Investigate missed deadcode elimination optimization

[mohammed90]

The linker's deadcode elimination may be partially disabled for variety of reasons, e.g. using MethodByName of the reflect package. It appears that Caddy pays this price due to the text/template package. The tool https://github.com/aarzilli/whydeadcode shows the call graph leading to a function that disables deadcode elimination.

It's not clear if avoiding those functions is possible in Caddy, but worth investigating and discussing. These are the ones I found in my attempts:

✗ $ go build -ldflags=-dumpdep . |& whydeadcode
text/template.(*state).evalField reachable from:
         text/template.(*state).evalFieldChain
         text/template.(*state).evalCommand
         text/template.(*state).evalPipeline
         text/template.(*state).walk
         text/template.(*Template).execute
         text/template.(*Template).Execute
         github.com/caddyserver/caddy/v2/cmd.init.0.func1.(*Command).SetHelpTemplate.tmpl.2
         github.com/caddyserver/caddy/v2/cmd.init.0.func1
         github.com/caddyserver/caddy/v2/cmd.init.0.func1·f
         github.com/caddyserver/caddy/v2/cmd.init.0
         github.com/caddyserver/caddy/v2/cmd..inittask
         go:main.inittasks
         _

$ go build -ldflags=-dumpdep . |& whydeadcode
text/template.(*state).evalField reachable from:
         text/template.(*state).evalFieldChain
         text/template.(*state).evalCommand
         text/template.(*state).evalPipeline
         text/template.(*state).walk
         text/template.(*Template).execute
         text/template.(*Template).Execute
         github.com/caddyserver/caddy/v2/cmd.init.0.func1.(*Command).SetVersionTemplate.tmpl.1
         github.com/caddyserver/caddy/v2/cmd.init.0.func1
         github.com/caddyserver/caddy/v2/cmd.init.0.func1·f
         github.com/caddyserver/caddy/v2/cmd.init.0
         github.com/caddyserver/caddy/v2/cmd..inittask
         go:main.inittasks
         _

$ go build -ldflags=-dumpdep . |& whydeadcode
text/template.(*state).evalField reachable from:
         text/template.(*state).evalFieldChain
         text/template.(*state).evalCommand
         text/template.(*state).evalPipeline
         text/template.(*state).walk
         text/template.(*Template).execute
         github.com/caddyserver/caddy/v2/modules/caddyhttp.buildHTTPServer
         github.com/caddyserver/caddy/v2/modules/caddyhttp.cmdRespond
         github.com/caddyserver/caddy/v2/modules/caddyhttp.cmdRespond·f
         github.com/caddyserver/caddy/v2/modules/caddyhttp.init.9.func1
         github.com/caddyserver/caddy/v2/modules/caddyhttp.init.9.func1·f
         github.com/caddyserver/caddy/v2/modules/caddyhttp.init.9
         github.com/caddyserver/caddy/v2/modules/caddyhttp..inittask
         go:main.inittasks
         _

[ottenhoff]

I can't find the origin of this one, but it doesn't look easy to eliminate:

text/template.(*state).evalField reachable from:
         text/template.(*state).evalFieldChain
         text/template.(*state).evalCommand
         text/template.(*state).evalPipeline
         text/template.(*state).walk
         text/template.(*Template).execute
         html/template.(*Template).Execute
         golang.org/x/net/trace.RenderEvents
         golang.org/x/net/trace.Events
         golang.org/x/net/trace.Events·f
         golang.org/x/net/trace.init.0
         golang.org/x/net/trace..inittask
         go:main.inittasks

https://github.com/golang/go/issues/62024

[mohammed90]

but it doesn't look easy to eliminate

Sigh. Thanks for taking a shot! I see now there's discussion at golang/go#72895.

[pgimalac]

I randomly stumbled on this issue and I don't know much about your code so I can't guarantee it's the same thing, but I know google.golang.org/grpc (which appears in your go.mod) imports golang.org/x/net/trace which uses a template in their init function, thus disabling DCE.

google.golang.org/grpc provides a build tag grpcnotrace to remove the import https://github.com/grpc/grpc-go/pull/6954.

Also for the record there is ongoing work to remove this use of templates https://github.com/golang/go/issues/62024.

[pgimalac]

While I'm here thanks to https://github.com/spf13/cobra/pull/1956, github.com/spf13/cobra now doesn't disable DCE by default, so if you can remove the uses of SetHelpTemplate and SetVersionTemplate then you might be able to fix this 👍