On Thu, 17 Sep 2026 20:57:05 GMT, Boris Ulasevich <[email protected]>
wrote:
>> A for-each loop over ArrayList.Itr carries two phases of one recurrence
>> through the loop: cursor (after the increment) and lastRet (before it). Both
>> are live at the same time, so the allocator puts a copying mov in the loop
>> body - on every iteration, for a value the loop never reads. This can be
>> dealt with at the Java level, see #32344, but this change is an attempt to
>> improve it at the level of C2 compilation.
>>
>> In a loop compiled by C2, lastRet is never written to memory: the iterator
>> is scalarized and the field lives in a register. In a plain for-each nobody
>> reads it (it is only used by set() and remove()) so its only consumers are
>> the Phi at the loop exit and the safepoint's debug info.
>>
>> C2 already knows how to get rid of a second index:
>> PhaseIdealLoop::replace_parallel_iv looks for a variable that walks
>> alongside the trip counter and rewrites its uses in terms of the counter.
>> It only recognizes PARALLEL shape:
>>
>>
>> PARALLEL: int a = 5; for (int iv = 0; iv < limit; iv++) { use(a); a
>> += 3; }
>> PREVIOUS: int prev = -1; for (int iv = 0; iv < limit; iv++) { use(prev);
>> prev = iv; }
>>
>>
>> The patch teaches the recognition step the second (PREVIOUS) shape: a phi in
>> the loop head whose backedge value is the trip counter plus a constant. Its
>> uses are then rewritten in terms of the trip counter, the phi becomes dead
>> code, and the copy in the loop body is no longer generated.
>>
>> With `-XX:LoopMaxUnroll=1` the resulting aarch64 assembly looks like the
>> following:
>>
>> @Benchmark
>> public void list_foreach(Blackhole bh) {
>> for (Object o : list) {
>> bh.consume(o);
>> }
>> }
>>
>> BEFORE AFTER
>> mov w14, w11 -
>> add x11, x10, w14, sxtw #2 add x12, x10, w16, sxtw #2
>> ldr w11, [x11, #0xc] ldr w12, [x12, #0xc]
>> lsl x11, x11, #3 add w16, w16, #0x1
>> add w11, w14, #0x1 lsl x12, x12, #3
>> cmp w11, w12 cmp w16, w14
>> b.lt #-0x18 b.lt #-0x14
>>
>>
>> Removing one instruction gives a speedup of up to 40% with
>> `-XX:LoopMaxUnroll=1`, while with default VM options the picture is mixed: a
>> 11–15% gain on some CPUs and nothing measurable on others.
>>
>> ---------
>> - [x] I confirm that I make this contribution in accordance with the
>> [OpenJDK Interim AI Policy](https://openjdk.org/legal/ai).
>
> Boris Ulasevich has updated the pull request incrementally with two
> additional commits since the last revision:
>
> - TestParallelIvInIntCountedLoop: assert no loop is left, and test the
> peeled case
> - generalize the entry value check: any two values differing by a constant
src/hotspot/share/opto/loopnode.cpp line 4567:
> 4565:
> 4566: // If (n2 - n1) is a constant, return it in *offset.
> 4567: static bool is_constant_difference(Node* n2, Node* n1, jint* offset) {
It should be preferable to use a reference here unless you want `offset` to be
nullable.
test/hotspot/jtreg/compiler/loopopts/parallel_iv/TestParallelIvInIntCountedLoop.java
line 405:
> 403: @Test
> 404: @IR(failOn = { IRNode.LOOP, IRNode.COUNTED_LOOP })
> 405: private static int testIntCountedLoopWithDoubledIv(int stop) {
Can you add a test case where `prev = i + c`? All these cases are `prev = i`.
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/32549#discussion_r4043047987
PR Review Comment: https://git.openjdk.org/jdk/pull/32549#discussion_r4043087930
