Short-circuit evaluation of && and || operators.

We now follow C's approach of evaluating these: we don't evaluate
the second expression in the operator if the value of the first one
determines the overall result.  Thus, these can now be used 
idiomatically like (index < limit && array[index] > 0) and such.

For varying expressions, the mask is set appropriately when evaluating
the second expression.

(For expressions that can be determined to be both simple and safe to
evaluate with the mask all off, we still evaluate both sides and compute
the logical op result directly, which saves a number of branches and tests.
However, the effect of this should never be visible to the programmer.)

Issue #4.

ast.cpp

@@ -315,3 +315,116 @@ EstimateCost(ASTNode *root) {
     return cost;
 }
 
+
+/** Given an AST node, check to see if it's safe if we happen to run the
+    code for that node with the execution mask all off.
+ */
+static bool
+lCheckAllOffSafety(ASTNode *node, void *data) {
+    bool *okPtr = (bool *)data;
+
+    if (dynamic_cast<FunctionCallExpr *>(node) != NULL) {
+        // FIXME: If we could somehow determine that the function being
+        // called was safe (and all of the args Exprs were safe, then it'd
+        // be nice to be able to return true here.  (Consider a call to
+        // e.g. floatbits() in the stdlib.)  Unfortunately for now we just
+        // have to be conservative.
+        *okPtr = false;
+        return false;
+    }
+
+    if (dynamic_cast<AssertStmt *>(node) != NULL) {
+        // While it's fine to run the assert for varying tests, it's not
+        // desirable to check an assert on a uniform variable if all of the
+        // lanes are off.
+        *okPtr = false;
+        return false;
+    }
+
+    if (dynamic_cast<NewExpr *>(node) != NULL ||
+        dynamic_cast<DeleteStmt *>(node) != NULL) {
+        // We definitely don't want to run the uniform variants of these if
+        // the mask is all off.  It's also worth skipping the overhead of
+        // executing the varying versions of them in the all-off mask case.
+        *okPtr = false;
+        return false;
+    }
+
+    if (g->target.allOffMaskIsSafe == true)
+        // Don't worry about memory accesses if we have a target that can
+        // safely run them with the mask all off
+        return true;
+
+    IndexExpr *ie;
+    if ((ie = dynamic_cast<IndexExpr *>(node)) != NULL && ie->baseExpr != NULL) {
+        const Type *type = ie->baseExpr->GetType();
+        if (type == NULL)
+            return true;
+        if (dynamic_cast<const ReferenceType *>(type) != NULL)
+            type = type->GetReferenceTarget();
+
+        ConstExpr *ce = dynamic_cast<ConstExpr *>(ie->index);
+        if (ce == NULL) {
+            // indexing with a variable... -> not safe
+            *okPtr = false;
+            return false;
+        }
+
+        const PointerType *pointerType = 
+            dynamic_cast<const PointerType *>(type);
+        if (pointerType != NULL) {
+            // pointer[index] -> can't be sure -> not safe
+            *okPtr = false;
+            return false;
+        }
+
+        const SequentialType *seqType = 
+            dynamic_cast<const SequentialType *>(type);
+        Assert(seqType != NULL);
+        int nElements = seqType->GetElementCount();
+        if (nElements == 0) {
+            // Unsized array, so we can't be sure -> not safe
+            *okPtr = false;
+            return false;
+        }
+
+        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) {
+                // Index is out of bounds -> not safe
+                *okPtr = false;
+                return false;
+            }
+        }
+
+        // All indices are in-bounds
+        return true;
+    }
+
+    MemberExpr *me;
+    if ((me = dynamic_cast<MemberExpr *>(node)) != NULL &&
+        me->dereferenceExpr) {
+        *okPtr = false;
+        return false;
+    }
+
+    DereferenceExpr *de;
+    if ((de = dynamic_cast<DereferenceExpr *>(node)) != NULL) {
+        const Type *exprType = de->expr->GetType();
+        if (dynamic_cast<const PointerType *>(exprType) != NULL) {
+            *okPtr = false;
+            return false;
+        }
+    }
+
+    return true;
+}
+
+
+bool
+SafeToRunWithMaskAllOff(ASTNode *root) {
+    bool safe = true;
+    WalkAST(root, lCheckAllOffSafety, NULL, &safe);
+    return safe;
+}