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Re: [RFC] optimize x - y cmp 0 with undefined overflow
- From: Eric Botcazou <ebotcazou at adacore dot com>
- To: Richard Biener <richard dot guenther at gmail dot com>
- Cc: gcc-patches at gcc dot gnu dot org
- Date: Tue, 27 May 2014 18:12:58 +0200
- Subject: Re: [RFC] optimize x - y cmp 0 with undefined overflow
- Authentication-results: sourceware.org; auth=none
- References: <1466969 dot VvFMuDKXoD at polaris> <53896104 dot oWY7sHq2zD at polaris> <CAFiYyc07qb6BtaGerCoojJercTnO4=b184O7km-rge=CyBjq2g at mail dot gmail dot com>
> I'm asking to merge them (move them to fold_comparison).
Done (in the end the patch removes more lines than it adds :-).
Tested on x86_64-suse-linux with no regressions.
2014-05-27 Eric Botcazou <ebotcazou@adacore.com>
* fold-const.c (fold_comparison): Clean up and extend X +- C1 CMP C2
to X CMP C2 -+ C1 transformation to EQ_EXPR/NE_EXPR.
Add X - Y CMP 0 to X CMP Y transformation.
(fold_binary_loc) <EQ_EXPR/NE_EXPR>: Remove same transformations.
2014-05-27 Eric Botcazou <ebotcazou@adacore.com>
* gcc.dg/fold-compare-8.c: New test.
* gcc.dg/Wstrict-overflow-25.c: Likewise.
--
Eric BotcazouIndex: fold-const.c
===================================================================
--- fold-const.c (revision 210911)
+++ fold-const.c (working copy)
@@ -8904,6 +8904,7 @@ static tree
fold_comparison (location_t loc, enum tree_code code, tree type,
tree op0, tree op1)
{
+ const bool equality_code = (code == EQ_EXPR || code == NE_EXPR);
tree arg0, arg1, tem;
arg0 = op0;
@@ -8920,28 +8921,24 @@ fold_comparison (location_t loc, enum tr
if (tree_swap_operands_p (arg0, arg1, true))
return fold_build2_loc (loc, swap_tree_comparison (code), type, op1, op0);
- /* Transform comparisons of the form X +- C1 CMP C2 to X CMP C2 +- C1. */
+ /* Transform comparisons of the form X +- C1 CMP C2 to X CMP C2 -+ C1. */
if ((TREE_CODE (arg0) == PLUS_EXPR || TREE_CODE (arg0) == MINUS_EXPR)
- && (TREE_CODE (TREE_OPERAND (arg0, 1)) == INTEGER_CST
- && !TREE_OVERFLOW (TREE_OPERAND (arg0, 1))
- && TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (arg1)))
- && (TREE_CODE (arg1) == INTEGER_CST
- && !TREE_OVERFLOW (arg1)))
+ && (equality_code || TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (arg0)))
+ && TREE_CODE (TREE_OPERAND (arg0, 1)) == INTEGER_CST
+ && !TREE_OVERFLOW (TREE_OPERAND (arg0, 1))
+ && TREE_CODE (arg1) == INTEGER_CST
+ && !TREE_OVERFLOW (arg1))
{
+ const enum tree_code
+ reverse_op = TREE_CODE (arg0) == PLUS_EXPR ? MINUS_EXPR : PLUS_EXPR;
tree const1 = TREE_OPERAND (arg0, 1);
- tree const2 = arg1;
+ tree const2 = fold_convert_loc (loc, TREE_TYPE (const1), arg1);
tree variable = TREE_OPERAND (arg0, 0);
- tree lhs;
- int lhs_add;
- lhs_add = TREE_CODE (arg0) != PLUS_EXPR;
-
- lhs = fold_build2_loc (loc, lhs_add ? PLUS_EXPR : MINUS_EXPR,
- TREE_TYPE (arg1), const2, const1);
+ tree new_const = int_const_binop (reverse_op, const2, const1);
/* If the constant operation overflowed this can be
simplified as a comparison against INT_MAX/INT_MIN. */
- if (TREE_CODE (lhs) == INTEGER_CST
- && TREE_OVERFLOW (lhs))
+ if (TREE_OVERFLOW (new_const))
{
int const1_sgn = tree_int_cst_sgn (const1);
enum tree_code code2 = code;
@@ -8961,29 +8958,48 @@ fold_comparison (location_t loc, enum tr
/* We now can look at the canonicalized case
VARIABLE + 1 CODE2 INT_MIN
and decide on the result. */
- if (code2 == LT_EXPR
- || code2 == LE_EXPR
- || code2 == EQ_EXPR)
- return omit_one_operand_loc (loc, type, boolean_false_node, variable);
- else if (code2 == NE_EXPR
- || code2 == GE_EXPR
- || code2 == GT_EXPR)
- return omit_one_operand_loc (loc, type, boolean_true_node, variable);
- }
+ switch (code2)
+ {
+ case EQ_EXPR:
+ case LT_EXPR:
+ case LE_EXPR:
+ return
+ omit_one_operand_loc (loc, type, boolean_false_node, variable);
+
+ case NE_EXPR:
+ case GE_EXPR:
+ case GT_EXPR:
+ return
+ omit_one_operand_loc (loc, type, boolean_true_node, variable);
- if (TREE_CODE (lhs) == TREE_CODE (arg1)
- && (TREE_CODE (lhs) != INTEGER_CST
- || !TREE_OVERFLOW (lhs)))
+ default:
+ gcc_unreachable ();
+ }
+ }
+ else
{
- if (code != EQ_EXPR && code != NE_EXPR)
+ if (!equality_code)
fold_overflow_warning ("assuming signed overflow does not occur "
"when changing X +- C1 cmp C2 to "
- "X cmp C1 +- C2",
+ "X cmp C2 -+ C1",
WARN_STRICT_OVERFLOW_COMPARISON);
- return fold_build2_loc (loc, code, type, variable, lhs);
+ return fold_build2_loc (loc, code, type, variable, new_const);
}
}
+ /* Transform comparisons of the form X - Y CMP 0 to X CMP Y. */
+ if (TREE_CODE (arg0) == MINUS_EXPR
+ && (equality_code || TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (arg0)))
+ && integer_zerop (arg1))
+ {
+ if (!equality_code)
+ fold_overflow_warning ("assuming signed overflow does not occur "
+ "when changing X - Y cmp 0 to X cmp Y",
+ WARN_STRICT_OVERFLOW_COMPARISON);
+ return fold_build2_loc (loc, code, type, TREE_OPERAND (arg0, 0),
+ TREE_OPERAND (arg0, 1));
+ }
+
/* For comparisons of pointers we can decompose it to a compile time
comparison of the base objects and the offsets into the object.
This requires at least one operand being an ADDR_EXPR or a
@@ -9111,8 +9127,7 @@ fold_comparison (location_t loc, enum tr
|| POINTER_TYPE_OVERFLOW_UNDEFINED))
{
- if (code != EQ_EXPR
- && code != NE_EXPR
+ if (!equality_code
&& bitpos0 != bitpos1
&& (pointer_may_wrap_p (base0, offset0, bitpos0)
|| pointer_may_wrap_p (base1, offset1, bitpos1)))
@@ -9146,7 +9161,7 @@ fold_comparison (location_t loc, enum tr
object and overflow on pointer differences is undefined as of
6.5.6/8 and /9 with respect to the signed ptrdiff_t. */
else if (bitpos0 == bitpos1
- && ((code == EQ_EXPR || code == NE_EXPR)
+ && (equality_code
|| (indirect_base0 && DECL_P (base0))
|| POINTER_TYPE_OVERFLOW_UNDEFINED))
{
@@ -9164,8 +9179,7 @@ fold_comparison (location_t loc, enum tr
else
offset1 = fold_convert_loc (loc, ssizetype, offset1);
- if (code != EQ_EXPR
- && code != NE_EXPR
+ if (!equality_code
&& (pointer_may_wrap_p (base0, offset0, bitpos0)
|| pointer_may_wrap_p (base1, offset1, bitpos1)))
fold_overflow_warning (("assuming pointer wraparound does not "
@@ -12888,21 +12902,6 @@ fold_binary_loc (location_t loc,
type);
}
- /* If this is an EQ or NE comparison of a constant with a PLUS_EXPR or
- a MINUS_EXPR of a constant, we can convert it into a comparison with
- a revised constant as long as no overflow occurs. */
- if (TREE_CODE (arg1) == INTEGER_CST
- && (TREE_CODE (arg0) == PLUS_EXPR
- || TREE_CODE (arg0) == MINUS_EXPR)
- && TREE_CODE (TREE_OPERAND (arg0, 1)) == INTEGER_CST
- && 0 != (tem = const_binop (TREE_CODE (arg0) == PLUS_EXPR
- ? MINUS_EXPR : PLUS_EXPR,
- fold_convert_loc (loc, TREE_TYPE (arg0),
- arg1),
- TREE_OPERAND (arg0, 1)))
- && !TREE_OVERFLOW (tem))
- return fold_build2_loc (loc, code, type, TREE_OPERAND (arg0, 0), tem);
-
/* Similarly for a NEGATE_EXPR. */
if (TREE_CODE (arg0) == NEGATE_EXPR
&& TREE_CODE (arg1) == INTEGER_CST
@@ -12956,13 +12955,6 @@ fold_binary_loc (location_t loc,
TREE_OPERAND (arg0, 1), arg1);
}
- /* If we have X - Y == 0, we can convert that to X == Y and similarly
- for !=. Don't do this for ordered comparisons due to overflow. */
- if (TREE_CODE (arg0) == MINUS_EXPR
- && integer_zerop (arg1))
- return fold_build2_loc (loc, code, type,
- TREE_OPERAND (arg0, 0), TREE_OPERAND (arg0, 1));
-
/* Convert ABS_EXPR<x> == 0 or ABS_EXPR<x> != 0 to x == 0 or x != 0. */
if (TREE_CODE (arg0) == ABS_EXPR
&& (integer_zerop (arg1) || real_zerop (arg1)))