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BranchMonitor.md

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The branches Monitor

Wizard's built-in branches monitor analyzes the dynamic control flow behavior of a program by instrumenting all branches and reporting a summary. It is enabled with the --monitors=branches flag and accepts a small number of flags.

Basic usage

% bin/wizeng --monitors=branches test/monitors/branch_monitor.wasm 
func "start":
  +18 br_if:	 [1, 4]
  +22 br_if:	 [1, 0]
func "foo":
  +22 br_table:	 [4, 4, 4, 8]
  +41 br_if:	 [1, 19]

As we can see above, the default behavior is to instrument every branch in the program and report the count for each destination. For conditional branch instructions such as br_if and if, the counts reported are the not taken/taken and false/true counts. For br_table, the count is per destination, starting with 0.

Annotated code display

The {c} or {code} suboption displays the branch information as annotations on the bytecode (including nice effects like color and underlines if your terminal supports it).

% bin/wizeng --monitors=branches{c} test/monitors/branch_monitor.wasm 
func "start":
   +3     i32.const[5]                     
   +5     local.set[0]                     
   +7     loop[]                             <-- +18: 4, +22: 0
   +9     local.get[0]                     
  +11     i32.const[1]                     
  +13     i32.sub                          
  +14     local.set[0]                     
  +16     local.get[0]                     
  +18     br_if[depth=0]                     --> +7: 4, +20: 1
  +20     local.get[0]                       <-- +18: 1
  +22     br_if[depth=0]                     --> +7: 0, +24: 1
  +24     end                                <-- +22: 1
  +25     call[func=2]                     
  +27     end                              
func "foo":
   +3     i32.const[20]                    
   +5     local.set[0]                     
   +7     loop[]                             <-- +41: 19
   +9     block[]                          
  +11     block[]                          
  +13     block[]                          
  +15     block[]                          
  +17     local.get[0]                     
  +19     i32.const[5]                     
  +21     i32.rem_s                        
  +22     br_table[4...]                     --> +30: 4, +29: 4, +28: 4, +27: 8
  +28     end                                <-- +22: 4
  +29     end                                <-- +22: 4
  +30     end                                <-- +22: 4
  +31     end                              
  +32     local.get[0]                     
  +34     i32.const[1]                     
  +36     i32.sub                          
  +37     local.set[0]                     
  +39     local.get[0]                     
  +41     br_if[depth=0]                     --> +7: 19, +43: 1
  +43     end                                <-- +41: 1
  +44     end

CSV display

The branch monitor can also output its data in a comma-separated-value format, which makes it useful for processing by other tools.

% bin/wizeng --monitors=branches -csv test/monitors/branch_monitor.wasm 
branch,func,pc,instruction,taken_paths
branch,"start",18,br_if,[1 4]
branch,"start",22,br_if,[1 0]
branch,"foo",22,br_table,[4 4 4 8]
branch,"foo",41,br_if,[1 19]