With many thanks to H.J. for doing all the hard work, this patch resolves
two P1 regressions; PR target/106933 and PR target/106959.

Although superficially similar, the i386 backend's two scalar-to-vector
(STV) passes perform their transformations in importantly different ways.
The original pass converting SImode and DImode operations to V4SImode
or V2DImode operations is "soft", allowing values to be maintained in
both integer and vector hard registers.  The newer pass converting TImode
operations to V1TImode is "hard" (all or nothing) that converts all uses
of a pseudo to vector form.  To implement this it invokes powerful ju-ju
calling SET_MODE on a REG_rtx, which due to RTL sharing, often updates
this pseudo's mode everywhere in the RTL chain.  Hence, TImode STV can only
be performed when all uses of a pseudo are convertible to V1TImode form.
To ensure this the STV passes currently use data-flow analysis to inspect
all DEFs and USEs in a chain.  This works fine for chains that are in
the usual single assignment form, but the occurrence of uninitialized
variables, or multiple assignments that split a pseudo's usage into
several independent chains (lifetimes) can lead to situations where
some but not all of a pseudo's occurrences need to be updated.  This is
safe for the SImode/DImode pass, but leads to the above bugs during
the TImode pass.

My one minor tweak to HJ's patch from comment #4 of bugzilla PR106959
is to only perform the new single_def_chain_p check for TImode STV; it
turns out that STV of SImode/DImode min/max operates safely on multiple-def
chains, and prohibiting this leads to testsuite regressions.  We don't
(yet) support V1TImode min/max, so this idiom isn't an issue during the
TImode STV pass.

For the record, the two alternate possible fixes are (i) make the TImode
STV pass "soft", by eliminating use of SET_MODE, instead using replace_rtx
with a new pseudo, or (ii) merging "chains" so that multiple DFA
chains/lifetimes are considered a single STV chain.

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?


2022-12-22  H.J. Lu  <hjl.tools@gmail.com>
            Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR target/106933
	PR target/106959
	* config/i386/i386-features.cc (single_def_chain_p): New predicate
	function to check that a pseudo's use-def chain is in SSA form.
	(timode_scalar_to_vector_candidate_p): Check that TImode regs that
	are SET_DEST or SET_SRC of an insn match/are single_def_chain_p.

gcc/testsuite/ChangeLog
	PR target/106933
	PR target/106959
	* gcc.target/i386/pr106933-1.c: New test case.
	* gcc.target/i386/pr106933-2.c: Likewise.
	* gcc.target/i386/pr106959-1.c: Likewise.
	* gcc.target/i386/pr106959-2.c: Likewise.
	* gcc.target/i386/pr106959-3.c: Likewise.

Thanks in advance,
Roger
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