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
>
