[PATCH v3 0/2] Improve diffcore-pickaxe documentation

[PATCH v3 0/2] Improve diffcore-pickaxe documentation

[Ramkumar Ramachandra]

[1/2] replaces "pickaxe regex" with "regex", as suggested by Junio.

[2/2] is now co-authored by Junio.  I've taken what I think are the
best changes introduced by both versions and merged them.  The
inter-diff follows.

diff --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
index b61a666..2835eef 100644
--- a/Documentation/diff-options.txt
+++ b/Documentation/diff-options.txt
@@ -383,18 +383,19 @@ ifndef::git-format-patch[]
 	that matches other criteria, nothing is selected.
 
 -S<string>::
-	Look for commits that change the number of occurrences of the
-	specified string (i.e. addition/ deletion) in a file.
+	Look for differences that change the number of occurrences of
+	the specified string (i.e. addition/deletion) in a file.
 	Intended for the scripter's use.
 +
 It is especially useful when you're looking for an exact block of code
 (like a struct), and want to know the history of that block since it
-first came into being.
+first came into being: use the feature iteratively to feed the
+interesting block in the preimage back into `-S`, and keep going until
+you get the very first version of the block.
 
 -G<regex>::
-	Grep through the patch text of commits for added/removed lines
-	that match <regex>.  `--pickaxe-regex` is implied in this
-	mode.
+	Look for differences whose patch text contains added/removed
+	lines that match <regex>.
 +
 To illustrate the difference between `-S<regex> --pickaxe-regex` and
 `-G<regex>`, consider a commit with the following diff in the same
@@ -408,7 +409,7 @@ file:
 +
 While `git log -G"regexec\(regexp"` will show this commit, `git log
 -S"regexec\(regexp" --pickaxe-regex` will not (because the number of
-occurrences of that string didn't change).
+occurrences of that string did not change).
 +
 See the 'pickaxe' entry in linkgit:gitdiffcore[7] for more
 information.
@@ -419,8 +420,8 @@ information.
 	in <string>.
 
 --pickaxe-regex::
-	Treat the <string> not as a plain string, but an extended
-	POSIX regex to match.  It is implied when `-G` is used.
+	Treat the <string> given to `-S` as an extended POSIX regular
+	expression to match.
 endif::git-format-patch[]
 
 -O<orderfile>::
diff --git a/Documentation/gitdiffcore.txt b/Documentation/gitdiffcore.txt
index d0f2b91..ef4c04a 100644
--- a/Documentation/gitdiffcore.txt
+++ b/Documentation/gitdiffcore.txt
@@ -225,31 +225,29 @@ diffcore-pickaxe: For Detecting Addition/Deletion of Specified String
 There are two kinds of pickaxe: the S kind (corresponding to 'git log
 -S') and the G kind (mnemonic: grep; corresponding to 'git log -G').
 
-The S kind detects filepairs whose "result" side and "origin" side
-have different number of occurrences of specified string.  By
-definition, it will not detect in-file moves.  Also, when a commit
-moves a file wholesale without affecting the string being looked at,
-rename detection kicks in as usual, and 'git log -S' omits the commit
-(since the number of occurrences of that string didn't change in that
-rename-detected filepair).  The implementation essentially runs a
-count, and is significantly cheaper than the G kind.
-
-The G kind detects filepairs whose patch text has an added or a
-deleted line that matches the given regexp.  This means that it can
-detect in-file (or what rename-detection considers the same file)
-moves.  The implementation of 'git log -G' runs diff twice and greps,
-and this can be quite expensive.
-
-When `--pickaxe-regex` is used with `-S`, treat the <string> not as a
-plain string, but an extended POSIX regex to match.  It is implied
-when `-G` is used.
-
-When `--pickaxe-all` is not in effect, diffcore-pickaxe leaves only
-the filepairs that touch the specified string in its output.  When in
-effect, diffcore-pickaxe leaves all filepairs intact if there is such
-a filepair, or makes the output empty otherwise.  The latter behavior
-is designed to make reviewing of the changes in the context of the
-whole changeset easier.
+"-S<block of text>" detects filepairs whose preimage and postimage
+have different number of occurrences of the specified block of text.
+By definition, it will not detect in-file moves.  Also, when a
+changeset moves a file wholesale without affecting the interesting
+string, rename detection kicks in as usual, and `-S` omits the
+filepair (since the number of occurrences of that string didn't change
+in that rename-detected filepair).  The implementation essentially
+runs a count, and is significantly cheaper than the G kind.  When used
+with `--pickaxe-regex`, treat the <block of text> as an extended POSIX
+regular expression to match, instead of a literal string.
+
+"-G<regular expression>" detects filepairs whose textual diff has an
+added or a deleted line that matches the given regular expression.
+This means that it can detect in-file (or what rename-detection
+considers the same file) moves.  The implementation runs diff twice
+and greps, and this can be quite expensive.
+
+When `-S` or `-G` are used without `--pickaxe-all`, only filepairs
+that match their respective criterion are kept in the output.  When
+`--pickaxe-all` is used, if even one filepair matches their respective
+criterion in a changeset, the entire changeset is kept.  This behavior
+is designed to make reviewing changes in the context of the whole
+changeset easier.
 
 diffcore-order: For Sorting the Output Based on Filenames
 ---------------------------------------------------------


Ramkumar Ramachandra (2):
  diffcore-pickaxe: make error messages more consistent
  diffcore-pickaxe doc: document -S and -G properly

 Documentation/diff-options.txt | 38 +++++++++++++++++++++++++++++--------
 Documentation/gitdiffcore.txt  | 43 ++++++++++++++++++++++++------------------
 diffcore-pickaxe.c             |  4 ++--
 3 files changed, 57 insertions(+), 28 deletions(-)

-- 
1.8.3.rc3.17.gd95ec6c.dirty

[PATCH 1/2] diffcore-pickaxe: make error messages more consistent

[Ramkumar Ramachandra]

Currently, diffcore-pickaxe reports two distinct errors for the same
user error:

    $ git log --pickaxe-regex -S'\1'
    fatal: invalid pickaxe regex: Invalid back reference

    $ git log -G'\1' # --pickaxe-regex is implied
    fatal: invalid log-grep regex: Invalid back reference

Since the error has nothing to do with "log-grep", change the -G and -S
error messages to say "invalid regex".

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
---
 diffcore-pickaxe.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/diffcore-pickaxe.c b/diffcore-pickaxe.c
index 63722f8..c97ac9b 100644
--- a/diffcore-pickaxe.c
+++ b/diffcore-pickaxe.c
@@ -122,7 +122,7 @@ static void diffcore_pickaxe_grep(struct diff_options *o)
 		char errbuf[1024];
 		regerror(err, &regex, errbuf, 1024);
 		regfree(&regex);
-		die("invalid log-grep regex: %s", errbuf);
+		die("invalid regex: %s", errbuf);
 	}
 
 	pickaxe(&diff_queued_diff, o, &regex, NULL, diff_grep);
@@ -246,7 +246,7 @@ static void diffcore_pickaxe_count(struct diff_options *o)
 			char errbuf[1024];
 			regerror(err, &regex, errbuf, 1024);
 			regfree(&regex);
-			die("invalid pickaxe regex: %s", errbuf);
+			die("invalid regex: %s", errbuf);
 		}
 		regexp = &regex;
 	} else {
-- 
1.8.3.rc3.17.gd95ec6c.dirty

[PATCH 2/2] diffcore-pickaxe doc: document -S and -G properly

[Ramkumar Ramachandra]

The documentation of -S and -G is very sketchy.  Completely rewrite the
sections in Documentation/diff-options.txt and
Documentation/gitdiffcore.txt.

References:
52e9578 ([PATCH] Introducing software archaeologist's tool "pickaxe".)
f506b8e (git log/diff: add -G<regexp> that greps in the patch text)

Inputs-from: Phil Hord <phil.hord@gmail.com>
Co-authored-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
---
 Documentation/diff-options.txt | 38 +++++++++++++++++++++++++++++--------
 Documentation/gitdiffcore.txt  | 43 ++++++++++++++++++++++++------------------
 2 files changed, 55 insertions(+), 26 deletions(-)

diff --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
index 104579d..2835eef 100644
--- a/Documentation/diff-options.txt
+++ b/Documentation/diff-options.txt
@@ -383,14 +383,36 @@ ifndef::git-format-patch[]
 	that matches other criteria, nothing is selected.
 
 -S<string>::
-	Look for differences that introduce or remove an instance of
-	<string>. Note that this is different than the string simply
-	appearing in diff output; see the 'pickaxe' entry in
-	linkgit:gitdiffcore[7] for more details.
+	Look for differences that change the number of occurrences of
+	the specified string (i.e. addition/deletion) in a file.
+	Intended for the scripter's use.
++
+It is especially useful when you're looking for an exact block of code
+(like a struct), and want to know the history of that block since it
+first came into being: use the feature iteratively to feed the
+interesting block in the preimage back into `-S`, and keep going until
+you get the very first version of the block.
 
 -G<regex>::
-	Look for differences whose added or removed line matches
-	the given <regex>.
+	Look for differences whose patch text contains added/removed
+	lines that match <regex>.
++
+To illustrate the difference between `-S<regex> --pickaxe-regex` and
+`-G<regex>`, consider a commit with the following diff in the same
+file:
++
+----
++    return !regexec(regexp, two->ptr, 1, &regmatch, 0);
+...
+-    hit = !regexec(regexp, mf2.ptr, 1, &regmatch, 0);
+----
++
+While `git log -G"regexec\(regexp"` will show this commit, `git log
+-S"regexec\(regexp" --pickaxe-regex` will not (because the number of
+occurrences of that string did not change).
++
+See the 'pickaxe' entry in linkgit:gitdiffcore[7] for more
+information.
 
 --pickaxe-all::
 	When `-S` or `-G` finds a change, show all the changes in that
@@ -398,8 +420,8 @@ ifndef::git-format-patch[]
 	in <string>.
 
 --pickaxe-regex::
-	Make the <string> not a plain string but an extended POSIX
-	regex to match.
+	Treat the <string> given to `-S` as an extended POSIX regular
+	expression to match.
 endif::git-format-patch[]
 
 -O<orderfile>::
diff --git a/Documentation/gitdiffcore.txt b/Documentation/gitdiffcore.txt
index 568d757..ef4c04a 100644
--- a/Documentation/gitdiffcore.txt
+++ b/Documentation/gitdiffcore.txt
@@ -222,26 +222,33 @@ version prefixed with '+'.
 diffcore-pickaxe: For Detecting Addition/Deletion of Specified String
 ---------------------------------------------------------------------
 
-This transformation is used to find filepairs that represent
-changes that touch a specified string, and is controlled by the
--S option and the `--pickaxe-all` option to the 'git diff-*'
-commands.
-
-When diffcore-pickaxe is in use, it checks if there are
-filepairs whose "result" side and whose "origin" side have
-different number of specified string.  Such a filepair represents
-"the string appeared in this changeset".  It also checks for the
-opposite case that loses the specified string.
-
-When `--pickaxe-all` is not in effect, diffcore-pickaxe leaves
-only such filepairs that touch the specified string in its
-output.  When `--pickaxe-all` is used, diffcore-pickaxe leaves all
-filepairs intact if there is such a filepair, or makes the
-output empty otherwise.  The latter behaviour is designed to
-make reviewing of the changes in the context of the whole
+There are two kinds of pickaxe: the S kind (corresponding to 'git log
+-S') and the G kind (mnemonic: grep; corresponding to 'git log -G').
+
+"-S<block of text>" detects filepairs whose preimage and postimage
+have different number of occurrences of the specified block of text.
+By definition, it will not detect in-file moves.  Also, when a
+changeset moves a file wholesale without affecting the interesting
+string, rename detection kicks in as usual, and `-S` omits the
+filepair (since the number of occurrences of that string didn't change
+in that rename-detected filepair).  The implementation essentially
+runs a count, and is significantly cheaper than the G kind.  When used
+with `--pickaxe-regex`, treat the <block of text> as an extended POSIX
+regular expression to match, instead of a literal string.
+
+"-G<regular expression>" detects filepairs whose textual diff has an
+added or a deleted line that matches the given regular expression.
+This means that it can detect in-file (or what rename-detection
+considers the same file) moves.  The implementation runs diff twice
+and greps, and this can be quite expensive.
+
+When `-S` or `-G` are used without `--pickaxe-all`, only filepairs
+that match their respective criterion are kept in the output.  When
+`--pickaxe-all` is used, if even one filepair matches their respective
+criterion in a changeset, the entire changeset is kept.  This behavior
+is designed to make reviewing changes in the context of the whole
 changeset easier.
 
-
 diffcore-order: For Sorting the Output Based on Filenames
 ---------------------------------------------------------
 
-- 
1.8.3.rc3.17.gd95ec6c.dirty

Re: [PATCH 2/2] diffcore-pickaxe doc: document -S and -G properly

[Junio C Hamano]

Ramkumar Ramachandra <artagnon@gmail.com> writes:

> The documentation of -S and -G is very sketchy.  Completely rewrite the
> sections in Documentation/diff-options.txt and
> Documentation/gitdiffcore.txt.
>
> References:
> 52e9578 ([PATCH] Introducing software archaeologist's tool "pickaxe".)
> f506b8e (git log/diff: add -G<regexp> that greps in the patch text)
>
> Inputs-from: Phil Hord <phil.hord@gmail.com>
> Co-authored-by: Junio C Hamano <gitster@pobox.com>
> Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
> ---
>  Documentation/diff-options.txt | 38 +++++++++++++++++++++++++++++--------
>  Documentation/gitdiffcore.txt  | 43 ++++++++++++++++++++++++------------------
>  2 files changed, 55 insertions(+), 26 deletions(-)
>
> diff --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
> index 104579d..2835eef 100644
> --- a/Documentation/diff-options.txt
> +++ b/Documentation/diff-options.txt
> @@ -383,14 +383,36 @@ ifndef::git-format-patch[]
>  	that matches other criteria, nothing is selected.
>  
>  -S<string>::
> -	Look for differences that introduce or remove an instance of
> -	<string>. Note that this is different than the string simply
> -	appearing in diff output; see the 'pickaxe' entry in
> -	linkgit:gitdiffcore[7] for more details.
> +	Look for differences that change the number of occurrences of
> +	the specified string (i.e. addition/deletion) in a file.
> +	Intended for the scripter's use.
> ++
> +It is especially useful when you're looking for an exact block of code
> +(like a struct), and want to know the history of that block since it
> +first came into being: use the feature iteratively to feed the
> +interesting block in the preimage back into `-S`, and keep going until
> +you get the very first version of the block.

OK, even though I would not say "especially" nor "useful" if I were
writing it, as it is the only use case it was designed for.

>  -G<regex>::
> +	Look for differences whose patch text contains added/removed
> +	lines that match <regex>.
> ++
> +To illustrate the difference between `-S<regex> --pickaxe-regex` and
> +`-G<regex>`, consider a commit with the following diff in the same
> +file:
> ++
> +----
> ++    return !regexec(regexp, two->ptr, 1, &regmatch, 0);
> +...
> +-    hit = !regexec(regexp, mf2.ptr, 1, &regmatch, 0);
> +----
> ++
> +While `git log -G"regexec\(regexp"` will show this commit, `git log
> +-S"regexec\(regexp" --pickaxe-regex` will not (because the number of
> +occurrences of that string did not change).
> ++
> +See the 'pickaxe' entry in linkgit:gitdiffcore[7] for more
> +information.

OK.

>  --pickaxe-regex::
> -	Make the <string> not a plain string but an extended POSIX
> -	regex to match.
> +	Treat the <string> given to `-S` as an extended POSIX regular
> +	expression to match.

OK.

> diff --git a/Documentation/gitdiffcore.txt b/Documentation/gitdiffcore.txt
> index 568d757..ef4c04a 100644
> --- a/Documentation/gitdiffcore.txt
> +++ b/Documentation/gitdiffcore.txt
> @@ -222,26 +222,33 @@ version prefixed with '+'.
>  diffcore-pickaxe: For Detecting Addition/Deletion of Specified String
>  ---------------------------------------------------------------------
>  
> -This transformation is used to find filepairs that represent
> -changes that touch a specified string, and is controlled by the
> --S option and the `--pickaxe-all` option to the 'git diff-*'
> -commands.
> -
> -When diffcore-pickaxe is in use, it checks if there are
> -filepairs whose "result" side and whose "origin" side have
> -different number of specified string.  Such a filepair represents
> -"the string appeared in this changeset".  It also checks for the
> -opposite case that loses the specified string.
> -
> -When `--pickaxe-all` is not in effect, diffcore-pickaxe leaves
> -only such filepairs that touch the specified string in its
> -output.  When `--pickaxe-all` is used, diffcore-pickaxe leaves all
> -filepairs intact if there is such a filepair, or makes the
> -output empty otherwise.  The latter behaviour is designed to
> -make reviewing of the changes in the context of the whole


> +There are two kinds of pickaxe: the S kind (corresponding to 'git log
> +-S') and the G kind (mnemonic: grep; corresponding to 'git log -G').

This is good as the beginning of a second paragraph or the second
sentence of the first paragraph.  This patch loses the description
of the general purpose of this machinery that should come at the
very beginning of the section (the original had a very good ut valid
only back when we had only -S; my "how about this" text did not have
a good one).

For example, the "rename" is about taking one set of filepairs and
expressing (some of) them as renames or copies by merging a deletion
filepair and a creation filepair into a rename-modify filepair, or
turning a creation filepair into a copy-modify filepair by finding a
preimage.  What does this transformation do?

Again here is my attempt for that missing first paragraph:

	This transformation limits the set of filepairs to those
	that change specified strings between the preimage and the
	postimage in a certain way.

        -S<block of text> and -G<regex> options are used to specify
	different ways these strings are sought.  Without
	--pickaxe-all, only the filepairs matching the given
	criterion is left in the output; all filepairs are left in
	the output when --pickaxe-all is used and if at least one
	filepair matches the given criterion.

but I do not have enough time now to condense the above down to a
readable paragraph of reasonable length (I expect that the ideal
final form would be like 5-6 lines at most).

> +"-S<block of text>" detects filepairs whose preimage and postimage
> +have different number of occurrences of the specified block of text.
> +By definition, it will not detect in-file moves.  Also, when a
> +changeset moves a file wholesale without affecting the interesting
> +string, rename detection kicks in as usual, and `-S` omits the
> +filepair (since the number of occurrences of that string didn't change
> +in that rename-detected filepair).

I am not sure why it is necessary to say anything about what the
previous step (diffcore-rename) might have done.  The input of this
(or any other) step in the diffcore pipeline is a preimage-postimage
filepairs, and to this transformation the filename does not matter.
Whether a file was moved (either "wholesale", implying nothing
changed, or renamed with modification at the same time) without
touching the block of text, or a file did not get involved in any
renaming, the only thing that matters is what the preimage and the
postimage in a filepair has (or does not have).

> + The implementation essentially
> +runs a count, and is significantly cheaper than the G kind.  When used
> +with `--pickaxe-regex`, treat the <block of text> as an extended POSIX
> +regular expression to match, instead of a literal string.

Sure.  Is "essentially runs a count" needed, though?  The reader has
just read "number of occurrences of the specified block of text" so
it would be obvious that the implementation counts.  It may be true
that it is significantly cheaper, but because they serve different
purposes, I am not sure it is worth saying.  It is like saying that
a hammer is significantly faster to drive a nail into wood than a
screwdriver to drive a screw into wood, without saying "nail" and
"screw".  It only invites readers to use a hammer to drive a screw.

> +"-G<regular expression>" detects filepairs whose textual diff has an
> +added or a deleted line that matches the given regular expression.
> +This means that it can detect in-file (or what rename-detection
> +considers the same file) moves.

"it can" sounds as if it is always a merit, which is probably not
what you wanted to imply.

When you are trying to see how a particular line came into the
shape, you would want to know what the previous shape of it was, but
a literal move will also be shown, which is a noise for the purpose
of digging.

> +The implementation runs diff twice
> +and greps, and this can be quite expensive.

Unlike the "count" one above which was obvious, the "runs diff and
greps hence expensive" part is worth saying.

> +When `-S` or `-G` are used without `--pickaxe-all`, only filepairs
> +that match their respective criterion are kept in the output.  When
> +`--pickaxe-all` is used, if even one filepair matches their respective
> +criterion in a changeset, the entire changeset is kept.  This behavior
> +is designed to make reviewing changes in the context of the whole
>  changeset easier.

OK.

>  
> -
>  diffcore-order: For Sorting the Output Based on Filenames
>  ---------------------------------------------------------

Re: [PATCH 2/2] diffcore-pickaxe doc: document -S and -G properly

[Ramkumar Ramachandra]

Junio C Hamano wrote:
> [...]

I agree with everything else, and made changes accordingly.

>         This transformation limits the set of filepairs to those
>         that change specified strings between the preimage and the
>         postimage in a certain way.

Definitely good.

>         -S<block of text> and -G<regex> options are used to specify
>         different ways these strings are sought.

Definitely better than the "two kinds of pickaxe" thing.

>         Without
>         --pickaxe-all, only the filepairs matching the given
>         criterion is left in the output; all filepairs are left in
>         the output when --pickaxe-all is used and if at least one
>         filepair matches the given criterion.

Why do a poor-man's version of --pickaxe-all here, when the last
paragraph already does justice to this?

When `-S` or `-G` are used without `--pickaxe-all`, only filepairs
that match their respective criterion are kept in the output.  When
`--pickaxe-all` is used, if even one filepair matches their respective
criterion in a changeset, the entire changeset is kept.  This behavior
is designed to make reviewing changes in the context of the whole
changeset easier.

> I am not sure why it is necessary to say anything about what the
> previous step (diffcore-rename) might have done.  The input of this
> (or any other) step in the diffcore pipeline is a preimage-postimage
> filepairs, and to this transformation the filename does not matter.
> Whether a file was moved (either "wholesale", implying nothing
> changed, or renamed with modification at the same time) without
> touching the block of text, or a file did not get involved in any
> renaming, the only thing that matters is what the preimage and the
> postimage in a filepair has (or does not have).

While what you're saying is technically true, I think it is important
to explain the interaction between diffcore-pickaxe and
diffcore-rename as I have done.  Someone who wants to understand what
`git log -S` does will come to this page and read this section:
without reading diffcore-rename, she will have an incomplete picture;
what's the harm in explaining diffcore-rename in the context of
diffcore-pickaxe?

I did do a s/rename detection/diffcore-rename/ though, so the user
knows where to look for more on this rename thing.

Re: [PATCH 2/2] diffcore-pickaxe doc: document -S and -G properly

[Junio C Hamano]

Ramkumar Ramachandra <artagnon@gmail.com> writes:

>>         Without
>>         --pickaxe-all, only the filepairs matching the given
>>         criterion is left in the output; all filepairs are left in
>>         the output when --pickaxe-all is used and if at least one
>>         filepair matches the given criterion.
>
> Why do a poor-man's version of --pickaxe-all here, when the last
> paragraph already does justice to this?

The point of the first paragraph is to serve to help both:

 (1) people who read about this rather technical part of the
     diffcore pipeline machinery, to prepare them by listing what
     they will learn about in the section; and

 (2) those who already have read and want to skim over, by giving a
     concise summary.

It may have been "poor" because it was merely "something like this"
patch, though.


> While what you're saying is technically true, I think it is important
> to explain the interaction between diffcore-pickaxe and
> diffcore-rename as I have done.  Someone who wants to understand what
> `git log -S` does will come to this page and read this section:
> without reading diffcore-rename, she will have an incomplete picture;
> what's the harm in explaining diffcore-rename in the context of
> diffcore-pickaxe?

That is a red-herring; that is exactly why we upfront say that the
diffcore machinery is a pipeline and we describe upstream processing
like rename before we talk about pickaxe in the same document.

This document is the most accurate _technical_ documentation of how
the pipeline works (and it is not in section 1 of the manual set).
If you want to improve end-user documentation by adding explanation
for interactions between pipeline stages and also pathspec, I am all
for it, but I think that belongs to the larger "git help diff", not
"git help diffcore".

Re: [PATCH 2/2] diffcore-pickaxe doc: document -S and -G properly

[Ramkumar Ramachandra]

Junio C Hamano wrote:
>> Why do a poor-man's version of --pickaxe-all here, when the last
>> paragraph already does justice to this?
>
> The point of the first paragraph is to serve to help both:

My question pertains to whether or not the explanation of
--pickaxe-all can wait till the last paragraph.  You want to
explain all the options in the first paragraph, but not
--pickaxe-regex?

>> While what you're saying is technically true, I think it is important
>> to explain the interaction between diffcore-pickaxe and
>> diffcore-rename as I have done.  Someone who wants to understand what
>> `git log -S` does will come to this page and read this section:
>> without reading diffcore-rename, she will have an incomplete picture;
>> what's the harm in explaining diffcore-rename in the context of
>> diffcore-pickaxe?
>
> This document is the most accurate _technical_ documentation of how
> the pipeline works (and it is not in section 1 of the manual set).
> If you want to improve end-user documentation by adding explanation
> for interactions between pipeline stages and also pathspec, I am all
> for it, but I think that belongs to the larger "git help diff", not
> "git help diffcore".

It's impossible to explain in the diff-options, because I do not have
even have access to the word "filepair", and no diffcore-rename
machinery to point to.  I could attempt a vague approximation, but I
do not think it's worth it.  I'll remove it if you insist.