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8244086: Following 8241492, strip mined loop may run extra iterations
Reviewed-by: mdoerr, thartmann
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rwestrel committed Apr 29, 2020
1 parent ed538ea commit 01cfedf2c975ec099dcf0165f06a76468e8b982d
Showing with 85 additions and 4 deletions.
  1. +11 −4 src/hotspot/share/opto/loopnode.cpp
  2. +74 −0 test/hotspot/jtreg/compiler/loopstripmining/TestStripMinedLimitBelowInit.java
@@ -1684,16 +1684,23 @@ void OuterStripMinedLoopNode::adjust_strip_mined_loop(PhaseIterGVN* igvn) {
if (iv_phi != NULL) {
// Now adjust the inner loop's exit condition
Node* limit = inner_cl->limit();
Node* sub = NULL;
// If limit < init for stride > 0 (or limit > init for stride 0),
// the loop body is run only once. Given limit - init (init - limit resp.)
// would be negative, the unsigned comparison below would cause
// the loop body to be run for LoopStripMiningIter.
Node* max = NULL;
if (stride > 0) {
sub = igvn->transform(new SubINode(limit, iv_phi));
max = MaxNode::max_diff_with_zero(limit, iv_phi, TypeInt::INT, *igvn);
} else {
sub = igvn->transform(new SubINode(iv_phi, limit));
max = MaxNode::max_diff_with_zero(iv_phi, limit, TypeInt::INT, *igvn);
}
// sub is positive and can be larger than the max signed int
// value. Use an unsigned min.
Node* const_iters = igvn->intcon(scaled_iters);
Node* min = MaxNode::unsigned_min(sub, const_iters, TypeInt::make(0, scaled_iters, Type::WidenMin), *igvn);
Node* min = MaxNode::unsigned_min(max, const_iters, TypeInt::make(0, scaled_iters, Type::WidenMin), *igvn);
// min is the number of iterations for the next inner loop execution:
// unsigned_min(max(limit - iv_phi, 0), scaled_iters) if stride > 0
// unsigned_min(max(iv_phi - limit, 0), scaled_iters) if stride < 0

Node* new_limit = NULL;
if (stride > 0) {
@@ -0,0 +1,74 @@
/*
* Copyright (c) 2020, Red Hat, Inc. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/

/**
* @test
* @bug 8244086
* @summary Following 8241492, strip mined loop may run extra iterations
* @requires vm.compiler2.enabled
*
* @run main/othervm -XX:-BackgroundCompilation -XX:LoopUnrollLimit=0 -XX:-TieredCompilation TestStripMinedLimitBelowInit
*
*/

public class TestStripMinedLimitBelowInit {
public static void main(String[] args) {
for (int i = 0; i < 20_000; i++) {
test1(0, 1000);
test2(1000, 0);
}
int sum = test1(1000, 0);
if (sum != 1000) {
throw new RuntimeException("wrong result: " + sum);
}
sum = test2(0, 1000);
if (sum != 0) {
throw new RuntimeException("wrong result: " + sum);
}
}

private static int test1(int start, int stop) {
int sum = 0;
int i = start;
do {
// So ciTypeFlow doesn't clone the loop head
synchronized (new Object()) {
}
sum += i;
i++;
} while (i < stop);
return sum;
}

private static int test2(int start, int stop) {
int sum = 0;
int i = start;
do {
synchronized (new Object()) {
}
sum += i;
i--;
} while (i >= stop);
return sum;
}
}

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