> I'm trying to understand from the above and the patch, but fail. What's
> the set of stmts we avoid to externalize because we can swap the
> (immediate) parent?
>
> As said in the PR for the initial patch for the greedy search we
> perform any heuristic will have downsides and for BB vectorization
> swapping is as good as not swapping if operand zero discovery
> ends up with an externalized node.
>
> Thanks,
> Richard.
Hi Richard,
In this regression case, operand-zero discovery does end up
externalized either way. I'm not trying to avoid externalization
itself here; what I'm trying to avoid is introducing additional
parent-level externalization.
In this case, the immediate-parent swap retry is harmful. It swaps
lanes that are already aligned with the first-lane operand columns,
breaks that local shape, and then the externalization propagates
upward. So we do not just externalize the child, we externalize the
parent as well, which gives a shorter SLP tree and worse codegen.
Without this patch, the dump shows:
```
Build SLP failed: different BIT_FIELD_REF arguments in
stmp_isum10_93.49_256 = BIT_FIELD_REF <vect__30.48_255, 64, 0>;
SLP discovery for node ... failed
SLP discovery for node ... failed
Re-trying with swapped operands of stmts 1 2 3
Build SLP for stmp_rsum00_88.79_304 = rsum00_106 + stmp_rsum00_88.79_303;
Build SLP for stmp_isum00_89.75_298 = BIT_FIELD_REF <vect__18.74_295, 64, 64>;
Build SLP failed: different operation in stmt
stmp_isum00_89.75_298 = BIT_FIELD_REF <vect__18.74_295, 64, 64>;
original stmt
stmp_rsum00_88.79_304 = rsum00_106 + stmp_rsum00_88.79_303;
...
Building vector operands from scalars
Building parent vector operands from scalars instead
...
SLP size 15 vs. limit 176.
```
With this patch, the child may still externalize, but we skip that
harmful immediate-parent retry, so the larger parent stays internal.
The dump then looks like:
```
Build SLP failed: different BIT_FIELD_REF arguments in
stmp_isum10_93.49_256 = BIT_FIELD_REF <vect__30.48_255, 64, 0>;
SLP discovery for node ... failed
Building vector operands from scalars
SLP discovery for node ... succeeded
...
Build SLP for rsum00_147 = _148 + rsum00_181;
Build SLP for isum00_143 = _144 + isum00_180;
Build SLP for rsum10_131 = _132 + rsum10_177;
Build SLP for isum10_127 = _128 + isum10_176;
...
SLP size 19 vs. limit 176.
```
So for this PR, "it ends up externalized anyway" is not the same as
"swapping is as good as not swapping". The old retry makes the
externalization spread to the parent and leaves a worse final SLP tree.
--
Regards,
Zhongyao
