Re: VAX: Another small GAS bug?

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

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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


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