added basic dataflow analysis to skslc

tests/SkSLErrorTest.cpp

Index: tests/SkSLErrorTest.cpp
diff --git a/tests/SkSLErrorTest.cpp b/tests/SkSLErrorTest.cpp
index 400ab6dc57d343e16619eb0127b33581c21efc68..5ec73c595b70a4ee213ddf1906802702e22f702a 100644
--- a/tests/SkSLErrorTest.cpp
+++ b/tests/SkSLErrorTest.cpp
@@ -181,7 +181,7 @@ DEF_TEST(SkSLUsingInvalidValue, r) {
 }
 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");
 }
@@ -278,3 +278,42 @@ DEF_TEST(SkSLInterfaceBlockStorageModifiers, 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");
+}