Re: [PATCH] Fix documentation about rot agent expression bytecode

[Simon Marchi <simon dot marchi at ericsson dot com>]

On 2017-07-19 05:16 PM, Simon Marchi wrote:
> The rot agent expression bytecode rotates the three items on the top of
> the stack.  It is not clear which way the rotation is.  However, the
> documentation currently shows this as the effect of the instructions:
> 
>    a b c => c b a
> 
> which doesn't make sense, since the value b doesn't move.  The two
> valid possibilities I see are
> 
>   a b c => b c a
>   a b c => c a b
> 
> depending on which way you rotate.
> 
> When looking at the gdbserver code, the top of the stack becomes the
> third item, and the next-to-top item becomes the top.  So the second
> form would be the right one, since in this notation the top of the stack
> is the rightmost element:
> 
>   a b c => c a b
> 
> I adjusted the symbolic description and added a bit of text to make it
> more obvious.
> 
> gdb/doc/ChangeLog:
> 
> 	* agentexpr.texi (rot): Fix symbolic description, improve
> 	textual description.
> ---
>  gdb/doc/agentexpr.texi | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/gdb/doc/agentexpr.texi b/gdb/doc/agentexpr.texi
> index 5668e9c..aef1973 100644
> --- a/gdb/doc/agentexpr.texi
> +++ b/gdb/doc/agentexpr.texi
> @@ -396,8 +396,10 @@ is zero, this is the same as @code{dup}; if @var{n} is one, it copies
>  the item under the top item, etc.  If @var{n} exceeds the number of
>  items on the stack, terminate with an error.
>  
> -@item @code{rot} (0x33): @var{a} @var{b} @var{c} => @var{c} @var{b} @var{a}
> -Rotate the top three items on the stack.
> +@item @code{rot} (0x33): @var{a} @var{b} @var{c} => @var{c} @var{a} @var{b}
> +Rotate the top three items on the stack.  The top (first) item becomes
> +the third item, the next-to-top (second) item becomes the top (first) item
> +and the third item becomes the next-to-top (second) item.
>  
>  @item @code{if_goto} (0x20) @var{offset}: @var{a} @result{}
>  Pop an integer off the stack; if it is non-zero, branch to the given
> 

I pushed this patch, though with small changes in the description to try to make it clearer.


>From 791fb3d7a58b86e5f30229625dfd33659f33fd6e Mon Sep 17 00:00:00 2001
From: Simon Marchi <simon.marchi@ericsson.com>
Date: Mon, 31 Jul 2017 11:22:21 +0200
Subject: [PATCH] Fix documentation about rot agent expression bytecode

The rot agent expression bytecode rotates the three items on the top of
the stack.  It is not clear which way the rotation is.  However, the
documentation currently shows this as the effect of the instructions:

   a b c => c b a

which doesn't make sense, since the value b doesn't move.  The two
valid possibilities I see are

  a b c => b c a
  a b c => c a b

depending on which way you rotate.

When looking at the gdbserver code, the top of the stack becomes the
third item, and the next-to-top item becomes the top.  So the second
form would be the right one, since in this notation the top of the stack
is the rightmost element:

  a b c => c a b

I adjusted the symbolic description and added a bit of text to make it
more obvious.

gdb/doc/ChangeLog:

	* agentexpr.texi (rot): Fix symbolic description, improve
	textual description.
---
 gdb/doc/ChangeLog      | 5 +++++
 gdb/doc/agentexpr.texi | 6 ++++--
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/gdb/doc/ChangeLog b/gdb/doc/ChangeLog
index d43ae35..6a3d061 100644
--- a/gdb/doc/ChangeLog
+++ b/gdb/doc/ChangeLog
@@ -1,3 +1,8 @@
+2017-07-31  Simon Marchi  <simon.marchi@ericsson.com>
+
+	* agentexpr.texi (rot): Fix symbolic description, improve
+	textual description.
+
 2017-07-26  Yao Qi  <yao.qi@linaro.org>

 	* gdb.texinfo (Maintenance Commands): Document command
diff --git a/gdb/doc/agentexpr.texi b/gdb/doc/agentexpr.texi
index 5668e9c..081e5fa 100644
--- a/gdb/doc/agentexpr.texi
+++ b/gdb/doc/agentexpr.texi
@@ -396,8 +396,10 @@ is zero, this is the same as @code{dup}; if @var{n} is one, it copies
 the item under the top item, etc.  If @var{n} exceeds the number of
 items on the stack, terminate with an error.

-@item @code{rot} (0x33): @var{a} @var{b} @var{c} => @var{c} @var{b} @var{a}
-Rotate the top three items on the stack.
+@item @code{rot} (0x33): @var{a} @var{b} @var{c} => @var{c} @var{a} @var{b}
+Rotate the top three items on the stack.  The top item (c) becomes the third
+item, the next-to-top item (b) becomes the top item and the third item (a) from
+the top becomes the next-to-top item.

 @item @code{if_goto} (0x20) @var{offset}: @var{a} @result{}
 Pop an integer off the stack; if it is non-zero, branch to the given
-- 
2.7.4