https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118012
--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Georg-Johann Lay from comment #6)
> (In reply to Andrew Pinski from comment #5)
> > /* Expand X*Y as X&-Y when Y must be zero or one. */
> > ...
> > if (bit0_p || bit1_p)
> > {
> > bool speed = optimize_insn_for_speed_p ();
> > int cost = add_cost (speed, mode) + neg_cost (speed, mode);
> > struct algorithm algorithm;
> > enum mult_variant variant;
> > if (CONST_INT_P (op1)
> > ? !choose_mult_variant (mode, INTVAL (op1),
> > &algorithm, &variant, cost)
> > : cost < mul_cost (speed, mode))
> >
> >
> > Note this should have a debug print in dump_file but that is a different
> > story.
> >
> > So maybe we are not doing the right costs here.
> That place just replaces a multiplication by some other tricks like negate +
> bit_and on the extended mode. I don't see how tweaking the costs there
> would give a branch-on-bit around the target operation (bit_xor in the
> example).
>
> You cannot roll back that in the backend by, say, a combine pattern that
> yould read
>
> (xor:HI (and:HI (neg:HI (sign_extract:HI (subreg:QI (reg:HI) 0)
> (const_int 1)
> (const_int 0))))
> (reg:HI))
>
> > Or we should have another choice of how to expand `a * onezero` into
> > `onezero ? a : 0`
>
> The propblem is that this is just the preparation for the 2nd xor operand to
> be 0u (when the bit is 0) or -1u (when the bit is 1). What you want is a
> branch depending on the bit, avoiding the 2nd operand altogether. Just do
>
> if (bit == 0)
> goto skip;
> c ^= a;
> skip:;
>
> instead of:
>
> // Do crazy things to coax bit == 1 into some operand coax and then
> c ^= coax;
>
> I don't see where the payload operation (the c ^= a) is in the expr.cc code
> from above, it's just setting up coax.
>
> And this doesn't only occur with bit == 1 but also with bit == 0.
> And this doesn't only occur wihh bit_xor but also with bit_ior and plus.
I can look into improving this situtation but it won't be until GCC 16's
stage1. RISCV also is looking into having this improved too.
