[Bug target/122018] x86 doesn't use BTS bitmanip instruction to both set and test bit

[ubizjak at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122018

--- Comment #20 from Uroš Bizjak <ubizjak at gmail dot com> ---
(In reply to uis from comment #18)
> I still don't know if ix86_pre_reload_split() should be here, which I copied
> from other insn_and_split.
> 
> I also noticed how fragile bts combine can be, here I can move one line and
> both gcc 15 and trunk fail:
> 
> void g();
> int f(int a, int b) {
>     bool t = false;
>     t = (a >> b) & 1;
>     if(!t) {
>         a = a | (1 << b);
>         g();
>     }
>     return a;
> }

Combine wants to create:

Trying 12, 13 -> 14:
   12: r107:SI=0x1
   13: {r106:SI=r107:SI<<r103:SI#0;clobber flags:CC;}
      REG_DEAD r107:SI
      REG_DEAD r103:SI
      REG_UNUSED flags:CC
      REG_EQUAL 0x1<<r103:SI#0
   14: {r102:SI=r102:SI|r106:SI;clobber flags:CC;}
      REG_DEAD r106:SI
      REG_UNUSED flags:CC
Successfully matched this instruction:
(parallel [
        (set (zero_extract:SI (reg/v:SI 102 [ a ])
                (const_int 1 [0x1])
                (subreg:QI (reg/v:SI 103 [ b ]) 0))
            (const_int 1 [0x1]))
        (clobber (reg:CC 17 flags))
    ])

which is another (and probably canonical) form of bit set RTX.