[gcc r16-6243] Fix profile_probability constructor arg [PR123153]

[Victor Do Nascimento]

https://gcc.gnu.org/g:24bf392581ea6d931afc1e97d90d6a2d6bdaa183

commit r16-6243-g24bf392581ea6d931afc1e97d90d6a2d6bdaa183
Author: Victor Do Nascimento <victor.donascimento@arm.com>
Date:   Wed Dec 17 14:43:41 2025 +0000

    Fix profile_probability constructor arg [PR123153]
    
    Given that profile probability is computed as an unsigned integer
    value in the [0, max_probability = (uint32_t) 1 << (n_bits - 2)]
    range (as opposed to a [0, 1] float), 50/50 likeihoods are encoded as
    `even()', mapping to `max_probability / 2'.
    
    The previous use of 0.5 for an even probability was, as a consequence
    of the implicit `double' -> `uint32_t' conversion, silently set to 0 by
    GCC when not using the `-Wconversion' flag.
    
    We therefore replace the erroneous `probability (0.5, GUESSED)'
    initialization with its correct `profile_probability::even ()'
    counterpart.
    
    gcc/ChangeLog:
    
            PR tree-optimization/123153
            * tree-vect-loop-manip.cc
            (slpeel_tree_duplicate_loop_to_edge_cfg): use
            profile_probability::even () for even likelihood.

Diff:
---
 gcc/tree-vect-loop-manip.cc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/gcc/tree-vect-loop-manip.cc b/gcc/tree-vect-loop-manip.cc
index 5d7d599b9749..e2ea0426050e 100644
--- a/gcc/tree-vect-loop-manip.cc
+++ b/gcc/tree-vect-loop-manip.cc
@@ -1976,7 +1976,7 @@ slpeel_tree_duplicate_loop_to_edge_cfg (class loop *loop, edge loop_exit,
 	  edge dup_exit = make_edge (bbcond, new_exit->dest, latch_is_false
 				? EDGE_TRUE_VALUE : EDGE_FALSE_VALUE);
 
-	  profile_probability probability (0.5, GUESSED);
+	  profile_probability probability = profile_probability::even ();
 	  to_latch_e->probability = dup_exit->probability = probability;
 
 	  set_immediate_dominator (CDI_DOMINATORS, dup_exit->src,