[Bug middle-end/82940] Suboptimal code for (a & 0x7f) | (b & 0x80) on powerpc

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82940

--- Comment #8 from Segher Boessenkool <segher at gcc dot gnu.org> ---
(In reply to Segher Boessenkool from comment #7)
> (In reply to Peter Cordes from comment #6)
> > # power64 GCC 9.2.1 (ATI13.0)
> >         rlwimi 3,4,0,255    # bit-blend according to mask, rotate count=0
> >         rldicl 3,3,0,32     # Is this zero-extension to 64-bit redundant?
> 
> It is: the rlwinm does an AND with 0xff already, so that clears the top 32
> bits for sure.

Wow I cannot read, it is an rlwimi, so scratch that.

The rlwimi here keeps the top 56 bits intact, and they already were 0, so the
insn still is redundant.