Hi,
As quoted from the function difference_cannot_overflow_p,
/* TODO: deeper inspection may be necessary to prove the equality. */
switch (code)
{
case PLUS_EXPR:
return expr_equal_p (e1, offset) || expr_equal_p (e2, offset);
case POINTER_PLUS_EXPR:
return expr_equal_p (e2, offset);
default:
return false;
}
The overflow check can be improved by using deeper inspection to prove the
equality. This patch deals with that by making below two improvements:
a) Handles constant cases.
b) Uses affine expansion as deeper inspection to check the equality.
As a result, functions strip_wrap_conserving_type_conversions and
expr_equal_p can be removed now. A test case is also added to illustrate iv
elimination opportunity captured by this patch.
Thanks,
bin
2014-07-17 Bin Cheng <bin.ch...@arm.com>
* tree-ssa-loop-ivopts.c (ivopts_data): New field name_expansion.
(tree_ssa_iv_optimize_init): Initialize name_expansion.
(tree_ssa_iv_optimize_finalize): Free name_expansion.
(strip_wrap_conserving_type_conversions, expr_equal_p): Delete.
(difference_cannot_overflow_p): New parameter. Handle constant
cases. Use affine expansion for equality check.
(iv_elimination_compare_lt): Pass new argument.
gcc/testsuite/ChangeLog
2014-07-17 Bin Cheng <bin.ch...@arm.com>
* gcc.dg/tree-ssa/ivopts-lt-2.c: New test.
Index: gcc/testsuite/gcc.dg/tree-ssa/ivopts-lt-2.c
===================================================================
--- gcc/testsuite/gcc.dg/tree-ssa/ivopts-lt-2.c (revision 0)
+++ gcc/testsuite/gcc.dg/tree-ssa/ivopts-lt-2.c (revision 0)
@@ -0,0 +1,20 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-ivopts" } */
+
+void
+f1 (int *p, unsigned int i)
+{
+ p += i;
+ do
+ {
+ *p = 0;
+ p += 1;
+ i++;
+ }
+ while (i < 100);
+}
+
+/* { dg-final { scan-tree-dump-times "PHI" 1 "ivopts" } } */
+/* { dg-final { scan-tree-dump-times "PHI <p_" 1 "ivopts"} } */
+/* { dg-final { scan-tree-dump-times "p_\[0-9\]* <" 1 "ivopts" } } */
+/* { dg-final { cleanup-tree-dump "ivopts" } } */
Index: gcc/tree-ssa-loop-ivopts.c
===================================================================
--- gcc/tree-ssa-loop-ivopts.c (revision 212387)
+++ gcc/tree-ssa-loop-ivopts.c (working copy)
@@ -323,6 +323,9 @@ struct ivopts_data
/* A bitmap of important candidates. */
bitmap important_candidates;
+ /* Cache used by tree_to_aff_combination_expand. */
+ struct pointer_map_t *name_expansion;
+
/* The maximum invariant id. */
unsigned max_inv_id;
@@ -877,6 +880,7 @@ tree_ssa_iv_optimize_init (struct ivopts_data *dat
data->iv_candidates.create (20);
data->inv_expr_tab = new hash_table<iv_inv_expr_hasher> (10);
data->inv_expr_id = 0;
+ data->name_expansion = NULL;
decl_rtl_to_reset.create (20);
}
@@ -4449,76 +4453,40 @@ iv_elimination_compare (struct ivopts_data *data,
return (exit->flags & EDGE_TRUE_VALUE ? EQ_EXPR : NE_EXPR);
}
-static tree
-strip_wrap_conserving_type_conversions (tree exp)
-{
- while (tree_ssa_useless_type_conversion (exp)
- && (nowrap_type_p (TREE_TYPE (exp))
- == nowrap_type_p (TREE_TYPE (TREE_OPERAND (exp, 0)))))
- exp = TREE_OPERAND (exp, 0);
- return exp;
-}
+/* Returns true if we can prove that BASE - OFFSET does not overflow. For now,
+ we only check either the case BASE and OFFSET are integer constants, or the
+ situation that BASE = SOMETHING + OFFSET, where the calculation is performed
+ in non-wrapping type. For the latter case, we use affine expansion for
+ further equality check.
-/* Walk the SSA form and check whether E == WHAT. Fairly simplistic, we
- check for an exact match. */
+ TODO: More generally, we could test for the situation that
+ BASE = SOMETHING + OFFSET' and OFFSET is between OFFSET' and zero.
+ This would require knowing the sign of OFFSET. */
static bool
-expr_equal_p (tree e, tree what)
+difference_cannot_overflow_p (struct ivopts_data *data, tree base, tree offset)
{
- gimple stmt;
enum tree_code code;
+ tree e1, e2;
+ aff_tree aff_e1, aff_e2, aff_offset;
- e = strip_wrap_conserving_type_conversions (e);
- what = strip_wrap_conserving_type_conversions (what);
-
- code = TREE_CODE (what);
- if (TREE_TYPE (e) != TREE_TYPE (what))
- return false;
-
- if (operand_equal_p (e, what, 0))
+ /* No overflow if offset is zero. */
+ if (offset == integer_zero_node)
return true;
- if (TREE_CODE (e) != SSA_NAME)
- return false;
-
- stmt = SSA_NAME_DEF_STMT (e);
- if (gimple_code (stmt) != GIMPLE_ASSIGN
- || gimple_assign_rhs_code (stmt) != code)
- return false;
-
- switch (get_gimple_rhs_class (code))
+ /* Overflow can be checked easily for constant values. */
+ if (TREE_CODE (base) == INTEGER_CST && TREE_CODE (offset) == INTEGER_CST)
{
- case GIMPLE_BINARY_RHS:
- if (!expr_equal_p (gimple_assign_rhs2 (stmt), TREE_OPERAND (what, 1)))
- return false;
- /* Fallthru. */
+ bool overflow = false;
+ tree type = TREE_TYPE (base);
+ signop sign = TYPE_SIGN (type);
- case GIMPLE_UNARY_RHS:
- case GIMPLE_SINGLE_RHS:
- return expr_equal_p (gimple_assign_rhs1 (stmt), TREE_OPERAND (what, 0));
- default:
- return false;
+ wide_int arg2 = wide_int::from (offset, TYPE_PRECISION (type),
+ TYPE_SIGN (TREE_TYPE (offset)));
+ (void) wi::sub (base, arg2, sign, &overflow);
+ return overflow;
}
-}
-/* Returns true if we can prove that BASE - OFFSET does not overflow. For now,
- we only detect the situation that BASE = SOMETHING + OFFSET, where the
- calculation is performed in non-wrapping type.
-
- TODO: More generally, we could test for the situation that
- BASE = SOMETHING + OFFSET' and OFFSET is between OFFSET' and zero.
- This would require knowing the sign of OFFSET.
-
- Also, we only look for the first addition in the computation of BASE.
- More complex analysis would be better, but introducing it just for
- this optimization seems like an overkill. */
-
-static bool
-difference_cannot_overflow_p (tree base, tree offset)
-{
- enum tree_code code;
- tree e1, e2;
-
if (!nowrap_type_p (TREE_TYPE (base)))
return false;
@@ -4547,13 +4515,27 @@ static bool
e2 = TREE_OPERAND (base, 1);
}
- /* TODO: deeper inspection may be necessary to prove the equality. */
+ /* Use affine expansion as deeper inspection to prove the equality. */
+ tree_to_aff_combination_expand (e2, TREE_TYPE (e2),
+ &aff_e2, &data->name_expansion);
+ tree_to_aff_combination_expand (offset, TREE_TYPE (offset),
+ &aff_offset, &data->name_expansion);
+ aff_combination_scale (&aff_offset, -1);
switch (code)
{
case PLUS_EXPR:
- return expr_equal_p (e1, offset) || expr_equal_p (e2, offset);
+ aff_combination_add (&aff_e2, &aff_offset);
+ if (aff_combination_zero_p (&aff_e2))
+ return true;
+
+ tree_to_aff_combination_expand (e1, TREE_TYPE (e1),
+ &aff_e1, &data->name_expansion);
+ aff_combination_add (&aff_e1, &aff_offset);
+ return aff_combination_zero_p (&aff_e1);
+
case POINTER_PLUS_EXPR:
- return expr_equal_p (e2, offset);
+ aff_combination_add (&aff_e2, &aff_offset);
+ return aff_combination_zero_p (&aff_e2);
default:
return false;
@@ -4677,7 +4659,7 @@ iv_elimination_compare_lt (struct ivopts_data *dat
offset = fold_build2 (MULT_EXPR, TREE_TYPE (cand->iv->step),
cand->iv->step,
fold_convert (TREE_TYPE (cand->iv->step), a));
- if (!difference_cannot_overflow_p (cand->iv->base, offset))
+ if (!difference_cannot_overflow_p (data, cand->iv->base, offset))
return false;
/* Determine the new comparison operator. */
@@ -6805,6 +6787,7 @@ tree_ssa_iv_optimize_finalize (struct ivopts_data
data->iv_candidates.release ();
delete data->inv_expr_tab;
data->inv_expr_tab = NULL;
+ free_affine_expand_cache (&data->name_expansion);
}
/* Returns true if the loop body BODY includes any function calls. */
