On 2024-08-16 08:08, Pip Cet wrote:

> /* Work around GCC bug 54561.  */
> #if GNUC_PREREQ (4, 3, 0)
> # pragma GCC diagnostic ignored "-Wclobbered"
> #endif
> 
> which means we won't get any warnings at all about such bugs.
> 
> However, even with that part removed, I don't get a compiler warning
> about 'c'.

With that part removed I got a warning about 'kb', not 'c'. The 'kb' 
warning was a false positive, the 'c' warning a false negative. The most 
likely explanation, I think, is that GCC got confused and put the wrong 
identifier name in its diagnostic. This would help to explain a lot of 
the -Wclobbered false positives we've gotten over the years, which has 
caused us to ignore -Wclobbered.


> What's weird about this is that the store at +5463 that's causing our
> problem isn't necessary, and neither is the load at +5479. The variable
> in question, 'kb', is not marked volatile, so it seems strange to me it's
> stored, then loaded, unnecessarily.

My guess is that GCC internally coalesced the two variables, and got 
confused because one was clobbered and the other was not, and issued a 
warning that it clobbered the variable but mistakenly said 'kb' not 'c'. 
The C standard entitles GCC to clobber 'c' so this isn't wrong code, 
it's just a bad (a *really* bad) diagnostic.


> Paul, do you agree that we should simply mark 'c' as volatile, or do you
> think this needs further investigation on the GCC side (because of the
> lack of a -Wclobbered warning being issued, or because of the weird
> code)?

We definitely should mark 'c' as volatile, or do something equivalent, 
because the Emacs code violates the C standard.

Also, if my hypothesis of the mistaken identifier is correct, we should 
stop ignoring -Wclobbered, and instead pacify GCC without using 
-Wclobbered. That way, when we screw up in setjmp related code, we'll 
get *some* warning that *something* is being clobbered, and can investigate.

To do that, I installed the attached patches into the Emacs master 
branch. While looking into this I noticed several uses of clobbered 
variables (i.e., violations of the C standard) and fixed them. Patches 
0001-0004 fix these similar bugs elsewhere. Patch 0005 implements your 
suggestion and should fix Bug#71744. Patch 0006 is a minor performance 
improvement over Patch 0005.

Please give these patches a try. Patch 0005 is simple and if it works 
for the original bug reporter we should be able to close the bug report 
as fixed.