RFR: 8365991: AArch64: Ignore BlockZeroingLowLimit when UseBlockZeroing is false [v4]

[Patrick Zhang]

On Tue, 28 Oct 2025 08:17:15 GMT, Andrew Haley <aph at openjdk.org> wrote:

> > I would like to reiterate that I have no objection to the functions when the `-XX:+UseBlockZeroing` option is set, everything can keep as is. My point is that `BlockZeroingLowLimit` serves literally/specifically as a switch to control whether DC ZVA instructions are generated for clearing instances under a specified bytes size limitation, rather than for deciding between unrolling and callout. Therefore, it should NOT affect the code-gen results any longer when `-XX:-UseBlockZeroing` is set, should it?
> 
> It does not. When `-XX:-UseBlockZeroing` is set, `BlockZeroingLowLimit` is ignored.


zero_words does not check `UseBlockZeroing`, it directly compares `cnt` and `BlockZeroingLowLimit / BytesPerWord`.

https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp#L6198C1-L6204C16


address MacroAssembler::zero_words(Register base, uint64_t cnt)
{
  assert(wordSize <= BlockZeroingLowLimit,
            "increase BlockZeroingLowLimit");
  address result = nullptr;
  if (cnt <= (uint64_t)BlockZeroingLowLimit / BytesPerWord) {
#ifndef PRODUCT


In contrast, the inner stub function does so.

https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/aarch64/stubGenerator_aarch64.cpp#L669

  address generate_zero_blocks() {
    Label done;
    Label base_aligned;

    Register base = r10, cnt = r11;

    __ align(CodeEntryAlignment);
    StubId stub_id = StubId::stubgen_zero_blocks_id;
    StubCodeMark mark(this, stub_id);
    address start = __ pc();

    if (UseBlockZeroing) {

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PR Comment: https://git.openjdk.org/jdk/pull/26917#issuecomment-3455265314