Allow linker to perform deadcode elimination for program using Cobra #1956

aarzilli · 2023-05-05T11:49:51Z

Cobra, in its default configuration, will execute a template to generate help, usage and version outputs. Text/template execution calls MethodByName and MethodByName disables dead code elimination in the Go linker, therefore all programs that make use of cobra will be linked with dead code elimination disabled, even if they end up replacing the default usage, help and version formatters with a custom function and no actual text/template evaluations are ever made at runtime.

Dead code elimination in the linker helps reduce disk space and memory utilization of programs. For example, for the simple example program used by TestDeadcodeElimination 40% of the final executable size is dead code. For a more realistic example, 12% of the size of Delve's executable is deadcode.

This PR changes Cobra so that, in its default configuration, it does not automatically inhibit deadcode elimination by:

changing Cobra's default behavior to emit output for usage and help using simple Go functions instead of template execution
quarantining all calls to template execution into SetUsageTemplate, SetHelpTemplate and SetVersionTemplate so that the linker can statically determine if they are reachable

CLAassistant · 2023-05-05T11:49:56Z

All committers have signed the CLA.

github-actions · 2023-05-05T11:50:10Z

This PR exceeds the recommended size of 200 lines. Please make sure you are NOT addressing multiple issues with one PR. Note this PR might be rejected due to its size.

aarzilli · 2023-05-05T14:53:00Z

The CLA thing isn't working for some reason.

github-actions · 2023-05-05T20:14:54Z

This PR exceeds the recommended size of 200 lines. Please make sure you are NOT addressing multiple issues with one PR. Note this PR might be rejected due to its size.

marckhouzam · 2023-06-12T12:39:11Z

Thanks @aarzilli !
I like the idea of optimizing things.
This will require some thought so I can understand clearly the behaviour change. Thanks for adding a test it should help with this.

aarzilli · 2023-06-12T12:51:37Z

Unless I made a mistake there shouldn't be any behavior changes (as in, observable from the outside). Happy to explain anything about this if needed.

aarzilli · 2023-07-28T07:01:10Z

Ping?

aarzilli · 2023-10-10T09:33:54Z

Ping?

github-actions · 2024-02-10T13:44:09Z

This PR exceeds the recommended size of 200 lines. Please make sure you are NOT addressing multiple issues with one PR. Note this PR might be rejected due to its size.

github-actions · 2024-04-10T08:01:12Z

This PR exceeds the recommended size of 200 lines. Please make sure you are NOT addressing multiple issues with one PR. Note this PR might be rejected due to its size.

github-actions · 2024-05-10T14:50:30Z

This PR exceeds the recommended size of 200 lines. Please make sure you are NOT addressing multiple issues with one PR. Note this PR might be rejected due to its size.

github-actions · 2024-06-13T10:01:15Z

This PR exceeds the recommended size of 200 lines. Please make sure you are NOT addressing multiple issues with one PR. Note this PR might be rejected due to its size.

github-actions · 2024-08-10T11:31:30Z

This PR exceeds the recommended size of 200 lines. Please make sure you are NOT addressing multiple issues with one PR. Note this PR might be rejected due to its size.

marckhouzam · 2024-10-14T01:58:22Z

@aarzilli I apologize for such a long delay, but I had misunderstood the value of this PR. I now realize that it may help all programs using Cobra, so I'm very interested.

I'm trying to convince myself that that this change actually has an impact.
Would you be able to explain how I can see the difference in a program that has dead code, before and after this PR?

Here is what I tried.

I used the program you have added to the tests in this PR
I added an "Unused()" function that I don't call in the program.
I ran go build -o myprog . (using cobra 1.8.1 without your PR)
I ran go tool nm ./myprog | grep Unused

I would have expected to see the Unused function but I don't.
I used go version go1.22.8 darwin/arm64

Could you clarify what I should expect?

This is the program:

package main

import (
	"fmt"
	"os"

	"github.com/spf13/cobra"
)

func Unused() {
	fmt.Println("not called")
}

var rootCmd = &cobra.Command{
	Version: "1.0",
	Use:     "example_program",
	Short:   "example_program - test fixture to check that deadcode elimination is allowed",
	Run: func(cmd *cobra.Command, args []string) {
		fmt.Println("hello world")
	},
	Aliases: []string{"alias1", "alias2"},
	Example: "stringer --help",
}

func main() {
	if err := rootCmd.Execute(); err != nil {
		fmt.Fprintf(os.Stderr, "Whoops. There was an error while executing your CLI '%s'", err)
		os.Exit(1)
	}
}

aarzilli · 2024-10-14T13:18:20Z

One easy way to see the impact of this change is to compile your example program with and without this PR:

$ ls -lh
total 8.4M
-rwxr-xr-x. 1 a a 5.2M Oct 14 14:45 example.withoutpr*
-rwxr-xr-x. 1 a a 3.2M Oct 14 14:45 example.withpr*
-rw-r--r--. 1 a a  596 Oct 14 14:44 main.go

This is an extreme example, where the size of the executable is reduced by 38%, but reductions of 10% are realistic.
I used go1.23 but you should get very similar results with go1.22.8.

As to your question, saying that deadcode elimination gets disabled is incorrect: in reality it continues to function but in a reduced capacity. Specifically if reflect.Value.MethodByName or reflect.Value.Method are reachable it can not always prove that exported methods are unreachable.

Your Unused function is not an exported method so, unless it is called by an exported method, it can always be deadcode eliminated.

If you want to see the Unused function make it all the way to the executable you have to do something like this:

package main

import (
	"fmt"
	"os"

	"github.com/spf13/cobra"
)

type Astruct struct {
	n int
}

//go:noinline
func Unused() {
	fmt.Println("not called")
}

//go:noinline
func (a *Astruct) Unused() {
	fmt.Println("not called", a.n)
	Unused()
}

//go:noinline
func (a *Astruct) Used() {
	fmt.Println("used", a.n)
}

var rootCmd = &cobra.Command{
	Version: "1.0",
	Use:     "example_program",
	Short:   "example_program - test fixture to check that deadcode elimination is allowed",
	Run: func(cmd *cobra.Command, args []string) {
		fmt.Println("hello world")
	},
	Aliases: []string{"alias1", "alias2"},
	Example: "stringer --help",
}

func main() {
	if err := rootCmd.Execute(); err != nil {
		fmt.Fprintf(os.Stderr, "Whoops. There was an error while executing your CLI '%s'", err)
		os.Exit(1)
	}
	var a Astruct
	fmt.Println(a)
	a.Used()
}

All of the noinline directives are needed because small functions like this would be removed by the inliner and the fmt.Println call is needed to make the Astruct type itself reachable (if all calls to the methods of Astruct are static the linker can still prove that Unused is unreachable).

With these changes you get:

$ go build main2.go && go tool nm ./main2 | grep 'Unused'
  589c00 T main.(*Astruct).Unused
  589ba0 T main.Unused
  765570 D runtime.adviseUnused
  4179e0 T runtime.sysUnusedOS

without the PR and:

$ go build main2.go && go tool nm ./main2 | grep 'Unused'
  602564 D runtime.adviseUnused
  4144a0 T runtime.sysUnusedOS

with the PR.

marckhouzam · 2024-10-15T03:28:15Z

Thanks for the explanation @aarzilli, I can now see the benefits.

So, with this PR, programs that don't set new templates (usage, help or version) will be able to do dead code elimination fully.

What about programs that do modify those templates? If they want to get full usage of dead code elimination they should convert their template use to a go function like you have done, IIUC? My next step for this review is to try to override the help/usage/version in that fashion and see if it works as expected.

I believe that using SetHelpFunc() and SetUsageFunc() will allow programs to do this, but I don't think we have a way to do this for the version template. But it is ok, it is still better than not being able to do it at all. Once this is merged, we can discuss adding a SetVersionFunc().

marckhouzam

This looks great.
I still want to do some testing, but I don't expect any big surprises.
Here are some comments for the PR.
The PR also needs to be rebased.

Thanks again for your patience, I think we can get this in soon.

marckhouzam · 2024-10-14T01:18:19Z

cobra_test.go

+		dirname  = "test_deadcode"
+		progname = "test_deadcode_elimination"
+	)
+	os.Mkdir(dirname, 0770)


I'm getting a warning here.
Can you change line to _ = os.Mkdir(dirname, 0770)

marckhouzam · 2024-10-14T01:18:41Z

cobra_test.go

+		os.Exit(1)
+	}
+}
+`), 0660)


I'm getting a warning here.
Can you change the permissions to 0600

marckhouzam · 2024-10-15T02:40:12Z

cobra_test.go

+	if err != nil {
+		t.Fatalf("could not write test program: %v", err)
+	}
+	defer os.RemoveAll(dirname)


Can you move this defer to right after the os.Mkdir call to guarantee it gets called even if the t.Fatalf above gets triggered.

marckhouzam · 2024-10-15T02:54:00Z

cobra_test.go

@@ -222,3 +226,57 @@ func TestRpad(t *testing.T) {
 		})
 	}
 }
+


Can you add a comment above this test explaining that deadcode elimination is reduced when MethodByName is present which will affect all programs using cobra; so this test make sure we don't introduce code that depends on MethodByName().

Also please put a comment with the URL of this PR.

See if you like what I wrote.

marckhouzam · 2024-10-15T03:34:55Z

What do you think about updating the documentation in the three sections starting here https://github.com/spf13/cobra/blob/main/site/content/user_guide.md#defining-your-own-help to explain the side-effect of overriding the template, and therefore that it is recommended to set a function instead?

Cobra, in its default configuration, will execute a template to generate help, usage and version outputs. Text/template execution calls MethodByName and MethodByName disables dead code elimination in the Go linker, therefore all programs that make use of cobra will be linked with dead code elimination disabled, even if they end up replacing the default usage, help and version formatters with a custom function and no actual text/template evaluations are ever made at runtime. Dead code elimination in the linker helps reduce disk space and memory utilization of programs. For example, for the simple example program used by TestDeadcodeElimination 40% of the final executable size is dead code. For a more realistic example, 12% of the size of Delve's executable is deadcode. This PR changes Cobra so that, in its default configuration, it does not automatically inhibit deadcode elimination by: 1. changing Cobra's default behavior to emit output for usage and help using simple Go functions instead of template execution 2. quarantining all calls to template execution into SetUsageTemplate, SetHelpTemplate and SetVersionTemplate so that the linker can statically determine if they are reachable

github-actions · 2024-10-15T08:19:54Z

This PR exceeds the recommended size of 200 lines. Please make sure you are NOT addressing multiple issues with one PR. Note this PR might be rejected due to its size.

aarzilli · 2024-10-15T08:24:21Z

Thanks for the explanation @aarzilli, I can now see the benefits.

So, with this PR, programs that don't set new templates (usage, help or version) will be able to do dead code elimination fully.

What about programs that do modify those templates? If they want to get full usage of dead code elimination they should convert their template use to a go function like you have done, IIUC?

Yes, this is correct.

I believe that using SetHelpFunc() and SetUsageFunc() will allow programs to do this, but I don't think we have a way to do this for the version template. But it is ok, it is still better than not being able to do it at all. Once this is merged, we can discuss adding a SetVersionFunc().

It imagine should be trivial.

github-actions · 2024-10-15T08:33:47Z

This PR exceeds the recommended size of 200 lines. Please make sure you are NOT addressing multiple issues with one PR. Note this PR might be rejected due to its size.

aarzilli force-pushed the dereflect branch from 99d7f09 to de6570f Compare May 5, 2023 20:14

aarzilli mentioned this pull request May 16, 2023

cmd/link: there should be a way to determine why deadcode elimination was not performed golang/go#60221

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aarzilli force-pushed the dereflect branch from de6570f to 8159848 Compare February 10, 2024 13:43

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marckhouzam changed the title ~~Restructure code to let linker perform deadcode elimination step~~ Allow linker to perform deadcode elimination for program using Cobra Oct 14, 2024

marckhouzam reviewed Oct 15, 2024

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aarzilli force-pushed the dereflect branch from ee7aba8 to 4da4d97 Compare October 15, 2024 08:19

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Allow linker to perform deadcode elimination for program using Cobra #1956

Allow linker to perform deadcode elimination for program using Cobra #1956

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@@ @@ -222,3 +226,57 @@ func TestRpad(t *testing.T) { @@
               		})
               	}
               }

Allow linker to perform deadcode elimination for program using Cobra #1956

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Allow linker to perform deadcode elimination for program using Cobra #1956

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