https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118125
--- Comment #10 from Martin Jambor <jamborm at gcc dot gnu.org> ---
For some reason, unlikely_executed_stmt_p (and thus
unlikely_executed_bb_p) do not see that __builtin_unreachable is a
cold function:
(gdb) pt decl
<function_decl 0x7fffbf5e7400 __builtin_unreachable
type <function_type 0x7fffbf5e6498
type <void_type 0x7fffbf61cf18 void VOID
align:8 warn_if_not_align:0 symtab:0 alias-set -1
structural-equality
pointer_to_this <pointer_type 0x7fffbf624000>>
QI
size <integer_cst 0x7fffbf602eb8 constant 8>
unit-size <integer_cst 0x7fffbf602ed0 constant 1>
align:8 warn_if_not_align:0 symtab:0 alias-set -1 structural-equality
arg-types <tree_list 0x7fffbf61bde8 value <void_type 0x7fffbf61cf18
void>>
pointer_to_this <pointer_type 0x7fffbe62e540>>
readonly volatile nothrow public external built-in QI <built-in>:0:0
align:8 warn_if_not_align:0 built-in: BUILT_IN_NORMAL:BUILT_IN_UNREACHABLE
context <translation_unit_decl 0x7fffbf4a6000
source/base/auto_derivative_function.cc>
attributes <tree_list 0x7fffbf5e4a78
purpose <identifier_node 0x7fffbf649e38 nothrow>
chain <tree_list 0x7fffbf5e4a50
purpose <identifier_node 0x7fffbf649e60 leaf>
chain <tree_list 0x7fffbf5e4a28
purpose <identifier_node 0x7fffbf649e10 noreturn>
chain <tree_list 0x7fffbf5e4a00
purpose <identifier_node 0x7fffbf649d48 const>>>>>>
(gdb) pt 0x7fffbf5e4a00
<tree_list 0x7fffbf5e4a00
purpose <identifier_node 0x7fffbf649d48 const>>
Patching the function with the following also restores the run-time
performance of the benchmark:
diff --git a/gcc/predict.cc b/gcc/predict.cc
index ef31c48bfe2..d3fc2f85414 100644
--- a/gcc/predict.cc
+++ b/gcc/predict.cc
@@ -843,6 +843,8 @@ unlikely_executed_stmt_p (gimple *stmt)
if (lookup_attribute ("cold", DECL_ATTRIBUTES (decl))
&& !lookup_attribute ("cold", DECL_ATTRIBUTES (current_function_decl)))
return true;
+ if (fndecl_built_in_p (decl, BUILT_IN_UNREACHABLE))
+ return true;
cgraph_node *n = cgraph_node::get (decl);
if (!n)
