On Wed Oct 14, 2026 at 2:10 AM PDT, Sarvesh Chandra wrote:
> On Mon, Oct 12, 2026 at 10:57 PM, Hongtao Liu <[email protected]> wrote:
>> On Sat Oct 3, 2026 at 7:02 AM PDT, Sarvesh Chandra wrote:
>> > + /* Bundle alignment replaces generic loop, jump, and label padding. */
>> > + if (opts->x_ix86_bundle_align_loops > 0)
>> > + {
>> > + SET_OPTION_IF_UNSET (opts, opts_set, str_align_loops, "1");
>> > + SET_OPTION_IF_UNSET (opts, opts_set, str_align_jumps, "1");
>> > + SET_OPTION_IF_UNSET (opts, opts_set, str_align_labels, "1");
>>
>> If we just set loop/jump/label align to 1, and keep align tight loops,
>> can we get similar code size reduction without performance drop?
>
> We do get similar code size reduction, but not without a performance
> drop on znver5. I am attaching the Cauldron page here, which contains
> the slides referring to the impact of bundle align against tight align
> in isolation:
> https://conf.gnu-tools-cauldron.org/prg26/talk/HS9RPC/
>
>> It sounds to me bundle alignments is an enhancement to current tightloop
>> alignment with dynamic decision in assembler instead of hard-coded
>> .palign. so what's the real gain of the new pass against the original
>> align_tight_loop?
>
> Bundle not only wins due to hardcoded palign, but also due to the
> crossing hot regions present in layout which need smarter insertion of
> padding. We saw the impact of this in the microbenchmarks in the
> Appendix section as well as the tupledealloc loop of cpython_r, which
> had commit-to-commit variation, fixed by bundle align which identifies
> the entire region in one bundle, vs tight which chooses one of them.
>
>> The pass manually setting all other alignments to 1, if the assembler
>> doesn't support the new directive .bundle_{lock, unlock}, do we have a
>> way to fallback? or just lose all alignment with some performance loss?
>
> The fallback when the assembler doesn't support flex bundles is the
> current default, that is, 16B loop/jump/label align + tight align,
> because we could not rule out the impact of any of the four flags on
> znver5. In the absence of bundle align, we can expect performance
> impacted negatively on frontend bound benchmarks. There are cases when
> setting loop/jump/label align to 1 also has a negative impact, in both
> real world benchmarks and microbenchmarks. Bundle align is one way to
> address all of these regressions under one umbrella.
>
But that macro you add for the check of flex bundle support is only used
inside the two i386.md output template, but ix86_default_align still
forces -falign-loops, -falign-jumps and -falign-labels to 1 with -march=znver5
on older assembler?
So you need to check the macro in ix86_default_bundle_align insead of the
i386.md?
