added basic dataflow analysis to skslc

tests/SkSLErrorTest.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkSLCompiler.h"
 
 #include "Test.h"
@@ skipping 163 matching lines @@
 DEF_TEST(SkSLUsingInvalidValue, r) {
     test_failure(r,
                  "void main() { int x = int; }", 
                  "error: 1: expected '(' to begin constructor invocation\n1 error\n");
     test_failure(r,
                  "int test() { return 1; } void main() { int x = test; }", 
                  "error: 1: expected '(' to begin function call\n1 error\n");
 }
 DEF_TEST(SkSLDifferentReturnType, r) {
     test_failure(r,
-                 "int main() { } void main() { }", 
+                 "int main() { return 1; } void main() { }", 
                  "error: 1: functions 'void main()' and 'int main()' differ only in return type\n1 "
                  "error\n");
 }
 
 DEF_TEST(SkSLDifferentModifiers, r) {
     test_failure(r,
                  "void test(int x); void test(out int x) { }", 
                  "error: 1: modifiers on parameter 1 differ between declaration and definition\n1 "
                  "error\n");
 }
@@ skipping 76 matching lines @@
     test_failure(r,
                  "void main() { int x = 5 > 2 ? true : 1.0; }",
                  "error: 1: ternary operator result mismatch: 'bool', 'float'\n1 error\n");
 }
 
 DEF_TEST(SkSLInterfaceBlockStorageModifiers, r) {
     test_failure(r,
                  "uniform foo { out int x; };",
                  "error: 1: interface block fields may not have storage qualifiers\n1 error\n");
 }
+
+DEF_TEST(SkSLUseWithoutInitialize, r) {
+    test_failure(r,
+                 "void main() { int x; if (5 == 2) x = 3; x++; }",

[dogben, 2016/10/13 03:55:43]

Would be good to add a few more test cases:

1. expressions in lvalues:
void main() {
    int x[2][2];
    x[i][1] = 4;
}

2. expressions in return statement:
int main() { return x; }

3. expressions in variable assignments:
void main() { int x = y; }

4. logical and and or
void main() { if (true && (false || x)) { return; } }

[ethannicholas, 2016/10/13 17:41:28]

Done.

+                 "error: 1: 'x' has not been assigned\n1 error\n");
+}
+
+DEF_TEST(SkSLUnreachable, r) {
+    test_failure(r,
+                 "void main() { return; return; }",
+                 "error: 1: unreachable\n1 error\n");
+    test_failure(r,
+                 "void main() { for (;;) { continue; int x = 1; } }",
+                 "error: 1: unreachable\n1 error\n");
+    test_failure(r,
+                 "void main() { for (;;) { } return; }",
+                 "error: 1: unreachable\n1 error\n");
+    test_failure(r,
+                 "void main() { if (true) return; else discard; return; }",

[dogben, 2016/10/13 03:55:43]

Maybe add this test case: (I think this works correctly, but I originally
thought it didn't work.)
void main() {
    return;
    while (true);
}

[ethannicholas, 2016/10/13 17:41:28]

Done.

+                 "error: 1: unreachable\n1 error\n");
+}
+
+DEF_TEST(SkSLNoReturn, r) {
+    test_failure(r,
+                 "int foo() { if (2 > 5) return 3; }",
+                 "error: 1: function can exit without returning a value\n1 error\n");
+}
+
+DEF_TEST(SkSLBreakOutsideLoop, r) {
+    test_failure(r,
+                 "void foo() { while(true) {} if (true) break; }",
+                 "error: 1: break statement must be inside a loop\n1 error\n");
+}
+
+DEF_TEST(SkSLContinueOutsideLoop, r) {
+    test_failure(r,
+                 "void foo() { for(;;); continue; }",
+                 "error: 1: continue statement must be inside a loop\n1 error\n");
+}