Re: [PATCH] phiopt: Optimize (x <=> y) cmp z [PR94589]

[Jonathan Wakely <jwakely@redhat.com>]

On 04/05/21 09:44 +0200, Jakub Jelinek wrote:
>Hi!
>
>genericize_spaceship genericizes i <=> j to approximately
>({ int c; if (i == j) c = 0; else if (i < j) c = -1; else c = 1; c; })
>for strong ordering and
>({ int c; if (i == j) c = 0; else if (i < j) c = -1; else if (i > j) c = 1; else c = 2; c; })
>for partial ordering.
>The C++ standard supports then == or != comparisons of that against
>strong/partial ordering enums, or </<=/==/!=/>/>= comparisons of <=> result
>against literal 0.
>
>In some cases we already optimize that but in many cases we keep performing
>all the 2 or 3 comparisons, compute the spaceship value and then compare
>that.
>
>The following patch recognizes those patterns if the <=> operands are
>integral types or floating point (the latter only for -ffast-math) and
>optimizes it to the single comparison that is needed (plus adds debug stmts
>if needed for the spaceship result).
>
>Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
>
>There are two things I'd like to address in a follow-up:
>1) if (HONOR_NANS (TREE_TYPE (lhs1)) || HONOR_SIGNED_ZEROS (TREE_TYPE (lhs1)))
>is what I've copied from elsewhere in phiopt, but thinking about it,
>alll we care is probably only HONOR_NANS, the matched pattern starts with
>== or != comparison and branches to the PHI bb with -1/0/1/2 result if it is
>equal, which should be the case for signed zero differences.
>2) the pr94589-2.C testcase should be matching just 12 times each, but runs
>into operator>=(strong_ordering, unspecified) being defined as

Should this say s/strong/partial/  ?