[Bug target/113652] [14 regression] Failed bootstrap on ppc unrecognized opcode: `lfiwzx' with -mcpu=7450

["bergner at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113652

Peter Bergner <bergner at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |meissner at gcc dot gnu.org

--- Comment #15 from Peter Bergner <bergner at gcc dot gnu.org> ---
(In reply to Kewen Lin from comment #11)
> In gcc, lfiwzx is guarded with TARGET_LFIWZX => TARGET_POPCNTD (ISA2.06),
> while -mvsx will guarantee TARGET_POPCNTD (ISA_2_6_MASKS_SERVER) set, so it
> considers lfiwzx is supported. IMHO the underlying philosophy is that having
> the capability of vsx the supported ISA level is at least 2.06, lfiwzx is
> supported from 2.06, so it's supported.
> 
> But binutils seems not to follow it:
> {"xvadddp",     XX3(60,96),     XX3_MASK,    PPCVSX,    PPCVLE,        
> {XT6, XA6, XB6}},
> {"lfiwzx",      X(31,887),      X_MASK,   POWER7|PPCA2, 0,             
> {FRT, RA0, RB}},
> Both are guarded with different masks and apparently PPCVSX doesn't enable
> POWER7.

That's because xvadddp is a VSX instruction (ie, mentioned in the VSX section
of the ISA), while lfiwzx is a floating point instruction and part of the base
ISA (for Power7 and above).  To me, that means the -mvsx assembler option is
correct to not enable lfiwzx.  ...and as Alan mentioned, even changing the
assembler to have -mvsx enable lfiwzx isn't a solution, since old already
released assemblers would still be broken.

The problem seems to be that the GCC option -mvsx enables some base (ie,
non-vsx) instructions not included in the 7450 which seems dangerous to me.  If
the vsx support in the compiler really needs those base power7 instructions to
function correctly, then we should be emitting an error when the user does
-mcpu=CPU -mvsx and CPU is something less the power7.  If the vsx support
doesn't really need those base power7 instructions to operate, then we
shouldn't be enabling them.   

Mike, can you confirm whether our -mvsx VSX support requires those base power7
instructions or not?