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[7/8] Use a single comparison for index-based alias checks
- From: Richard Sandiford <richard dot sandiford at arm dot com>
- To: gcc-patches at gcc dot gnu dot org
- Date: Mon, 11 Nov 2019 18:51:22 +0000
- Subject: [7/8] Use a single comparison for index-based alias checks
- References: <mpt36eui76b.fsf@arm.com>
This patch rewrites the index-based alias checks to use conditions
of the form:
(unsigned T) (a - b + bias) <= limit
E.g. before the patch:
struct s { int x[100]; };
void
f1 (struct s *s1, int a, int b)
{
for (int i = 0; i < 32; ++i)
s1->x[i + a] += s1->x[i + b];
}
used:
add w3, w1, 3
cmp w3, w2
add w3, w2, 3
ccmp w1, w3, 0, ge
ble .L2
whereas after the patch it uses:
sub w3, w1, w2
add w3, w3, 3
cmp w3, 6
bls .L2
The patch also fixes the seg_len1 and seg_len2 negation for cases in
which seg_len is a "negative unsigned" value narrower than 64 bits,
like it is for 32-bit targets. Previously we'd end up with values
like 0xffffffff000000001 instead of 1.
2019-11-11 Richard Sandiford <richard.sandiford@arm.com>
gcc/
* tree-data-ref.c (create_intersect_range_checks_index): Rewrite
the index tests to have the form (unsigned T) (B - A + bias) <= limit.
gcc/testsuite/
* gcc.dg/vect/vect-alias-check-18.c: New test.
* gcc.dg/vect/vect-alias-check-19.c: Likewise.
* gcc.dg/vect/vect-alias-check-20.c: Likewise.
Index: gcc/tree-data-ref.c
===================================================================
--- gcc/tree-data-ref.c 2019-11-11 18:31:01.263118514 +0000
+++ gcc/tree-data-ref.c 2019-11-11 18:31:05.099091743 +0000
@@ -1744,7 +1744,9 @@ prune_runtime_alias_test_list (vec<dr_wi
We can create expression based on index rather than address:
- (i_0 + 4 < j_0 || j_0 + 4 < i_0)
+ (unsigned) (i_0 - j_0 + 3) <= 6
+
+ i.e. the indices are less than 4 apart.
Note evolution step of index needs to be considered in comparison. */
@@ -1781,15 +1783,8 @@ create_intersect_range_checks_index (cla
if (neg_step)
{
abs_step = -abs_step;
- seg_len1 = -seg_len1;
- seg_len2 = -seg_len2;
- }
- else
- {
- /* Include the access size in the length, so that we only have one
- tree addition below. */
- seg_len1 += dr_a.access_size;
- seg_len2 += dr_b.access_size;
+ seg_len1 = (-wi::to_poly_wide (dr_a.seg_len)).force_uhwi ();
+ seg_len2 = (-wi::to_poly_wide (dr_b.seg_len)).force_uhwi ();
}
/* Infer the number of iterations with which the memory segment is accessed
@@ -1803,16 +1798,13 @@ create_intersect_range_checks_index (cla
|| !can_div_trunc_p (seg_len2 + abs_step - 1, abs_step, &niter_len2))
return false;
- poly_uint64 niter_access1 = 0, niter_access2 = 0;
- if (neg_step)
- {
- /* Divide each access size by the byte step, rounding up. */
- if (!can_div_trunc_p (dr_a.access_size - abs_step - 1,
- abs_step, &niter_access1)
- || !can_div_trunc_p (dr_b.access_size + abs_step - 1,
- abs_step, &niter_access2))
- return false;
- }
+ /* Divide each access size by the byte step, rounding up. */
+ poly_uint64 niter_access1, niter_access2;
+ if (!can_div_trunc_p (dr_a.access_size + abs_step - 1,
+ abs_step, &niter_access1)
+ || !can_div_trunc_p (dr_b.access_size + abs_step - 1,
+ abs_step, &niter_access2))
+ return false;
unsigned int i;
for (i = 0; i < DR_NUM_DIMENSIONS (dr_a.dr); i++)
@@ -1852,38 +1844,87 @@ create_intersect_range_checks_index (cla
index of data reference. Like segment length, index length is
linear function of the number of iterations with index_step as
the coefficient, i.e, niter_len * idx_step. */
- tree idx_len1 = fold_build2 (MULT_EXPR, TREE_TYPE (min1), idx_step,
- build_int_cst (TREE_TYPE (min1),
- niter_len1));
- tree idx_len2 = fold_build2 (MULT_EXPR, TREE_TYPE (min2), idx_step,
- build_int_cst (TREE_TYPE (min2),
- niter_len2));
- tree max1 = fold_build2 (PLUS_EXPR, TREE_TYPE (min1), min1, idx_len1);
- tree max2 = fold_build2 (PLUS_EXPR, TREE_TYPE (min2), min2, idx_len2);
- /* Adjust ranges for negative step. */
+ offset_int abs_idx_step = offset_int::from (wi::to_wide (idx_step),
+ SIGNED);
if (neg_step)
- {
- /* IDX_LEN1 and IDX_LEN2 are negative in this case. */
- std::swap (min1, max1);
- std::swap (min2, max2);
-
- /* As with the lengths just calculated, we've measured the access
- sizes in iterations, so multiply them by the index step. */
- tree idx_access1
- = fold_build2 (MULT_EXPR, TREE_TYPE (min1), idx_step,
- build_int_cst (TREE_TYPE (min1), niter_access1));
- tree idx_access2
- = fold_build2 (MULT_EXPR, TREE_TYPE (min2), idx_step,
- build_int_cst (TREE_TYPE (min2), niter_access2));
-
- /* MINUS_EXPR because the above values are negative. */
- max1 = fold_build2 (MINUS_EXPR, TREE_TYPE (max1), max1, idx_access1);
- max2 = fold_build2 (MINUS_EXPR, TREE_TYPE (max2), max2, idx_access2);
- }
- tree part_cond_expr
- = fold_build2 (TRUTH_OR_EXPR, boolean_type_node,
- fold_build2 (LE_EXPR, boolean_type_node, max1, min2),
- fold_build2 (LE_EXPR, boolean_type_node, max2, min1));
+ abs_idx_step = -abs_idx_step;
+ poly_offset_int idx_len1 = abs_idx_step * niter_len1;
+ poly_offset_int idx_len2 = abs_idx_step * niter_len2;
+ poly_offset_int idx_access1 = abs_idx_step * niter_access1;
+ poly_offset_int idx_access2 = abs_idx_step * niter_access2;
+
+ gcc_assert (known_ge (idx_len1, 0)
+ && known_ge (idx_len2, 0)
+ && known_ge (idx_access1, 0)
+ && known_ge (idx_access2, 0));
+
+ /* Each access has the following pattern, with lengths measured
+ in units of INDEX:
+
+ <-- idx_len -->
+ <--- A: -ve step --->
+ +-----+-------+-----+-------+-----+
+ | n-1 | ..... | 0 | ..... | n-1 |
+ +-----+-------+-----+-------+-----+
+ <--- B: +ve step --->
+ <-- idx_len -->
+ |
+ min
+
+ where "n" is the number of scalar iterations covered by the segment
+ and where each access spans idx_access units.
+
+ A is the range of bytes accessed when the step is negative,
+ B is the range when the step is positive.
+
+ When checking for general overlap, we need to test whether
+ the range:
+
+ [min1 + low_offset1, min2 + high_offset1 + idx_access1 - 1]
+
+ overlaps:
+
+ [min2 + low_offset2, min2 + high_offset2 + idx_access2 - 1]
+
+ where:
+
+ low_offsetN = +ve step ? 0 : -idx_lenN;
+ high_offsetN = +ve step ? idx_lenN : 0;
+
+ This is equivalent to testing whether:
+
+ min1 + low_offset1 <= min2 + high_offset2 + idx_access2 - 1
+ && min2 + low_offset2 <= min1 + high_offset1 + idx_access1 - 1
+
+ Converting this into a single test, there is an overlap if:
+
+ 0 <= min2 - min1 + bias <= limit
+
+ where bias = high_offset2 + idx_access2 - 1 - low_offset1
+ limit = (high_offset1 - low_offset1 + idx_access1 - 1)
+ + (high_offset2 - low_offset2 + idx_access2 - 1)
+ i.e. limit = idx_len1 + idx_access1 - 1 + idx_len2 + idx_access2 - 1
+
+ Combining the tests requires limit to be computable in an unsigned
+ form of the index type; if it isn't, we fall back to the usual
+ pointer-based checks. */
+ poly_offset_int limit = (idx_len1 + idx_access1 - 1
+ + idx_len2 + idx_access2 - 1);
+ tree utype = unsigned_type_for (TREE_TYPE (min1));
+ if (!wi::fits_to_tree_p (limit, utype))
+ return false;
+
+ poly_offset_int low_offset1 = neg_step ? -idx_len1 : 0;
+ poly_offset_int high_offset2 = neg_step ? 0 : idx_len2;
+ poly_offset_int bias = high_offset2 + idx_access2 - 1 - low_offset1;
+
+ tree subject = fold_build2 (MINUS_EXPR, utype,
+ fold_convert (utype, min2),
+ fold_convert (utype, min1));
+ subject = fold_build2 (PLUS_EXPR, utype, subject,
+ wide_int_to_tree (utype, bias));
+ tree part_cond_expr = fold_build2 (GT_EXPR, boolean_type_node, subject,
+ wide_int_to_tree (utype, limit));
if (*cond_expr)
*cond_expr = fold_build2 (TRUTH_AND_EXPR, boolean_type_node,
*cond_expr, part_cond_expr);
Index: gcc/testsuite/gcc.dg/vect/vect-alias-check-18.c
===================================================================
--- /dev/null 2019-09-17 11:41:18.176664108 +0100
+++ gcc/testsuite/gcc.dg/vect/vect-alias-check-18.c 2019-11-11 18:31:05.099091743 +0000
@@ -0,0 +1,64 @@
+#define N 200
+#define DIST 32
+
+typedef signed char sc;
+typedef unsigned char uc;
+typedef signed short ss;
+typedef unsigned short us;
+typedef int si;
+typedef unsigned int ui;
+typedef signed long long sll;
+typedef unsigned long long ull;
+
+#define FOR_EACH_TYPE(M) \
+ M (sc) M (uc) \
+ M (ss) M (us) \
+ M (si) M (ui) \
+ M (sll) M (ull) \
+ M (float) M (double)
+
+#define TEST_VALUE(I) ((I) * 11 / 2)
+
+#define ADD_TEST(TYPE) \
+ TYPE a_##TYPE[N * 2]; \
+ void __attribute__((noinline, noclone)) \
+ test_##TYPE (int x, int y) \
+ { \
+ for (int i = 0; i < N; ++i) \
+ a_##TYPE[x - i] += a_##TYPE[y - i]; \
+ }
+
+#define DO_TEST(TYPE) \
+ for (int i = 0; i < DIST * 2; ++i) \
+ { \
+ for (int j = 0; j < N + DIST * 2; ++j) \
+ a_##TYPE[j] = TEST_VALUE (j); \
+ test_##TYPE (i + N - 1, DIST + N - 1); \
+ for (int j = 0; j < N + DIST * 2; ++j) \
+ { \
+ TYPE expected; \
+ if (j < i || j >= i + N) \
+ expected = TEST_VALUE (j); \
+ else if (i >= DIST) \
+ expected = ((TYPE) TEST_VALUE (j) \
+ + (TYPE) TEST_VALUE (j + DIST - i)); \
+ else \
+ expected = ((TYPE) TEST_VALUE (j) \
+ + a_##TYPE[j + DIST - i]); \
+ if (expected != a_##TYPE[j]) \
+ __builtin_abort (); \
+ } \
+ }
+
+FOR_EACH_TYPE (ADD_TEST)
+
+int
+main (void)
+{
+ FOR_EACH_TYPE (DO_TEST)
+ return 0;
+}
+
+/* { dg-final { scan-tree-dump {flags: *WAR\n} "vect" { target vect_int } } } */
+/* { dg-final { scan-tree-dump "using an index-based overlap test" "vect" } } */
+/* { dg-final { scan-tree-dump-not "using an address-based" "vect" } } */
Index: gcc/testsuite/gcc.dg/vect/vect-alias-check-19.c
===================================================================
--- /dev/null 2019-09-17 11:41:18.176664108 +0100
+++ gcc/testsuite/gcc.dg/vect/vect-alias-check-19.c 2019-11-11 18:31:05.099091743 +0000
@@ -0,0 +1,62 @@
+#define N 200
+#define DIST 32
+
+typedef signed char sc;
+typedef unsigned char uc;
+typedef signed short ss;
+typedef unsigned short us;
+typedef int si;
+typedef unsigned int ui;
+typedef signed long long sll;
+typedef unsigned long long ull;
+
+#define FOR_EACH_TYPE(M) \
+ M (sc) M (uc) \
+ M (ss) M (us) \
+ M (si) M (ui) \
+ M (sll) M (ull) \
+ M (float) M (double)
+
+#define ADD_TEST(TYPE) \
+ TYPE a_##TYPE[N * 2]; \
+ void __attribute__((noinline, noclone)) \
+ test_##TYPE (int x, int y) \
+ { \
+ for (int i = 0; i < N; ++i) \
+ { \
+ a_##TYPE[i + x] = i; \
+ a_##TYPE[i + y] = 42 - i * 2; \
+ } \
+ }
+
+#define DO_TEST(TYPE) \
+ for (int i = 0; i < DIST * 2; ++i) \
+ { \
+ __builtin_memset (a_##TYPE, 0, sizeof (a_##TYPE)); \
+ test_##TYPE (DIST, i); \
+ for (int j = 0; j < N + DIST * 2; ++j) \
+ { \
+ TYPE expected = 0; \
+ if (i > DIST && j >= i && j < i + N) \
+ expected = 42 - (j - i) * 2; \
+ if (j >= DIST && j < DIST + N) \
+ expected = j - DIST; \
+ if (i <= DIST && j >= i && j < i + N) \
+ expected = 42 - (j - i) * 2; \
+ if (expected != a_##TYPE[j]) \
+ __builtin_abort (); \
+ } \
+ }
+
+FOR_EACH_TYPE (ADD_TEST)
+
+int
+main (void)
+{
+ FOR_EACH_TYPE (DO_TEST)
+ return 0;
+}
+
+/* { dg-final { scan-tree-dump {flags: *WAW\n} "vect" { target vect_int } } } */
+/* { dg-final { scan-tree-dump "using an index-based overlap test" "vect" } } */
+/* { dg-final { scan-tree-dump-not "using an address-based" "vect" } } */
Index: gcc/testsuite/gcc.dg/vect/vect-alias-check-20.c
===================================================================
--- /dev/null 2019-09-17 11:41:18.176664108 +0100
+++ gcc/testsuite/gcc.dg/vect/vect-alias-check-20.c 2019-11-11 18:31:05.099091743 +0000
@@ -0,0 +1,66 @@
+#define N 200
+#define DIST 32
+
+typedef signed char sc;
+typedef unsigned char uc;
+typedef signed short ss;
+typedef unsigned short us;
+typedef int si;
+typedef unsigned int ui;
+typedef signed long long sll;
+typedef unsigned long long ull;
+
+#define FOR_EACH_TYPE(M) \
+ M (sc) M (uc) \
+ M (ss) M (us) \
+ M (si) M (ui) \
+ M (sll) M (ull) \
+ M (float) M (double)
+
+#define TEST_VALUE(I) ((I) * 11 / 2)
+
+#define ADD_TEST(TYPE) \
+ TYPE a_##TYPE[N * 2]; \
+ TYPE __attribute__((noinline, noclone)) \
+ test_##TYPE (int x, int y) \
+ { \
+ TYPE res = 0; \
+ for (int i = 0; i < N; ++i) \
+ { \
+ a_##TYPE[i + x] = i; \
+ res += a_##TYPE[i + y]; \
+ } \
+ return res; \
+ }
+
+#define DO_TEST(TYPE) \
+ for (int i = 0; i < DIST * 2; ++i) \
+ { \
+ for (int j = 0; j < N + DIST * 2; ++j) \
+ a_##TYPE[j] = TEST_VALUE (j); \
+ TYPE res = test_##TYPE (DIST, i); \
+ for (int j = 0; j < N; ++j) \
+ if (a_##TYPE[j + DIST] != (TYPE) j) \
+ __builtin_abort (); \
+ TYPE expected_res = 0; \
+ for (int j = i; j < i + N; ++j) \
+ if (i <= DIST && j >= DIST && j < DIST + N) \
+ expected_res += j - DIST; \
+ else \
+ expected_res += TEST_VALUE (j); \
+ if (expected_res != res) \
+ __builtin_abort (); \
+ }
+
+FOR_EACH_TYPE (ADD_TEST)
+
+int
+main (void)
+{
+ FOR_EACH_TYPE (DO_TEST)
+ return 0;
+}
+
+/* { dg-final { scan-tree-dump {flags: *RAW\n} "vect" { target vect_int } } } */
+/* { dg-final { scan-tree-dump "using an index-based overlap test" "vect" } } */
+/* { dg-final { scan-tree-dump-not "using an address-based" "vect" } } */