adds gnark/profile that outputs pprof compatible circuit profiling data

[gbotrel]

gnark/profile is a profiling package, which creates pprof compatible profiling files. Once the .pprof file is generated, one can visualize it using go tool pprof like standard Golang CPU or Memory profiles.

A gnark Profile measures constraint added in a circuit (for now, all constraints are equal, but we may differentiate them in the future), example output (from examples/rollup):

Usage:

type Circuit struct {
	A frontend.Variable
}

func (circuit *Circuit) Define(api frontend.API) error {
	api.AssertIsEqual(api.Mul(circuit.A, circuit.A), circuit.A)
	return nil
}

func Example() {
	// default options generate gnark.pprof in current dir
	// use pprof as usual (go tool pprof -http=:8080 gnark.pprof) to read the profile file
	// overlapping profiles are allowed (define profiles inside Define or subfunction to profile
	// part of the circuit only)
	p := profile.Start()
	_, _ = frontend.Compile(ecc.BN254.ScalarField(), r1cs.NewBuilder, &Circuit{})
	p.Stop()

	fmt.Println(p.NbConstraints())
	fmt.Println(p.Top())
	// Output:
	// 2
	// Showing nodes accounting for 2, 100% of 2 total
	//       flat  flat%   sum%        cum   cum%
	//          1 50.00% 50.00%          2   100%  profile_test.(*Circuit).Define profile/profile_test.go:17
	//          1 50.00%   100%          1 50.00%  r1cs.(*r1cs).AssertIsEqual frontend/cs/r1cs/api_assertions.go:37
}

This should make api.Counter and api.Tag useless.

[ivokub]

Suggested edit:

diff --git a/profile/profile.go b/profile/profile.go
index 6ac2042e..3d3778d9 100644
--- a/profile/profile.go
+++ b/profile/profile.go
@@ -41,19 +41,22 @@ type Profile struct {
 	chDone chan struct{}
 }
 
-// ProfilePath controls the profile destination file. If blank, profile is not written.
+// Option defines configuration Options for Profile.
+type Option func(*Profile)
+
+// WithPath controls the profile destination file. If blank, profile is not written.
 //
 // Defaults to ./gnark.pprof.
-func ProfilePath(path string) func(*Profile) {
+func WithPath(path string) Option {
 	return func(p *Profile) {
 		p.filePath = path
 	}
 }
 
-// ProfileNoOutput indicates that the profile is not going to be written to disk.
+// WithNoOutput indicates that the profile is not going to be written to disk.
 //
-// This is equivalent to ProfilePath("")
-func ProfileNoOutput() func(*Profile) {
+// This is equivalent to WithPath("")
+func WithNoOutput() Option {
 	return func(p *Profile) {
 		p.filePath = ""
 	}
@@ -65,7 +68,7 @@ func ProfileNoOutput() func(*Profile) {
 // All calls to profile.Start() and Stop() are meant to be executed in the same go routine (frontend.Compile).
 //
 // It is allowed to create multiple overlapping profiling sessions in one circuit.
-func Start(options ...func(*Profile)) *Profile {
+func Start(options ...Option) *Profile {
 
 	// start the worker first time a profiling session starts.
 	onceInit.Do(func() {

[ivokub]

Looks good. Maybe only thing which mildly caught my eye was that for Plonk we don't record if the constraint comes from Add/Mul. But I guess that in the future when custom gates arrive we have to rethink anyway.

I also changed the profile option function signature to look similar with other Option types.