Allow undefined behavior also for perf baseline

config/examples/performance_sensitive.yaml

@@ -3,6 +3,12 @@
 
 # You should choose the fastest setjmp/longjmp for your platform.
 
+# With the vast majority of compilers some of the 'undefined behavior'
+# assumptions are fine, and produce smaller and faster code, so enable
+# by default for performance oriented targets.  You may need to disable
+# this for some compilers.
+DUK_USE_ALLOW_UNDEFINED_BEHAVIOR: true
+
 DUK_USE_PREFER_SIZE: false
 DUK_USE_PACKED_TVAL: false  # packed duk_tval slower in most cases
 DUK_USE_FASTINT: true