Wizard's built-in control monitor analyzes the dynamic control flow behavior of a program by instrumenting all loops and branches.
Its primary purpose is to be a more informative display of the bytecode, annotated with execution counts.
It is enabled with the --monitors=control flag.
% bin/wizeng --monitors=control test/monitors/loop_monitor0.wasm
func #0: x 1
+1 | call[func=1]
+3 ╰>end x 1
func #1: x 1
+3 | i32.const[133]
+6 | local.set[0]
+8 | ╭ loop[] <─╮ x 133
+10 | | i32.const[3] │
+12 | | local.set[1] │
+14 | | ╭ loop[] <───╮ x 399
+16 | | | local.get[1] │ │
+18 | | | i32.const[1] │ │
+20 | | | i32.sub │ │
+21 | | | local.set[1] │ │
+23 | | | local.get[1] │ │
+25 | | | br_if[depth=0] ────╯ x 266
| | | │ x 133
+27 | | ╰ end │ x 133
+28 | | local.get[0] │
+30 | | i32.const[1] │
+32 | | i32.sub │
+33 | | local.set[0] │
+35 | | local.get[0] │
+37 | | br_if[depth=0] ──╯ x 132
| | x 1
+39 | ╰ end x 1
+40 | i32.const[123]
+43 | local.set[0]
+45 | ╭ loop[] <─╮ x 123
+47 | | local.get[0] │
+49 | | i32.const[1] │
+51 | | i32.sub │
+52 | | local.set[0] │
+54 | | local.get[0] │
+56 | | br_if[depth=0] ──╯ x 122
| | x 1
+58 | ╰ end x 1
+59 ╰>end x 1
As we can see above, the default behavior is to instrument every loop and branch in the program and display a control flow graph annotated with execution counts.
Functions are annotated with their invocation count on the first line.
Similarly, loop and end bytecodes are annotated with their execution and branches have their taken count, with a fallthrough count, if applicable.