cmd/compile: provide a way to auto-discover -d debug keys
Currently one needs to refer to the sources to have a list of accepted
debug keys. We can copy what 'ssa/help' does and introspect the list of
debug keys to print a more detailed help:
$ go tool compile -d help
usage: -d arg[,arg]* and arg is <key>[=<value>]
<key> is one of:
append print information about append compilation
closure print information about closure compilation
disablenil disable nil checks
dclstack run internal dclstack check
gcprog print dump of GC programs
nil print information about nil checks
panic do not hide any compiler panic
slice print information about slice compilation
typeassert print information about type assertion inlining
wb print information about write barriers
export print export data
pctab print named pc-value table
ssa/help print help about SSA debugging
<value> is key-specific.
Key "pctab" supports values:
"pctospadj", "pctofile", "pctoline", "pctoinline", "pctopcdata"
For '-d help' to be discoverable, a hint is given in the -d flag
description.
A last thing, today at least one go file needs to be provided to get to
the code printing ssa/help.
$ go tool compile -d ssa/help foo.go
Add a check so one can just do '-d help' or '-d ssa/help'
Caught by trybot: I needed to update fmt_test.go as I'm introducing the
usage of %-*s in a format string.
Fixes #20041
Change-Id: Ib2858b038c1bcbe644aa3b1a371009710c6d957d
Reviewed-on: https://go-review.googlesource.com/41091
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>