[gcc r17-3483] [AutoFDO] Do not leave entry block at zero after guessed-profile adjust

https://gcc.gnu.org/g:ab0712667b0461e34679cade62369a2c92fbc9be

commit r17-3483-gab0712667b0461e34679cade62369a2c92fbc9be
Author: Kugan Vivekanandarajah <[email protected]>
Date:   Fri Aug 21 10:57:09 2026 +1000

    [AutoFDO] Do not leave entry block at zero after guessed-profile adjust
    
    When afdo_adjust_guessed_profile runs scale_bbs, the entry block can be
    scaled to 0 (auto FDO) while inner basic blocks keep nonzero AFDO
    counts.
    
    Example from profile annotation (stats.ii, _M_get_insert_unique_pos):
      ... total:264 head:-1
      bb 0 count updated 118111600 (estimated locally) -> 0 (auto FDO)
    Example ICE at LTO (WPA merge of comdat clones):
      Merging profiles of _M_get_insert_unique_pos/10 count:64711123189
      (auto FDO) to _M_get_insert_unique_pos/1 count:0 (auto FDO)
      during IPA pass: modref
      lto1: internal compiler error: in apply_scale, at profile-count.h:1192
        ipa_merge_profiles (dstnum, dstden)
    
    Fix by setting a zero entry block count to half scaled sample after
    afdo_adjust_guessed_profile.
    
    gcc/ChangeLog:
    
    2026-08-21  Kugan Vivekanandarajah  <[email protected]>
    
            * auto-profile.cc (afdo_calculate_branch_prob): Set a zero entry
            block count to half a scaled sample after adjusting guessed profiles.

Diff:
---
 gcc/auto-profile.cc | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/gcc/auto-profile.cc b/gcc/auto-profile.cc
index 324a26cefc8c..643e0622df5b 100644
--- a/gcc/auto-profile.cc
+++ b/gcc/auto-profile.cc
@@ -4421,6 +4421,11 @@ afdo_calculate_branch_prob (bb_set *annotated_bb)
 	    }
       }
   afdo_adjust_guessed_profile (annotated_bb);
+  /* Avoid scaling with a zero entry count during IPA profile merging.  */
+  basic_block entry = ENTRY_BLOCK_PTR_FOR_FN (cfun);
+  if (!entry->count.nonzero_p ())
+    entry->count = profile_count::from_gcov_type
+      (MAX ((gcov_type) 1, autofdo::afdo_count_scale / 2)).afdo ();
   FOR_ALL_BB_FN (bb, cfun)
     {
       bb->aux = NULL;