> 2020-12-10 Jakub Jelinek <ja...@redhat.com>
>
> PR rtl-optimization/98212
> * dojump.c (do_compare_rtx_and_jump): Change computation of
> first_prob for and_them and don't invert prob around it.
>
> * gcc.dg/predict-8.c: Adjust expected probability.
>
> --- gcc/dojump.c.jj 2020-12-09 15:11:17.042888002 +0100
> +++ gcc/dojump.c 2020-12-10 12:24:56.991844956 +0100
> @@ -1138,19 +1138,21 @@ do_compare_rtx_and_jump (rtx op0, rtx op
> cprob = cprob.apply_scale (99, 100);
> else
> cprob = profile_probability::even ();
> - /* We want to split:
> + /* For and_them we want to split:
> if (x) goto t; // prob;
> + goto f;
> into
> - if (a) goto t; // first_prob;
> + if (a) ; else goto f; // first_prob for ;
> + // 1 - first_prob for goto
f;
> if (b) goto t; // prob;
> + goto f;
> such that the overall probability of jumping to t
> - remains the same and first_prob is prob * cprob. */
> + remains the same and first_prob is 1 - prob * (1 -
cprob). */
> if (and_them)
> {
> rtx_code_label *dest_label;
> - prob = prob.invert ();
> - profile_probability first_prob = prob.split
(cprob).invert ();
> - prob = prob.invert ();
> + profile_probability first_prob
> + = prob.split (cprob.invert ()).invert ();
> /* If we only jump if true, just bypass the second
jump. */
> if (! if_false_label)
> {
> @@ -1163,6 +1170,15 @@ do_compare_rtx_and_jump (rtx op0, rtx op
> do_compare_rtx_and_jump (op0, op1, first_code, unsignedp,
> mode, size, dest_label, NULL, first_prob);
> }
> + /* For !and_them we want to split:
> + if (x) goto t; // prob;
> + goto f;
> + into
> + if (a) goto t; // first_prob;
> + if (b) goto t; // prob;
> + goto f;
> + such that the overall probability of jumping to t
> + remains the same and first_prob is prob * cprob. */
> else
> {
> profile_probability first_prob = prob.split (cprob);
prob.split adjusts prob so this needs to be reflected in the comment (maybe
"adjusted prob" or the formula if it is simple). Otherwise looks good to me.
--
Eric Botcazou
