Re: FPU emulation incorrect for 68LC040?

[Finn Thain <fthain@telegraphics.com.au>]

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On Thu, 22 Jul 2010, Mattias Engdegård wrote:

> 22 jul 2010 kl. 03.05 skrev Finn Thain:
> 
> > But if you send me some inline asm, I'll patch it into busybox and test a
> > 2.6.34 initramfs for you.
> 
> Thanks! Try this:
> 
> void test_postinc(void)
> {
>        double x = 0.0;
>        double a[2] = {3.5, 0.0};
>        double *p = a;
>        __asm__ volatile  ("fneg.d (%0)+,%1" : "+a"(p), "+f"(x));
>        printf("postinc: result %f (expected %f), address %p (expected %p)\n",
>               x, -3.5, p, a + 1);
> }
> 
> void test_predec(void)
> {
>        double x = 0.0;
>        double a[2] = {-1.75, 0.0};
>        double *p = a + 1;
>        __asm__ volatile  ("fneg.d -(%0),%1" : "+a"(p), "+f"(x));
>        printf("predec: result %f (expected %f), address %p (expected %p)\n",
>               x, 1.75, p, a);
> }
> 

I ran this on a PowerBook 190cs:

postinc: result -3.500000 (expected -3.500000), address 0xef8ddcf8 (expected 0xef8ddcf8)
predec: result 1.750000 (expected 1.750000), address 0xef8ddcf0 (expected 0xef8ddcf0)

It is probably not relevant, but I believe that this CPU is free from the 
68LC040 erratum that affects page faults and FPU ops as I've tested for 
that before.

# cat /proc/cpuinfo
CPU:            68040
MMU:            68040
FPU:            none
Clocking:       32.9MHz
BogoMips:       21.96
Calibration:    109824 loops
# cat /proc/version
Linux version 2.6.34-mac (fthain@nippy) (gcc version 4.4.4 (GCC) ) #5 Fri Jul 23 13:32:53 EST 2010

Here's the disassembly:

8007da96 <test_predec>:
8007da96:	4e56 fff0      	linkw %fp,#-16
8007da9a:	2d7c bffc 0000 	movel #-1074003968,%fp@(-16)
8007daa0:	fff0 
8007daa2:	42ae fff4      	clrl %fp@(-12)
8007daa6:	42ae fff8      	clrl %fp@(-8)
8007daaa:	42ae fffc      	clrl %fp@(-4)
8007daae:	41ee fff8      	lea %fp@(-8),%a0
8007dab2:	f200 5c0f      	fmovecrx #15,%fp0
8007dab6:	f220 541a      	fnegd %a0@-,%fp0
8007daba:	486e fff0      	pea %fp@(-16)
8007dabe:	2f08           	movel %a0,%sp@-
8007dac0:	42a7           	clrl %sp@-
8007dac2:	2f3c 3ffc 0000 	movel #1073479680,%sp@-
8007dac8:	f227 7400      	fmoved %fp0,%sp@-
8007dacc:	4879 8009 b25a 	pea 8009b25a <isoformats.7051+0x29>
8007dad2:	2f39 800a 8e04 	movel 800a8e04 <_IO_stderr>,%sp@-
8007dad8:	4eb9 8000 e564 	jsr 8000e564 <__fprintf>
8007dade:	4fef 0020      	lea %sp@(32),%sp
8007dae2:	4e5e           	unlk %fp
8007dae4:	4e75           	rts

8007dae6 <test_postinc>:
8007dae6:	4e56 fff0      	linkw %fp,#-16
8007daea:	2d7c 400c 0000 	movel #1074528256,%fp@(-16)
8007daf0:	fff0 
8007daf2:	42ae fff4      	clrl %fp@(-12)
8007daf6:	42ae fff8      	clrl %fp@(-8)
8007dafa:	42ae fffc      	clrl %fp@(-4)
8007dafe:	41ee fff0      	lea %fp@(-16),%a0
8007db02:	f200 5c0f      	fmovecrx #15,%fp0
8007db06:	f218 541a      	fnegd %a0@+,%fp0
8007db0a:	486e fff8      	pea %fp@(-8)
8007db0e:	2f08           	movel %a0,%sp@-
8007db10:	42a7           	clrl %sp@-
8007db12:	2f3c c00c 0000 	movel #-1072955392,%sp@-
8007db18:	f227 7400      	fmoved %fp0,%sp@-
8007db1c:	4879 8009 b295 	pea 8009b295 <isoformats.7051+0x64>
8007db22:	2f39 800a 8e04 	movel 800a8e04 <_IO_stderr>,%sp@-
8007db28:	4eb9 8000 e564 	jsr 8000e564 <__fprintf>
8007db2e:	4fef 0020      	lea %sp@(32),%sp
8007db32:	4e5e           	unlk %fp
8007db34:	4e75           	rts

HTH

Finn