https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71413
--- Comment #3 from Bernd Schmidt <bernds at gcc dot gnu.org> ---
Also, to help narrow it down, can you try it with this patch?
Index: tree-ssa-strlen.c
===================================================================
--- tree-ssa-strlen.c (revision 237069)
+++ tree-ssa-strlen.c (working copy)
@@ -1860,7 +1860,7 @@ handle_builtin_memcmp (gimple_stmt_itera
use_operand_p use_p;
imm_use_iterator iter;
- if (!res)
+ if (1 || !res)
return true;
FOR_EACH_IMM_USE_FAST (use_p, iter, res)
