Re: port-sh3/60773: sh3 __sync_val_compare_and_swap_1 test failures

["Valery Ushakov via gnats" <gnats-admin%NetBSD.org@localhost>]

The following reply was made to PR toolchain/60773; it has been noted by GNATS.

From: Valery Ushakov <uwe%stderr.spb.ru@localhost>
To: gnats-bugs%netbsd.org@localhost
Cc: Taylor R Campbell <riastradh%NetBSD.org@localhost>,
	Nick Hudson <skrll%NetBSD.org@localhost>
Subject: Re: port-sh3/60773: sh3 __sync_val_compare_and_swap_1 test failures
Date: Thu, 24 Sep 2026 20:17:00 +0300

 On Thu, Sep 24, 2026 at 00:20:02 +0000, Valery Ushakov via gnats wrote:
 
 >  common/lib/libc/atomic/atomic_cas_8_cas.c defines
 >  sync_val_compare_and_swap_1 to return signed int8_t
 >  
 >  _atomic_cas_8 returns correct uin8_t value 0x88, then
 >  sync_val_compare_and_swap_1 sign extends it before returning.
 
 For the record, just to make it explicit.  Consider simplified:
 
 #include <inttypes.h>
 #include <stdint.h>
 #include <stdio.h>
 
 #define OLDVAL (0x1122334455667788UL)
 #define NEWVAL (0x8090a0b0c0d0e0f0UL)
 
 
 int
 main()
 {
     volatile uint8_t val;
     uint8_t oldval;
     uint8_t newval;
     uint8_t expval;
     uint8_t expres;
     uint8_t res;
 
 
     val    = (uint8_t)OLDVAL;
     oldval = (uint8_t)OLDVAL;
     newval = (uint8_t)NEWVAL;
     expval = (uint8_t)NEWVAL;
     expres = (uint8_t)OLDVAL;
     res = __sync_val_compare_and_swap_1(&val, oldval, newval);
 
     printf("expected %x, res = %x\n", expres, res);
     return 0;
 }
 
 
 
 => 0x7a095778 <_atomic_cas_8+16>:     jsr     @r0
 (gdb) si
 0x7a09563c in ?? () from /usr/lib/libc.so.12
 1: x/i $pc
 => 0x7a09563c:     extu.b  r5,r5
 
 We have entered _atomic_cas_8_up (static, so no symbol):
 
 (gdb) finish
 Run till exit from #0  0x7a09563c in ?? () from /usr/lib/libc.so.12
 0x7a09577c in _atomic_cas_8 () from /usr/lib/libc.so.12
 1: x/i $pc
 => 0x7a09577c <_atomic_cas_8+20>:     lds.l   @r15+,pr
 (gdb) p/x $r0
 $1 = 0x88
 
 We've got correct old uint8_t value (unsigned)
 
 (gdb) si
 0x7a09577e in _atomic_cas_8 () from /usr/lib/libc.so.12
 1: x/i $pc
 => 0x7a09577e <_atomic_cas_8+22>:     rts
 (gdb)
 0x7a031536 in __sync_val_compare_and_swap_1 () from /usr/lib/libc.so.12
 1: x/i $pc
 => 0x7a031536 <__sync_val_compare_and_swap_1+18>:     exts.b  r0,r0
 (gdb) si
 0x7a031538 in __sync_val_compare_and_swap_1 () from /usr/lib/libc.so.12
 1: x/i $pc
 => 0x7a031538 <__sync_val_compare_and_swap_1+20>:     lds.l   @r15+,pr
 (gdb) p/x $r0
 $2 = 0xffffff88
 
 Now we are back to __sync_val_compare_and_swap_1 and it needs to
 return int8_t (signed) value so it converts the uint8_t result of
 _atomic_cas_8_up by sign-extending (exts.b) the r0.
 
 
 -uwe