https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94567
--- 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 == GET_MODE_PRECISION (mode))
+ {
+ mode = GET_MODE_WIDER_MODE (mode).require ();
+ val = gen_lowpart (mode, val);
+ }
+
Which I think is roughly what you were suggesting. Mine does it with a SUBREG,
so it matches existing patterns... A ZERO_EXTEND may well require new
patterns.
