Re: [Freemint-discuss] Strange bug with asm clobbers

[Miro K. <mir...@gm...>]

Hmm, it reminds me of https://github.com/freemint/freemint/issues/285 but
this time it looks like behaving the other way around?

Dňa so 22. 3. 2025, 14:07 Thorsten Otto via Freemint-discuss <
fre...@li...> napísal(a):

> Hi,
>
> while trying to write a test program, i encountered a strange bug with gcc
> asm
> clobber list. The code in question is
>
>         register unsigned short *start __asm__("a0");
>         unsigned short *end;
>         oldsp = Super(0);
>         start = buf + sizeof(buf) / sizeof(buf[0]);
>         __asm__ __volatile__(
>                   ".dc.w 0xf320,0x4e71,0x4e71,0x4e71\n" /* fsave -(a0)
> */
>                   "move.l %1,%0\n"
>                 : "=a"(end)
>                 : "a"(start)
>                 : );
>         SuperToUser(oldsp);
>
>         printf("start: %08lx end: %08lx size: %ld\n", (long)end,
> (long)start, (long)start - (long)end);
>
> And the generated code is:
>
> [0000014a] 7000                      moveq.l    #0,d0
> [0000014c] 2f00                      move.l     d0,-(a7)
> [0000014e] 3f3c 0020                 move.w     #$0020,-(a7) ; Super
> [00000152] 4e41                      trap       #1
> [00000154] 5c8f                      addq.l     #6,a7
> [00000156] 41f9 0001 94c8            lea.l      $000193E4,a0
> [0000015c] f320                      fsave      -(a0)
> [0000015e] 4e71                      nop
> [00000160] 4e71                      nop
> [00000162] 4e71                      nop
> [00000164] 2648                      movea.l    a0,a3
> [00000166] 2a4f                      movea.l    a7,a5         <--
> apparently
> the backup_sp from SuperToUser macro
> [00000168] 2f00                      move.l     d0,-(a7)
> [0000016a] 3f3c 0020                 move.w     #$0020,-(a7) ; Super
> [0000016e] 4e41                      trap       #1
> [00000170] 2e4d                      movea.l    a5,a7
> [00000172] 2008                      move.l     a0,d0         <-- using
> a0,
> although it was declared as being clobbered???
> [00000174] 908b                      sub.l      a3,d0
> [00000176] 2f00                      move.l     d0,-(a7)
> [00000178] 2f08                      move.l     a0,-(a7)
> [0000017a] 2f0b                      move.l     a3,-(a7)
> [00000186] 487a ff3a                 pea.l      .LC0(pc)
> [0000018a] 4eb9 0000 2998            jsr        _printf
>
>
> As seen above, gcc seems to assume that "a0" still contains the value of
> start, although is was declared as being clobbered in the SuperToUser
> macro.
>
> Did i miss something, or what is going wrong there?
>
>
>
>
>
>
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