how does vrp2 rearrange this?

[Andrew Pinski]

On Tue, Oct 19, 2021 at 1:29 PM Andrew MacLeod via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> using testcase ifcvt-4.c:
>
>
> typedef int word __attribute__((mode(word)));
>
> word
> foo (word x, word y, word a)
> {
>    word i = x;
>    word j = y;
>    /* Try to make taking the branch likely.  */
>    __builtin_expect (x > y, 1);
>    if (x > y)
>      {
>        i = a;
>        j = i;
>      }
>    return i * j;
>
> The current VRP2 pass takes:
>
>    if (x_3(D) > y_4(D))
>      goto <bb 3>; [50.00%]        <<---  note the THEN target
>    else
>      goto <bb 4>; [50.00%]
> ;;    succ:       3 [50.0% (guessed)]  count:536870912 (estimated
> locally) (TRUE_VALUE,EXECUTABLE)
> ;;                4 [50.0% (guessed)]  count:536870912 (estimated
> locally) (FALSE_VALUE,EXECUTABLE)
>
> ;;   basic block 3, loop depth 0, count 536870912 (estimated locally),
> maybe hot
> ;;    prev block 2, next block 4, flags: (NEW, REACHABLE, VISITED)
> ;;    pred:       2 [50.0% (guessed)]  count:536870912 (estimated
> locally) (TRUE_VALUE,EXECUTABLE)
> ;;    succ:       4 [always]  count:536870912 (estimated locally)
> (FALLTHRU,EXECUTABLE)
>
> ;;   basic block 4, loop depth 0, count 1073741824 (estimated locally),
> maybe hot
> ;;    prev block 3, next block 1, flags: (NEW, REACHABLE, VISITED)
> ;;    pred:       2 [50.0% (guessed)]  count:536870912 (estimated
> locally) (FALSE_VALUE,EXECUTABLE)
> ;;                3 [always]  count:536870912 (estimated locally)
> (FALLTHRU,EXECUTABLE)
>    # i_1 = PHI <x_3(D)(2), a_5(D)(3)>
>    # j_2 = PHI <y_4(D)(2), a_5(D)(3)>
>    _6 = i_1 * j_2;
>    # VUSE <.MEM_7(D)>
>    return _6;
>
> and turns it into :
>
> ;;   basic block 2, loop depth 0, count 1073741824 (estimated locally),
> maybe hot
> ;;    prev block 0, next block 3, flags: (NEW, REACHABLE, VISITED)
> ;;    pred:       ENTRY [always]  count:1073741824 (estimated locally)
> (FALLTHRU,EXECUTABLE)
>    if (x_3(D) > y_4(D))
>      goto <bb 4>; [50.00%]        <<-- has been reversed.
>    else
>      goto <bb 3>; [50.00%]
> ;;    succ:       4 [50.0% (guessed)]  count:536870912 (estimated
> locally) (TRUE_VALUE,EXECUTABLE)
> ;;                3 [50.0% (guessed)]  count:536870912 (estimated
> locally) (FALSE_VALUE,EXECUTABLE)
>
> ;;   basic block 3, loop depth 0, count 536870912 (estimated locally),
> maybe hot
> ;;    prev block 2, next block 4, flags: (NEW, VISITED)
> ;;    pred:       2 [50.0% (guessed)]  count:536870912 (estimated
> locally) (FALSE_VALUE,EXECUTABLE)
> ;;    succ:       4 [always]  count:536870912 (estimated locally)
> (FALLTHRU,EXECUTABLE)
>
> ;;   basic block 4, loop depth 0, count 1073741824 (estimated locally),
> maybe hot
> ;;    prev block 3, next block 1, flags: (NEW, REACHABLE, VISITED)
> ;;    pred:       3 [always]  count:536870912 (estimated locally)
> (FALLTHRU,EXECUTABLE)
> ;;                2 [50.0% (guessed)]  count:536870912 (estimated
> locally) (TRUE_VALUE,EXECUTABLE)
>    # i_1 = PHI <x_3(D)(3), a_5(D)(2)>
>    # j_2 = PHI <y_4(D)(3), a_5(D)(2)>
>    _6 = i_1 * j_2;
>    # VUSE <.MEM_7(D)>
>    return _6;
>
> So the IF has reversed the targets (but not the condition), and the PHIs
> in the target block have been adjusted.
>
> Where does this happen? I cannot find it.  There doesnt seem to be
> anything in the IL which is reflecting the "expect" from the original
> code..  and if I run ranger vrp instead of classic_vrp, we don't do
> this...  so I'm missing something....

I suspect this is an artifact of inserting and removing the assert
expressions in VRP rather than anything else.

Thanks,
Andrew

>
> Thanks
>
> Andrew
>