Re: port-arm/56839: GCC emits wrong codes for compare_and_swap_1 bultins on armv5 (el & eb)

[Rin Okuyama <rokuyama.rk%gmail.com@localhost>]

The following reply was made to PR port-arm/56839; it has been noted by GNATS.

From: Rin Okuyama <rokuyama.rk%gmail.com@localhost>
To: Nick Hudson <nick.hudson%gmx.co.uk@localhost>, gnats-bugs%netbsd.org@localhost,
 port-arm-maintainer%netbsd.org@localhost, gnats-admin%netbsd.org@localhost,
 netbsd-bugs%netbsd.org@localhost
Cc: 
Subject: Re: port-arm/56839: GCC emits wrong codes for compare_and_swap_1
 bultins on armv5 (el & eb)
Date: Fri, 20 May 2022 10:32:32 +0900

 Hi,
 
 On 2022/05/18 20:48, Nick Hudson wrote:
 > I think this fixes things. It's a bit ugly...
 > 
 > https://www.netbsd.org/~skrll/pr56839.diff
 
 This does not work:
 
 (1) GCC expands __sync_bool_compare_and_swap_N() into
      __sync_val_compare_and_swap_N() as optimization.
 
 (2) The patch does not cover __sync_val_compare_and_swap_N()
      ( == _atomic_cas_8N() ).
 
 On the other hand, this patch for _atomic_cas_8N() fixes the problem:
 
 https://gist.github.com/rokuyama/3d791f2ceb2757dc389a95910162c9cb
 
 However, IMO, this is not the real fix:
 
 (a) I've found that MI part, not arm-dependent part, of GCC carries
      out sign-extension; something is wrong in
      expand_builtin_compare_and_swap() @ dist/gcc/gcc/builtins.c,
      but I've never fully understand what goes wrong yet...
      (Strangely enough, GCC itself recognizes that these builtins
      takes unsigned integers...)
 
 (b) Therefore, archs other than arm that do not have 1- and 2-byte
      atomic cas builtins (== missing equivalent instructions) should
      be affected.
 
 (c) It should be better to fix GCC rather than adding workaround to
      libc routines.
 
 Thanks,
 rin