bug#53058: etc/DEBUG could say more about --enable-check-lisp-object-type

[Sean Whitton <spwhitton@spwhitton.name>]

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Hello,

On Wed 05 Jan 2022 at 02:11PM +02, Eli Zaretskii wrote:

> Comparisons with structures, like foo == Qnil, aren't guaranteed to
> work in GDB.  --enable-check-lisp-object-type makes Lisp objects be
> structures rather than simple scalar values.

Thanks -- how about the attached?

-- 
Sean Whitton

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From eaf40f04614063010e60d14c9ba8ba48e4b842bc Mon Sep 17 00:00:00 2001
From: Sean Whitton <spwhitton@spwhitton.name>
Date: Thu, 6 Jan 2022 11:17:44 -0700
Subject: [PATCH] etc/DEBUG: Say more about --enable-check-lisp-object-type

* etc/DEBUG: Explain how --enable-check-lisp-object-type can obstruct
certain debugging strategies.  Don't claim that this switch has no
effect on debugging with GDB.
---
 etc/DEBUG | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/etc/DEBUG b/etc/DEBUG
index dd33b42f19..6d0e27cbce 100644
--- a/etc/DEBUG
+++ b/etc/DEBUG
@@ -28,12 +28,21 @@ exceptionally hard.
 Older versions of GCC may need more than just the -g3 flag.  For more,
 search for "analyze failed assertions" below.
 
-The 2 --enable-* switches are optional.  They don't have any effect on
-debugging with GDB, but will compile additional code that might catch
-the problem you are debugging much earlier, in the form of assertion
-violation.  The --enable-checking option also enables additional
-functionality useful for debugging display problems; see more about
-this below under "Debugging Emacs redisplay problems".
+The 2 --enable-* switches are optional.  They compile additional code
+that might catch the problem you are debugging much earlier, in the
+form of assertion violation.  The --enable-checking option also
+enables additional functionality useful for debugging display
+problems; see more about this below under "Debugging Emacs redisplay
+problems".
+
+You should be aware that the --enable-check-lisp-object-type option
+changes Lisp objects from scalar values into structs, and this can
+obstruct certain debugging strategies.  In particular, asking GDB to
+compare Lisp objects might not work.  For example, consider this
+attempt to conditionalize a break point: "condition 2 NILP (foo)".  In
+this case you are requesting that GDB make the comparison "foo ==
+Qnil", and you might need to recompile without
+--enable-check-lisp-object-type for that to work.
 
 Emacs needs not be installed to be debugged, you can debug the binary
 created in the 'src' directory.
-- 
2.30.2