Re: [PATCH] Adjust backwards threader costing of PHIs

[Jeff Law <jeffreyalaw@gmail.com>]

On 8/5/2022 7:58 AM, Richard Biener wrote:
> The following adjusts the costing of PHIs to match how I understand
> the comment and maybe the original intent.  The will be no
> non-degenerate PHI nodes remaining on the threaded path but when there
> are alternate path exits PHI nodes at their destinations will likely
> require extra copies on those edges and that's what we want to account
> for.
>
> Jeff added this code long time ago and at least the special-casing
> of (just) m_name does not make sense anymore.
>
> Unfortunately this tweaks heuristics enough to make the threader
> thread an unfortunate path in libgomp/team.c so that
> -Walloca-larger-than diagnoses an allocation of -1 elements.  I'm
> not 100% sure this condition is impossible so I've added a guard
> not allocating zero or "less" stack.  There's also an uninit
> diagnostic in opts.cc about best_math::m_best_candidate_len that
> looks like a false positive but all but this member are initialized
> so the patch initializes this one as well to avoid this false
> positive.
>
> I have yet to analyze some fallout as well:
>
> FAIL: gcc.dg/uninit-pred-9_b.c bogus warning (test for bogus messages, line 20)
> FAIL: gcc.dg/tree-ssa/phi_on_compare-4.c scan-tree-dump-times dom2 "Removing bas
> ic block" 1
> FAIL: gcc.dg/tree-ssa/pr59597.c scan-tree-dump-times threadfull1 "Registering jump thread" 4
> FAIL: gcc.dg/tree-ssa/pr61839_2.c scan-tree-dump-times evrp "%" 0
> FAIL: gcc.dg/tree-ssa/pr61839_2.c scan-tree-dump-times evrp "972195717 % " 0
> FAIL: gcc.dg/tree-ssa/pr61839_2.c scan-tree-dump-times evrp "972195717 / " 0
> FAIL: gcc.dg/tree-ssa/pr61839_2.c scan-tree-dump-times evrp "__builtin_abort" 1
>
> the early threading fails are because we now account for the mid-exit
> PHI copies and early threading has a limit of a single copied insn.
> But this testcase was never about threading but VRP which seemingly
> regressed meanwhile...
>
> Bootstrapped and tested (with the above FAILs) on
> x86_64-unknown-linux-gnu.
>
> Any comments besides the FAILout?
I don't recall adding that code, but I did find it in the archives.

https://gcc.gnu.org/pipermail/gcc-patches/2016-March/443452.html

But even with that and reviewing the PR, I still don't remember much 
about this particular chunk of code.  I do recall that I was never 
actually happy with pr69196 state and that's why we've kept it open all 
these years.

It doesn't look like tree-ssa/pr69196 has regressed, so if you're happy 
with the patch, I've got no objections.

jeff