This resolves PR tree-optimization/105668, a P1 ice-on-valid regression
triggered by my recent patch to add a vec_cmpeqv1tiv1ti define_expand
to the i386 backend.  The existence of this optab currently leads GCC
to incorrectly assume the existence of a corresponding vcond_mask for
V1TImode.

I believe the best solution (of the three possible fixes) is to allow
gimple_expand_vec_cond_expr to fail (return NULL) when a suitable optab
to generate a IFN_VCOND_MASK isn't available, but instead allow RTL
expansion to provide a default implementation using vector mode logic
operations.  On x86_64, the equivalent of a pblend can be generated in
three instructions using pand, pandn and pxor.  In fact, this fallback
implementation is already used in ix86_expand_sse_movcc when the -march
doesn't provide a suitable instruction.  This patch provides that
functionality to all targets in the middle-end, allowing the vectorizer(s)
to safely assume support for VEC_COND_EXPR (when the target has suitable
vector logic instructions).

I should point out (for the record) that the new expand_vec_cond_expr
function in expr.cc is very different from the function of the same name
removed by Matin Liska in June 2020.
https://gcc.gnu.org/pipermail/gcc-patches/2020-June/547097.html
https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=502d63b6d6141597bb18fd23c8
7736a1b384cf8f
That function simply expanded the vcond_mask optab and failed if it
wasn't available, which is currently the task of the gimple-isel pass.
The implementation here is a traditional RTL expander, sythesizing the
desired vector conditional move using bit-wise XOR and AND instructions
of the mask vector.

At some point in the future, gimple-isel could be enhanced to consider
alternative vector modes, as a V1TI blend/vec_cond_expr may be implemented
using V2DI, V4SI, V8HI or V16QI.  Alas, I couldn't figure out how to
conveniently iterate over the desired modes, so this enhancement is left
for a follow-up patch.

This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check, both with and without --target_board=unix{-m32} with
no new failures.  Ok for mainline?


2022-05-23  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR tree-optimization/105668
	* expr.cc (expand_vec_cond_expr): New function to expand
	VEC_COND_EXPR using vector mode logical instructions.
	(expand_expr_real_2) <case VEC_COND_EXPR>: Call the above.
	* gimple-isel.cc (gimple_expand_vec_cond_expr): Instead of
	asserting, retun NULL when the target's get_vcond_mask_icode
	returns CODE_FOR_nothing.

gcc/testsuite/ChangeLog
	PR tree-optimization/105668
	* gcc.target/i386/pr105668.c: New test case.

Roger
--