Subject: Re: [sv-ec] Re: [sv-bc] Packed arrays
From: Michael McNamara (mac@verisity.com)
Date: Wed Jan 29 2003 - 15:18:38 PST
Kevin Cameron x3251 writes:
> You would only have to do that with packed structs/unions, and the
> overhead difference is likely to be marginal.
>
> Using $rtb/$btr is definitely slower regardless of bit order than
> using wreal or a shared variable.
>
Come now. Realtobits is presuambly coded as:
typedef union {
double d;
long long l;
} rtb_u;
long long realtobits( double d ) {
rtb_u u;
u.d = d;
return(u.l);
}
which on the sparc is expanded to essentially a nop (three cheers for
register windows!):
0x10608 <realtobits>: retl
0x1060c <realtobits+4>: nop
and given the test program:
main ()
{
union {
long long l;
int a[2];
} u;
double d = 3.1415926459045;
long long l;
l = realtobits(d);
u.l = l;
printf("D is %f; $realtobits(D) is 0x%08x%08x\n",d,u.a[0],u.a[1]);
}
on the sparc you get:
kodiak 162 > a.out
D is 3.141593; $realtobits(D) is 0x400921fb533c1c8a
On the x86 it expands to:
0x8048460 <realtobits>: push %ebp
0x8048461 <realtobits+1>: mov %esp,%ebp
0x8048463 <realtobits+3>: mov 0x8(%ebp),%eax
0x8048466 <realtobits+6>: mov 0xc(%ebp),%edx
0x8048469 <realtobits+9>: pop %ebp
0x804846a <realtobits+10>: ret
0x804846b <realtobits+11>: nop
because I have a 32 bit linux port, and hence you can't do a 64 bit
move; and moreover without register windows you have to actually copy
the input to the output. Curiously the CISC takes more instructions
that the RISC, in this case!
On Linux, or course, as a little endian, you get a different
intermediate value:
D is 3.141593; $realtobits(D) is 0x533c1c8a400921fb
The point is realtobits and bitstoreal do precisely what a union would
do; and I believe this is what you are asking for in your packed
discussion; and so I tell you that in terms of execution speed, you
already have what you want, part of Verilog 1364-1995.
All that you could optimize away is the procedure call, which on the
sparc is the call and the return.
I do agree that it would be far nicer if one didn't have to call
$realtobits and $bitstoreal when passing things around.
However, if you do make this conversion to bits as union part of SV,
you will must then chose:
1) State that the intermediate bit pattern is undefined and
implementaion specific,
2) Require the little endian (x86) to do it the big endian (sparc)
way, making this operation more expensive on x86
3) Require the big endian (sparc) to do it the little endian (x86)
way, making this operation more expensive on sparc
-mac (beating a dead horse, and enjoying it enormously... You gave me
a brief bit of joy, allowing me to program in C again... *sigh* )
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