[Bug target/104581] [12 Regression] Huge compile-time regression building SPEC 2017 538.imagick_r with PGO

[rguenth at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104581

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Since targetm.mode_switching.needed is called as part of the mode-switching
dataflow problem I don't see how looking at reg refs should be needed at all.
What might be needed and what I'm not sure is available, is the mode-switching
state of uses.  The relevant call is

          FOR_BB_INSNS (bb, insn)
            {
              if (INSN_P (insn))
                {
                  int mode = targetm.mode_switching.needed (e, insn);

and indeed all the time is spent in

        /* Check if DEF_INSN is before INSN.  */
        rtx_insn *next;
        for (next = NEXT_INSN (def_insn);
             next != nullptr && next != end && next != insn;
             next = NEXT_INSN (next))
          ;