[PATCH 1 of 2 V5] delta: have a native implementation of _findsnapshot

[Yuya Nishihara]

On Fri, 4 Jan 2019 09:49:04 +0100, Boris FELD wrote:
> On 03/01/2019 09:40, Yuya Nishihara wrote:
> > On Wed, 02 Jan 2019 12:33:25 +0100, Boris Feld wrote:
> >> # HG changeset patch
> >> # User Boris Feld <boris.feld at octobus.net>
> >> # Date 1545297320 -3600
> >> #      Thu Dec 20 10:15:20 2018 +0100
> >> # Node ID aef438c9ecdbae3efb5f57f56041109cd5acda39
> >> # Parent  9e593db5f1a1b08f812591439c831ace55bca321
> >> # EXP-Topic sparse-revlog
> >> # Available At https://bitbucket.org/octobus/mercurial-devel/
> >> #              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r aef438c9ecdb
> >> delta: have a native implementation of _findsnapshot
> >>
> >> The function might traverse a lot of revision, a native implementation get
> >> significantly faster.
> >>
> >> example affected manifest write
> >> before: 0.114989
> >> after:  0.067141 (-42%)
> >>
> >> diff --git a/mercurial/cext/revlog.c b/mercurial/cext/revlog.c
> >> --- a/mercurial/cext/revlog.c
> >> +++ b/mercurial/cext/revlog.c
> >> @@ -1050,6 +1050,75 @@ static PyObject *index_issnapshot(indexO
> >>  	return PyBool_FromLong((long)issnap);
> >>  }
> >>  
> >> +static PyObject *index_findsnapshots(indexObject *self, PyObject *args)
> >> +{
> >> +	Py_ssize_t start_rev;
> >> +	PyObject *cache;
> >> +	Py_ssize_t base;
> >> +	Py_ssize_t rev;
> >> +	int issnap;
> >> +	PyObject *key = NULL;
> >> +	PyObject *allvalues = NULL;
> >> +	PyObject *value = NULL;
> > Nit: several variables can be moved inside the "for" loop.
> 
> Okay moving them.
> 
> A couple of weeks ago, you moved a variable declaration higher in the
> function because of some windows thingy. Can you elaborate on the
> special case you were trying to avoid?

In old C standards to which old MSC compilers conform, variables can't be
"declared" in the middle of the block.

Don't:

  {
    foo();
    int x = bar();

Do:

  {
    int x;
    foo();
    x = bar();

or:

  {
    foo();
    {
      int x = bar();