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From: "law at redhat dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/94567] [10 Regression] wrong code at -O2 and -O3 on
 x86_64-linux-gnu
Date: Tue, 14 Apr 2020 11:59:07 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94567

--- Comment #5 from Jeffrey A. Law <law at redhat dot com> ---
I've pondered just killing that pattern, but I'm pretty sure there'll be
notable regressions.  There was a clear regression we fixed in gcc-6 due to=
 not
handling QImode operands in that pattern.

What I'm playing with is looking at pos + len and if it hits the sign bit in
the operand's mode, then widening the mode.  Something like this:

+  /* If the mask is going to have the sign bit set in the mode
+     we want to do the comparison, then we must widen the target
+     mode.  Otherwise the flags will be incorrect when we split
+     this into a (compare (and (op0) (mask))) and a subsequent
+     test like LE will get the wrong result.  */
+  if (mode < E_DImode
+      && pos + len =3D=3D GET_MODE_PRECISION (mode))
+    {
+      mode =3D GET_MODE_WIDER_MODE (mode).require ();
+      val =3D gen_lowpart (mode, val);
+    }
+

Which I think is roughly what you were suggesting.  Mine does it with a SUB=
REG,
so it matches existing patterns...  A ZERO_EXTEND may well require new
patterns.=