switch lowering: limit number of cluster attemps

gcc/ChangeLog:

	PR tree-optimization/96979
	* doc/invoke.texi: Document new param max-switch-clustering-attempts.
	* params.opt: Add new parameter.
	* tree-switch-conversion.c (jump_table_cluster::find_jump_tables):
	Limit number of attempts.
	(bit_test_cluster::find_bit_tests): Likewise.

gcc/testsuite/ChangeLog:

	PR tree-optimization/96979
	* g++.dg/tree-ssa/pr96979.C: New test.
This commit is contained in:
Martin Liska 2020-09-22 12:23:35 +02:00
parent 1a84651d16
commit c6df6039e9
4 changed files with 75 additions and 0 deletions

View File

@ -13452,6 +13452,10 @@ The smallest number of different values for which it is best to use a
jump-table instead of a tree of conditional branches. If the value is
0, use the default for the machine.
@item max-switch-clustering-attempts
The maximum number of clustering attempts used
in bit-test and jump-table switch expansion.
@item jump-table-max-growth-ratio-for-size
The maximum code size growth ratio when expanding
into a jump table (in percent). The parameter is used when

View File

@ -82,6 +82,10 @@ The maximum length of a constant string for a builtin string cmp call eligible f
Common Joined UInteger Var(param_case_values_threshold) Param Optimization
The smallest number of different values for which it is best to use a jump-table instead of a tree of conditional branches, if 0, use the default for the machine.
-param=max-switch-clustering-attempts=
Common Joined UInteger Var(param_max_switch_clustering_attempts) Param Optimization Init(10000)
The maximum number of clustering attempts used in bit-test and jump-table switch expansion.
-param=comdat-sharing-probability=
Common Joined UInteger Var(param_comdat_sharing_probability) Init(20) Param Optimization
Probability that COMDAT function will be shared with different compilation unit.

View File

@ -0,0 +1,50 @@
/* PR tree-optimization/96979 */
/* { dg-do compile } */
/* { dg-options "-std=c++17 -O2 -fdump-tree-switchlower1" } */
using u64 = unsigned long long;
constexpr inline u64
foo (const char *str) noexcept
{
u64 value = 0xcbf29ce484222325ULL;
for (u64 i = 0; str[i]; i++)
value = (value ^ u64(str[i])) * 0x100000001b3ULL;
return value;
}
struct V
{
enum W
{
#define A(n) n,
#define B(n) A(n##0) A(n##1) A(n##2) A(n##3) A(n##4) A(n##5) A(n##6) A(n##7) A(n##8) A(n##9)
#define C(n) B(n##0) B(n##1) B(n##2) B(n##3) B(n##4) B(n##5) B(n##6) B(n##7) B(n##8) B(n##9)
#define D(n) C(n##0) C(n##1) C(n##2) C(n##3) C(n##4) C(n##5) C(n##6) C(n##7) C(n##8) C(n##9)
#define E D(foo1) D(foo2) D(foo3)
E
last
};
constexpr static W
bar (const u64 h) noexcept
{
switch (h)
{
#undef A
#define F(n) #n
#define A(n) case foo (F(n)): return n;
E
}
return last;
}
};
int
baz (const char *s)
{
const u64 h = foo (s);
return V::bar (h);
}
/* { dg-final { scan-tree-dump-times ";; Bail out: --param=max-switch-clustering-attempts reached" 2 "switchlower1" } } */

View File

@ -1183,6 +1183,7 @@ jump_table_cluster::find_jump_tables (vec<cluster *> &clusters)
min.quick_push (min_cluster_item (0, 0, 0));
HOST_WIDE_INT attempts = 0;
for (unsigned i = 1; i <= l; i++)
{
/* Set minimal # of clusters with i-th item to infinite. */
@ -1194,6 +1195,14 @@ jump_table_cluster::find_jump_tables (vec<cluster *> &clusters)
if (i - j < case_values_threshold ())
s += i - j;
if (attempts++ == param_max_switch_clustering_attempts)
{
if (dump_file)
fprintf (dump_file, ";; Bail out: "
"--param=max-switch-clustering-attempts reached\n");
return clusters.copy ();
}
/* Prefer clusters with smaller number of numbers covered. */
if ((min[j].m_count + 1 < min[i].m_count
|| (min[j].m_count + 1 == min[i].m_count
@ -1308,6 +1317,7 @@ bit_test_cluster::find_bit_tests (vec<cluster *> &clusters)
min.quick_push (min_cluster_item (0, 0, 0));
HOST_WIDE_INT attempts = 0;
for (unsigned i = 1; i <= l; i++)
{
/* Set minimal # of clusters with i-th item to infinite. */
@ -1315,6 +1325,13 @@ bit_test_cluster::find_bit_tests (vec<cluster *> &clusters)
for (unsigned j = 0; j < i; j++)
{
if (attempts++ == param_max_switch_clustering_attempts)
{
if (dump_file)
fprintf (dump_file, ";; Bail out: "
"--param=max-switch-clustering-attempts reached\n");
return clusters.copy ();
}
if (min[j].m_count + 1 < min[i].m_count
&& can_be_handled (clusters, j, i - 1))
min[i] = min_cluster_item (min[j].m_count + 1, j, INT_MAX);