Re: [x86 PATCH] PR target/106933: Limit TImode STV to SSA-like def-use chains.

[Uros Bizjak <ubizjak@gmail.com>]

On Fri, Dec 23, 2022 at 12:09 AM Roger Sayle <roger@nextmovesoftware.com> wrote:
>
>
> 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.

I assume these two alternatives would result in much more invasive
surgery, so let's consider these "for the future".

> 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.

OK.

Thanks,
Uros.