[PATCH] slp tree vectorizer: Re-calculate vectorization factor in the case of invalid choices [PR96974]

[Stam Markianos-Wright]

Hi all,

This patch resolves bug:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96974

This is achieved by forcing a re-calculation of *stmt_vectype_out if an
incompatible combination of TYPE_VECTOR_SUBPARTS is detected, but with 
an extra introduced max_nunits ceiling.

I am not 100% sure if this is the best way to go about fixing this, 
because this is my first look at the vectorizer and I lack knowledge of 
the wider context, so do let me know if you see a better way to do this!

I have added the previously ICE-ing reproducer as a new test.

This is compiled as "g++ -Ofast -march=armv8.2-a+sve 
-fdisable-tree-fre4" for GCC11 and "g++ -Ofast -march=armv8.2-a+sve" for 
GCC10.

(the non-fdisable-tree-fre4 version has gone latent on GCC11)

Bootstrapped and reg-tested on aarch64-linux-gnu.
Also reg-tested on aarch64-none-elf.


gcc/ChangeLog:

         * tree-vect-stmts.c (get_vectype_for_scalar_type): Add new
         parameter to core function and add new function overload.
         (vect_get_vector_types_for_stmt): Add re-calculation logic.

gcc/testsuite/ChangeLog:

         * g++.target/aarch64/sve/pr96974.C: New test.
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