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cmd/compile: document how to update builtin.go

No code changes.

Fixes #15835.

Change-Id: Ibae3f20882f976babc4093df5e9fea0b2cf0e9d9
Reviewed-on: https://go-review.googlesource.com/23443
Reviewed-by: Alan Donovan <adonovan@google.com>
This commit is contained in:
Robert Griesemer 2016-05-25 11:23:56 -07:00
parent b7d96b8e05
commit a689f6b8af
3 changed files with 42 additions and 3 deletions

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@ -6,7 +6,7 @@
// (see fmt.go, parser.go as "documentation" for how to use/setup data structures)
/*
Export data encoding:
1) Export data encoding principles:
The export data is a serialized description of the graph of exported
"objects": constants, types, variables, and functions. In general,
@ -49,7 +49,7 @@ Before exporting or importing, the type tables are populated with the
predeclared types (int, string, error, unsafe.Pointer, etc.). This way
they are automatically encoded with a known and fixed type index.
Encoding format:
2) Encoding format:
The export data starts with a single byte indicating the encoding format
(compact, or with debugging information), followed by a version string
@ -84,6 +84,43 @@ each encoding routine there is a matching and symmetric decoding routine.
This symmetry makes it very easy to change or extend the format: If a new
field needs to be encoded, a symmetric change can be made to exporter and
importer.
3) Making changes to the encoding format:
Any change to the encoding format requires a respective change in the
exporter below and a corresponding symmetric change to the importer in
bimport.go.
Furthermore, it requires a corresponding change to go/internal/gcimporter
and golang.org/x/tools/go/gcimporter15. Changes to the latter must preserve
compatibility with both the last release of the compiler, and with the
corresponding compiler at tip. That change is necessarily more involved,
as it must switch based on the version number in the export data file.
It is recommended to turn on debugFormat when working on format changes
as it will help finding encoding/decoding inconsistencies quickly.
Special care must be taken to update builtin.go when the export format
changes: builtin.go contains the export data obtained by compiling the
builtin/runtime.go and builtin/unsafe.go files; those compilations in
turn depend on importing the data in builtin.go. Thus, when the export
data format changes, the compiler must be able to import the data in
builtin.go even if its format has not yet changed. Proceed in several
steps as follows:
- Change the exporter to use the new format, and use a different version
string as well.
- Update the importer accordingly, but accept both the old and the new
format depending on the version string.
- all.bash should pass at this point.
- Run mkbuiltin.go: this will create a new builtin.go using the new
export format.
- go test -run Builtin should pass at this point.
- Remove importer support for the old export format and (maybe) revert
the version string again (it's only needed to mark the transition).
- all.bash should still pass.
Don't forget to set debugFormat to false.
*/
package gc

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@ -3,7 +3,8 @@
// license that can be found in the LICENSE file.
// Binary package import.
// Based loosely on x/tools/go/importer.
// See bexport.go for the export data format and how
// to make a format change.
package gc

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@ -8,6 +8,7 @@
// Run this after changing builtin/runtime.go and builtin/unsafe.go
// or after changing the export metadata format in the compiler.
// Either way, you need to have a working compiler binary first.
// See bexport.go for how to make an export metadata format change.
package main
import (