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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/108357] [13 Regression] Dead Code Elimination
 Regression at -O2 since r13-4607-g2dc5d6b1e7ec88
Date: Wed, 29 Mar 2023 09:03:24 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108357

--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Xi Ruoyao from comment #7)
> (In reply to Richard Biener from comment #6)
> > (In reply to Xi Ruoyao from comment #5)
> > > The test fails on loongarch64-linux-gnu.  foo is kept in 114t.threadf=
ull1,
> > > but removed in 135t.forwprop3.
> > >=20
> > > Does this mean something is wrong for LoongArch, or we should simply =
check
> > > the tree dump in a later pass (for e.g. 254t.optimized)?
> >=20
> > I guess it depends on LOGICAL_OP_NON_SHORT_CIRCUIT, can you try
> > --param logical-op-non-short-circuit=3D1 and see if that helps?
>=20
> Nope, the result is same.

Aha, the issue is missing promotions, already in .original:

-    short int g =3D a (5, (int) ((short int) b ^ 9854));
-  f ((int) g);
+    short int g =3D a (5, (short int) b ^ 9854);
+  f (g);

(+ is loongarch, - is x86_64)

That results in different IL into threadfull1.  On loongarch forwprop3
elides the branch, probably with the help of nonzero bits set by CCP.=