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received-spf: None (protection.outlook.com: microsoft.com does not designate permitted sender hosts) authentication-results: spf=none (sender IP is ) smtp.mailfrom=dstolee@microsoft.com; x-microsoft-antispam-message-info: C+yGC9V7DdpObnyjhoJPXvDWj/8bWHwJhkW2zfx6KWIeaOL1oQn/qQuYLdkTxJCh9kcgyN+uPMP/nU1WgvPKGOy7ccVVDRyfCku+Jy+n39mlbbPSf1JzasznZ75hQRVy1WT2n5CrpoQ5GgSqLFqVuK9brocfbBuOKv/6vJMqUFpyY6j1DOQ5oociPj6YZ/Yw spamdiagnosticoutput: 1:99 spamdiagnosticmetadata: NSPM Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Office365-Filtering-Correlation-Id: ecce9d55-32bd-4f18-558c-08d5af61b06a X-OriginatorOrg: microsoft.com X-MS-Exchange-CrossTenant-Network-Message-Id: ecce9d55-32bd-4f18-558c-08d5af61b06a X-MS-Exchange-CrossTenant-originalarrivaltime: 01 May 2018 12:47:25.2052 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 72f988bf-86f1-41af-91ab-2d7cd011db47 X-MS-Exchange-Transport-CrossTenantHeadersStamped: BL0PR2101MB1042 Sender: git-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org We now calculate generation numbers in the commit-graph file and use them in paint_down_to_common(). Expand the section on generation numbers to discuss how the three special generation numbers GENERATION_NUMBER_INFINITY, _ZERO, and _MAX interact with other generation numbers. Signed-off-by: Derrick Stolee --- Documentation/technical/commit-graph.txt | 29 ++++++++++++++++++++---- 1 file changed, 24 insertions(+), 5 deletions(-) diff --git a/Documentation/technical/commit-graph.txt b/Documentation/techn= ical/commit-graph.txt index 0550c6d0dc..e1a883eb46 100644 --- a/Documentation/technical/commit-graph.txt +++ b/Documentation/technical/commit-graph.txt @@ -77,6 +77,29 @@ in the commit graph. We can treat these commits as havin= g "infinite" generation number and walk until reaching commits with known generation number. =20 +We use the macro GENERATION_NUMBER_INFINITY =3D 0xFFFFFFFF to mark commits= not +in the commit-graph file. If a commit-graph file was written by a version +of Git that did not compute generation numbers, then those commits will +have generation number represented by the macro GENERATION_NUMBER_ZERO =3D= 0. + +Since the commit-graph file is closed under reachability, we can guarantee +the following weaker condition on all commits: + + If A and B are commits with generation numbers N amd M, respectively, + and N < M, then A cannot reach B. + +Note how the strict inequality differs from the inequality when we have +fully-computed generation numbers. Using strict inequality may result in +walking a few extra commits, but the simplicity in dealing with commits +with generation number *_INFINITY or *_ZERO is valuable. + +We use the macro GENERATION_NUMBER_MAX =3D 0x3FFFFFFF to for commits whose +generation numbers are computed to be at least this value. We limit at +this value since it is the largest value that can be stored in the +commit-graph file using the 30 bits available to generation numbers. This +presents another case where a commit can have generation number equal to +that of a parent. + Design Details -------------- =20 @@ -98,18 +121,14 @@ Future Work - The 'commit-graph' subcommand does not have a "verify" mode that is necessary for integration with fsck. =20 -- The file format includes room for precomputed generation numbers. These - are not currently computed, so all generation numbers will be marked as - 0 (or "uncomputed"). A later patch will include this calculation. - - After computing and storing generation numbers, we must make graph walks aware of generation numbers to gain the performance benefits they enable. This will mostly be accomplished by swapping a commit-date-order= ed priority queue with one ordered by generation number. The following operations are important candidates: =20 - - paint_down_to_common() - 'log --topo-order' + - 'tag --merged' =20 - Currently, parse_commit_gently() requires filling in the root tree object for a commit. This passes through lookup_tree() and consequently --=20 2.17.0.39.g685157f7fb