Re: [PATCH] Don't fold away division by zero (PR middle-end/66127)

[Marek Polacek <polacek at redhat dot com>]

On Wed, May 13, 2015 at 03:06:00PM +0000, Joseph Myers wrote:
> > It won't handle e.g. 0 * (unsigned) (1 / 0).
> 
> I think this illustrates why handling this in the folding-for-optimization 
> is a mistake.
> 
> For optimization, it's perfectly fine to fold away 0 * (something without 
> side effects), and division by 0 should only be considered to have side 
> effects if language semantic options were specified to that effect (such 
> as using ubsan), which should then have the effect of producing GENERIC 
> that's not a simple division but represents whatever checks are required.
> 
> Rather, how about having an extra argument to c_fully_fold_internal (e.g. 
> for_int_const) indicating that the folding is of an expression with 
> integer constant operands (so this argument would be true in the new case 
> of folding the contents of a C_MAYBE_CONST_EXPR with 
> C_MAYBE_CONST_EXPR_INT_OPERANDS set)?  In that special case, 
> c_fully_fold_internal would only fold the expression itself if all 
> evaluated operands folded to INTEGER_CSTs (so if something that doesn't 
> get folded, such as division by 0, is encountered, then all evaluated 
> expressions containing it also don't get folded).

Did you mean something like the following?  It is on top of the previous
c_fully_fold_internal patch; if it makes any sense, I'll squash these
into one.

With the following, I don't see any regressions in the C dg.exp testsuite.
This still doesn't reject enum { A = 0 * (unsigned) (1 / 0) };, but note
that we don't reject such an enum at present.

Thanks.

diff --git gcc/c-family/c-common.c gcc/c-family/c-common.c
index 24200f0..1dc181d 100644
--- gcc/c-family/c-common.c
+++ gcc/c-family/c-common.c
@@ -315,7 +315,7 @@ const struct fname_var_t fname_vars[] =
 /* Global visibility options.  */
 struct visibility_flags visibility_options;
 
-static tree c_fully_fold_internal (tree expr, bool, bool *, bool *);
+static tree c_fully_fold_internal (tree expr, bool, bool *, bool *, bool);
 static tree check_case_value (location_t, tree);
 static bool check_case_bounds (location_t, tree, tree, tree *, tree *);
 
@@ -1148,7 +1148,7 @@ c_fully_fold (tree expr, bool in_init, bool *maybe_const)
       expr = TREE_OPERAND (expr, 0);
     }
   ret = c_fully_fold_internal (expr, in_init, maybe_const,
-			       &maybe_const_itself);
+			       &maybe_const_itself, false);
   if (eptype)
     ret = fold_convert_loc (loc, eptype, ret);
   *maybe_const &= maybe_const_itself;
@@ -1161,11 +1161,13 @@ c_fully_fold (tree expr, bool in_init, bool *maybe_const)
    arithmetic overflow (for C90, *MAYBE_CONST_OPERANDS is carried from
    both evaluated and unevaluated subexpressions while
    *MAYBE_CONST_ITSELF is carried from only evaluated
-   subexpressions).  */
+   subexpressions).  FOR_INT_CONST indicates if EXPR is an expression
+   with integer constant operands, and if any of the operands doesn't
+   get folded to an integer constant, don't fold the expression itself.  */
 
 static tree
 c_fully_fold_internal (tree expr, bool in_init, bool *maybe_const_operands,
-		       bool *maybe_const_itself)
+		       bool *maybe_const_itself, bool for_int_const)
 {
   tree ret = expr;
   enum tree_code code = TREE_CODE (expr);
@@ -1212,7 +1214,7 @@ c_fully_fold_internal (tree expr, bool in_init, bool *maybe_const_operands,
 	{
 	  *maybe_const_itself = false;
 	  inner = c_fully_fold_internal (inner, in_init, maybe_const_operands,
-					 maybe_const_itself);
+					 maybe_const_itself, true);
 	}
       if (pre && !in_init)
 	ret = build2 (COMPOUND_EXPR, TREE_TYPE (expr), pre, inner);
@@ -1263,7 +1265,7 @@ c_fully_fold_internal (tree expr, bool in_init, bool *maybe_const_operands,
       op1 = TREE_OPERAND (expr, 1);
       op2 = TREE_OPERAND (expr, 2);
       op0 = c_fully_fold_internal (op0, in_init, maybe_const_operands,
-				   maybe_const_itself);
+				   maybe_const_itself, for_int_const);
       STRIP_TYPE_NOPS (op0);
       if (op0 != orig_op0)
 	ret = build3 (COMPONENT_REF, TREE_TYPE (expr), op0, op1, op2);
@@ -1280,10 +1282,10 @@ c_fully_fold_internal (tree expr, bool in_init, bool *maybe_const_operands,
       op2 = TREE_OPERAND (expr, 2);
       op3 = TREE_OPERAND (expr, 3);
       op0 = c_fully_fold_internal (op0, in_init, maybe_const_operands,
-				   maybe_const_itself);
+				   maybe_const_itself, for_int_const);
       STRIP_TYPE_NOPS (op0);
       op1 = c_fully_fold_internal (op1, in_init, maybe_const_operands,
-				   maybe_const_itself);
+				   maybe_const_itself, for_int_const);
       STRIP_TYPE_NOPS (op1);
       op1 = decl_constant_value_for_optimization (op1);
       if (op0 != orig_op0 || op1 != orig_op1)
@@ -1340,7 +1342,7 @@ c_fully_fold_internal (tree expr, bool in_init, bool *maybe_const_operands,
       orig_op0 = op0 = TREE_OPERAND (expr, 0);
       orig_op1 = op1 = TREE_OPERAND (expr, 1);
       op0 = c_fully_fold_internal (op0, in_init, maybe_const_operands,
-				   maybe_const_itself);
+				   maybe_const_itself, for_int_const);
       STRIP_TYPE_NOPS (op0);
       if (code != MODIFY_EXPR
 	  && code != PREDECREMENT_EXPR
@@ -1352,9 +1354,14 @@ c_fully_fold_internal (tree expr, bool in_init, bool *maybe_const_operands,
 	 expression for the sake of conversion warnings.  */
       if (code != MODIFY_EXPR)
 	op1 = c_fully_fold_internal (op1, in_init, maybe_const_operands,
-				     maybe_const_itself);
+				     maybe_const_itself, for_int_const);
       STRIP_TYPE_NOPS (op1);
       op1 = decl_constant_value_for_optimization (op1);
+
+      if (for_int_const && (TREE_CODE (op0) != INTEGER_CST
+			    || TREE_CODE (op1) != INTEGER_CST))
+	goto out;
+
       if (op0 != orig_op0 || op1 != orig_op1 || in_init)
 	ret = in_init
 	  ? fold_build2_initializer_loc (loc, code, TREE_TYPE (expr), op0, op1)
@@ -1424,7 +1431,7 @@ c_fully_fold_internal (tree expr, bool in_init, bool *maybe_const_operands,
       /* Unary operations.  */
       orig_op0 = op0 = TREE_OPERAND (expr, 0);
       op0 = c_fully_fold_internal (op0, in_init, maybe_const_operands,
-				   maybe_const_itself);
+				   maybe_const_itself, for_int_const);
       STRIP_TYPE_NOPS (op0);
       if (code != ADDR_EXPR && code != REALPART_EXPR && code != IMAGPART_EXPR)
 	op0 = decl_constant_value_for_optimization (op0);
@@ -1472,14 +1479,16 @@ c_fully_fold_internal (tree expr, bool in_init, bool *maybe_const_operands,
 	 arguments.  */
       orig_op0 = op0 = TREE_OPERAND (expr, 0);
       orig_op1 = op1 = TREE_OPERAND (expr, 1);
-      op0 = c_fully_fold_internal (op0, in_init, &op0_const, &op0_const_self);
+      op0 = c_fully_fold_internal (op0, in_init, &op0_const, &op0_const_self,
+				   for_int_const);
       STRIP_TYPE_NOPS (op0);
 
       unused_p = (op0 == (code == TRUTH_ANDIF_EXPR
 			  ? truthvalue_false_node
 			  : truthvalue_true_node));
       c_disable_warnings (unused_p);
-      op1 = c_fully_fold_internal (op1, in_init, &op1_const, &op1_const_self);
+      op1 = c_fully_fold_internal (op1, in_init, &op1_const, &op1_const_self,
+				   for_int_const);
       STRIP_TYPE_NOPS (op1);
       c_enable_warnings (unused_p);
 
@@ -1510,16 +1519,19 @@ c_fully_fold_internal (tree expr, bool in_init, bool *maybe_const_operands,
       orig_op0 = op0 = TREE_OPERAND (expr, 0);
       orig_op1 = op1 = TREE_OPERAND (expr, 1);
       orig_op2 = op2 = TREE_OPERAND (expr, 2);
-      op0 = c_fully_fold_internal (op0, in_init, &op0_const, &op0_const_self);
+      op0 = c_fully_fold_internal (op0, in_init, &op0_const, &op0_const_self,
+				   for_int_const);
 
       STRIP_TYPE_NOPS (op0);
       c_disable_warnings (op0 == truthvalue_false_node);
-      op1 = c_fully_fold_internal (op1, in_init, &op1_const, &op1_const_self);
+      op1 = c_fully_fold_internal (op1, in_init, &op1_const, &op1_const_self,
+				   for_int_const);
       STRIP_TYPE_NOPS (op1);
       c_enable_warnings (op0 == truthvalue_false_node);
 
       c_disable_warnings (op0 == truthvalue_true_node);
-      op2 = c_fully_fold_internal (op2, in_init, &op2_const, &op2_const_self);
+      op2 = c_fully_fold_internal (op2, in_init, &op2_const, &op2_const_self,
+				   for_int_const);
       STRIP_TYPE_NOPS (op2);
       c_enable_warnings (op0 == truthvalue_true_node);
 

	Marek