Re: VAX: Another small GAS bug?

[Kalvis Duckmanton <kalvisd%gmail.com@localhost>]

On 7/4/24 05:01, Jan-Benedict Glaw wrote:
> I haven't even _looked_ at the code, so I don't know what it's
> actually doing. I don't know the FP representation out of my head, but
> if an all-zero value is interpreted as 0.0, a CLRx would probably be
> enough? Still, I'd like to see that bug fixed (and maybe add a
> testcase for it to upstream Binutils.)
>
>    Unfortunately, I guess I'll only have time for this in a month or
> the like, maybe you, or Kalvin, or Anders, or Martin, or Maciej, or
> Mouse, or ... may want to have a look?  Esp, since it happened in a
> regular CI build, but I was unable (in a few-minutes attempt) to
> reproduce it. :-/  I'd like to make sure there isn't much more hidden
> behind this issue.

So I had a look today.  I don't believe GAS is at fault; it has 
assembled an F-floating zero (so 32 bits wide) ... into the lower part 
of a buffer big enough to hold a quadword (64 bits) or a D-floating 
value (again 64 bits).  The other 32 bits in that buffer, which are 
mantissa bits in the D-floating type, are not initialised.  As rin 
points out, VAX looks only at the sign and exponent fields to determine 
whether a value is a zero.

The easiest fix seems to be to change the constants from $0f0.0 to 
$0d0.0.  If I do that and reassemble unimpl_emul.S (using native GAS), I get

 478:   c8 02 ad 40     bisl2 $0x2,0x40(fp)
 47c:   d1 58 38        cmpl r8,$0x38
 47f:   19 0f           blss 490 <emodd+0x1e8>
 481:   7d 8f 00 00     movq $0x0000000000000000,0x20(ap)
 485:   00 00 00 00
 489:   00 00 ac 20
 48d:   17 af 78        jmp 508 <emodd+0x260>

as expected.  I've shown only one example but the other 2 are the same - 
no unexpectedly set bits.

Proposed patch below.

cheers

kalvis

--8<--

diff --git a/sys/arch/vax/vax/unimpl_emul.S b/sys/arch/vax/vax/unimpl_emul.S
index c3b7b83baa3a..a546ceda3e2b 100644
--- a/sys/arch/vax/vax/unimpl_emul.S
+++ b/sys/arch/vax/vax/unimpl_emul.S
@@ -683,7 +683,7 @@ emodd:      bsbw    touser
         * there aren't any bits left for the fraction.  Therefore we're
         * done here;  TMPFRAC1 is equal to TMPFRACTGT and TMPFRAC2 is 0.
         */
-       movq $0f0.0, TMPFRAC2
+       movq $0d0.0, TMPFRAC2
        jmp 9f          /* we're done, move on */
 1:
        /*
@@ -727,7 +727,7 @@ emodd:      bsbw    touser
         * We are less than 1.0; TMPFRAC1 should be 0, and TMPFRAC2 should
         * be equal to TMPFRACTGT.
         */
-       movd $0f0.0, TMPFRAC1
+       movd $0d0.0, TMPFRAC1
        movd TMPFRACTGT, TMPFRAC2
 9:
        /*
@@ -763,7 +763,7 @@ zeroexit:
        bsbw getaddr_byte
        movl $0x0, (%r0)
        bsbw getaddr_byte
-       movd $0f0, (%r0)
+       movd $0d0, (%r0)
        brw goback


--8<--