Add GetConstantValue, fix const-variable ES2 loop bounds

Prior to this fix, the new test cases would report that the various loop
terms needed to be constant expressions.

Bug: skia:12472
Change-Id: Ic377ed0c4598136ae38fb2b65c93b6d8609d54cb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/452276
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>

resources/sksl/runtime/LoopFloat.rts

@@ -1,17 +1,19 @@
 uniform half4 colorRed, colorGreen;
 
 // Should return 5
+const float kZero = 0;
 float return_loop() {
-    for (float i = 0; i < 10; ++i) {
+    for (float i = kZero; i < 10; ++i) {
         if (i == 5) { return i; }
     }
     return 0;
 }
 
 // Should return 35
+const float kTen = kZero + 10;
 float continue_loop() {
     float sum = 0;
-    for (float i = 0; i < 10; ++i) {
+    for (float i = 0; i < kTen; ++i) {
         if (i < 5) { continue; }
         sum += i;
     }
@@ -21,7 +23,8 @@ float continue_loop() {
 // Should return 15
 float break_loop() {
     float sum = 0;
-    for (float i = 0; i < 10; ++i) {
+    const float kOne = 1;
+    for (float i = 0; i < 10; i += kOne) {
         if (i > 5) { break; }
         sum += i;
     }

resources/sksl/runtime/LoopInt.rts

@@ -1,17 +1,19 @@
 uniform half4 colorRed, colorGreen;
 
 // Should return 5
+const int kZero = 0;
 int return_loop() {
-    for (int i = 0; i < 10; ++i) {
+    for (int i = kZero; i < 10; ++i) {
         if (i == 5) { return i; }
     }
     return 0;
 }
 
 // Should return 35
+const int kTen = kZero + 10;
 int continue_loop() {
     int sum = 0;
-    for (int i = 0; i < 10; ++i) {
+    for (int i = 0; i < kTen; ++i) {
         if (i < 5) { continue; }
         sum += i;
     }
@@ -21,7 +23,8 @@ int continue_loop() {
 // Should return 15
 int break_loop() {
     int sum = 0;
-    for (int i = 0; i < 10; ++i) {
+    const int kOne = 1;
+    for (int i = 0; i < 10; i += kOne) {
         if (i > 5) { break; }
         sum += i;
     }

src/sksl/SkSLAnalysis.cpp

@@ -15,6 +15,7 @@
 #include "include/sksl/SkSLErrorReporter.h"
 #include "src/core/SkSafeMath.h"
 #include "src/sksl/SkSLCompiler.h"
+#include "src/sksl/SkSLConstantFolder.h"
 #include "src/sksl/ir/SkSLExpression.h"
 #include "src/sksl/ir/SkSLProgram.h"
 
@@ -1009,19 +1010,6 @@ bool Analysis::IsSameExpressionTree(const Expression& left, const Expression& ri
     }
 }
 
-static bool get_constant_value(const Expression& expr, double* val) {
-    const Expression* valExpr = expr.getConstantSubexpression(0);
-    if (!valExpr) {
-        return false;
-    }
-    if (valExpr->is<Literal>()) {
-        *val = valExpr->as<Literal>().value();
-        return true;
-    }
-    SkDEBUGFAILF("unexpected constant type (%s)", expr.type().description().c_str());
-    return false;
-}
-
 static const char* invalid_for_ES2(int offset,
                                    const Statement* loopInitializer,
                                    const Expression* loopTest,
@@ -1047,7 +1035,7 @@ static const char* invalid_for_ES2(int offset,
     if (!initDecl.value()) {
         return "missing loop index initializer";
     }
-    if (!get_constant_value(*initDecl.value(), &loopInfo.fStart)) {
+    if (!ConstantFolder::GetConstantValue(*initDecl.value(), &loopInfo.fStart)) {
         return "loop index initializer must be a constant expression";
     }
 
@@ -1084,7 +1072,7 @@ static const char* invalid_for_ES2(int offset,
             return "invalid relational operator";
     }
     double loopEnd = 0;
-    if (!get_constant_value(*cond.right(), &loopEnd)) {
+    if (!ConstantFolder::GetConstantValue(*cond.right(), &loopEnd)) {
         return "loop index must be compared with a constant expression";
     }
 
@@ -1106,7 +1094,7 @@ static const char* invalid_for_ES2(int offset,
             if (!is_loop_index(next.left())) {
                 return "expected loop index in loop expression";
             }
-            if (!get_constant_value(*next.right(), &loopInfo.fDelta)) {
+            if (!ConstantFolder::GetConstantValue(*next.right(), &loopInfo.fDelta)) {
                 return "loop index must be modified by a constant expression";
             }
             switch (next.getOperator().kind()) {

src/sksl/SkSLConstantFolder.cpp

@@ -145,6 +145,15 @@ bool ConstantFolder::GetConstantInt(const Expression& value, SKSL_INT* out) {
     return true;
 }
 
+bool ConstantFolder::GetConstantValue(const Expression& value, double* out) {
+    const Expression* expr = GetConstantValueForVariable(value);
+    if (!expr->is<Literal>()) {
+        return false;
+    }
+    *out = expr->as<Literal>().value();
+    return true;
+}
+
 static bool is_constant_scalar_value(const Expression& inExpr, double match) {
     const Expression* expr = ConstantFolder::GetConstantValueForVariable(inExpr);
     return (expr->is<Literal>() && expr->as<Literal>().value() == match);

src/sksl/SkSLConstantFolder.h

@@ -30,6 +30,12 @@ public:
      */
     static bool GetConstantInt(const Expression& value, SKSL_INT* out);
 
+    /**
+     * If value is a literal or const scalar variable with a known value, returns true and stores
+     * the value in out. Otherwise returns false.
+     */
+    static bool GetConstantValue(const Expression& value, double* out);
+
     /**
      * If the expression is a const variable with a known compile-time-constant value, returns that
      * value. If not, returns the original expression as-is.

tests/sksl/runtime/LoopFloat.stage

@@ -1,8 +1,10 @@
 uniform half4 colorRed;
 uniform half4 colorGreen;
+const float kZero_0 = 0.0;
+const float kTen_0 = 10.0;
 float return_loop_0()
 {
-	for (float i = 0.0;i < 10.0; ++i) 
+	for (float i = kZero_0;i < 10.0; ++i) 
 	{
 		if (i == 5.0) 
 		{
@@ -14,7 +16,7 @@ float return_loop_0()
 float continue_loop_0()
 {
 	float sum = 0.0;
-	for (float i = 0.0;i < 10.0; ++i) 
+	for (float i = 0.0;i < kTen_0; ++i) 
 	{
 		if (i < 5.0) 
 		{
@@ -27,7 +29,8 @@ float continue_loop_0()
 float break_loop_0()
 {
 	float sum = 0.0;
-	for (float i = 0.0;i < 10.0; ++i) 
+	const float kOne = 1.0;
+	for (float i = 0.0;i < 10.0; i += kOne) 
 	{
 		if (i > 5.0) 
 		{

tests/sksl/runtime/LoopInt.stage

@@ -1,8 +1,10 @@
 uniform half4 colorRed;
 uniform half4 colorGreen;
+const int kZero_0 = 0;
+const int kTen_0 = 10;
 int return_loop_0()
 {
-	for (int i = 0;i < 10; ++i) 
+	for (int i = kZero_0;i < 10; ++i) 
 	{
 		if (i == 5) 
 		{
@@ -14,7 +16,7 @@ int return_loop_0()
 int continue_loop_0()
 {
 	int sum = 0;
-	for (int i = 0;i < 10; ++i) 
+	for (int i = 0;i < kTen_0; ++i) 
 	{
 		if (i < 5) 
 		{
@@ -27,7 +29,8 @@ int continue_loop_0()
 int break_loop_0()
 {
 	int sum = 0;
-	for (int i = 0;i < 10; ++i) 
+	const int kOne = 1;
+	for (int i = 0;i < 10; i += kOne) 
 	{
 		if (i > 5) 
 		{