique_ptr<Expression> constantPropagate(
+                                                        const IRGenerator& irGenerator,
+                                                        const DefinitionMap& definitions) override {
+        auto exprIter = definitions.find(&fVariable);
+        if (exprIter != definitions.end() && exprIter->second) {
+            const Expression* expr = exprIter->second->get();
+            switch (expr->fKind) {
+                case Expression::kIntLiteral_Kind:
+                    return std::unique_ptr<Expression>(new IntLiteral(
+                                                                     irGenerator.fContext,
+                                                                     Position(),
+                                                                     ((IntLiteral*) expr)->fValue));
+                case Expression::kFloatLiteral_Kind:
+                    return std::unique_ptr<Expression>(new FloatLiteral(
+                                                                   irGenerator.fContext,
+                                                                   Position(),
+                                                                   ((FloatLiteral*) expr)->fValue));
+                default:
+                    break;
+            }
+        }
+        return nullptr;
+    }
+
     const Variable& fVariable;
 
+private:
+    RefKind fRefKind;
+
     typedef Expression INHERITED;
 };