Issue warning when compile-time constant out-of-bounds array index is used.

Issue #98.
Also fixes two examples that had bugs of this type that this warning
  uncovered!

expr.cpp

@@ -2681,6 +2681,22 @@ IndexExpr::GetLValue(FunctionEmitContext *ctx) const {
     if (!basePtr)
         return NULL;
 
+    // If the array index is a compile time constant, check to see if it
+    // may lead to an out-of-bounds access.
+    ConstExpr *ce = dynamic_cast<ConstExpr *>(index);
+    const SequentialType *seqType = dynamic_cast<const SequentialType *>(type);
+    assert(seqType != NULL);
+    int nElements = seqType->GetElementCount();
+    if (ce != NULL && nElements > 0) {
+        int32_t indices[ISPC_MAX_NVEC];
+        int count = ce->AsInt32(indices);
+        for (int i = 0; i < count; ++i) {
+            if (indices[i] < 0 || indices[i] >= nElements)
+                Warning(index->pos, "Array index \"%d\" may be out of bounds for "
+                        "\"%d\" element array.", indices[i], nElements);
+        }
+    }
+
     basePtr = lCastUniformVectorBasePtr(basePtr, ctx);
 
     ctx->SetDebugPos(pos);

tests/array-1.ispc

@@ -3,7 +3,7 @@ export uniform int width() { return programCount; }
 
 
 
-static float x[2][1];
+static float x[1][2];
 
 export void f_f(uniform float RET[], uniform float aFOO[]) {
     float a = aFOO[programIndex];

tests/array-scatter-vary.ispc

@@ -13,7 +13,7 @@ export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
 }
 
     
-export void result(uniform float RET[4]) { 
+export void result(uniform float RET[]) { 
     RET[programIndex] = 0;
     RET[3] = 4;
     RET[4] = 5;