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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/109154] [13 regression] jump threading
 de-optimizes nested floating point comparisons
Date: Wed, 29 Mar 2023 06:38:18 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109154

--- Comment #39 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Andrew Macleod from comment #37)
> Created attachment 54780 [details]
> in progress patch
>=20
> Well call me a liar.=20
>=20
> It took me a while to understand why, but if we leave it to single
> dependencies only, the impact is relatively linear.  I wrote a bunch of
> code, then deleted most of it as I found the engine was bypassing my code
> and doing it on its own.
>=20
> The attached patch is the core.  It actually works to a depth of 5
> recomputations. my sample of:
>   int a =3D left * 2;
>   int b =3D a - 4;
>   int c =3D b % 7;
>   func (a,b ,c);
>   int d =3D c * 4;
>   if (left =3D=3D 20)
>     {
>       func (b,c,d);
>=20
> produces=20
>   <bb 5> :
>   func (36, 1, 4);
>=20
> IT also changes your program somewhat.
>=20=20
> Try applying it and see if it does what you want. It bootstraps, regressi=
on
> are running.. but based on the minimal code impact, I wouldn't expect
> incorrect failures.
>=20
> Performance impact on building GCC is barely half a percent in VRP, and
> 0.05% overall compile time.  pretty minimal.
>=20
> Im still working with it to tweak it, I just wanted you to be able to see=
 if
> it helps.  I presume we dont want to add a new --param this late in the
> game.   But it seems we can set a reasonable number and not run into much
> trouble.

There is no problem with adding --params, and those are always better than
magic numbers.

Btw, I originally wondered why we don't re-compute zone1_12 because it's
in the imports of the successor (OK, the empty successors single successor
block) and expected those to trigger re-computes.=