[Simh] O2 optimization on Linux (was: VMS 4.6 crashes on Linux)

[Timothe Litt]

This test appears to be implementing the constraint that p0/1 base register
values must be in in system space (bit 31 set) & longword aligned (bits 0,1
clear).
 
I don't think it's parens; there seem to be enough.  
 
If this is a 64-bit environment, you might be seeing a sign extension /
unsigned promotion issue in the complier.
 
You might try this to make what's desired explicit:
 
#define ML_PXBR_TEST(r)  if (((((uint32)(r))&  0x80000000u) == 0) || \
                           ((r)&  0x00000003)) RSVD_OPND_FAULT

But it would really help to see the assembler generated for the two cases -
especially the one that fails.


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From: simh-bounces at trailing-edge.com [mailto:simh-bounces at trailing-edge.com]
On Behalf Of Peter Allan
Sent: Friday, December 31, 2010 12:32
To: simh at trailing-edge.com
Subject: Re: [Simh] O2 optimization on Linux (was: VMS 4.6 crashes on Linux)




On 28 December 2010 22:00, Bob Supnik <bob at supnik.org> wrote:


Jason Stevens wrote:

You didn't build it with -O2 on linux did you?  There is some weird
things with GCC and SIMH's VAX...  I can only speak to 4.X BSD, but I
was able to identify two procedures that when optimized with -O2 break
4BSD on SIMH....

I kind of detailed it here:

http://www.mail-archive.com/simh@trailing-edge.com/msg00463.html

And the procedures in question were:

op_ldpctx
op_mtpr

---

Interestingly, those routines are the ONLY places in the VAX simulator where
these macros are used:

/* Machine specific reserved operand tests (all NOPs) */

#define ML_PA_TEST(r)
#define ML_LR_TEST(r)
#define ML_SBR_TEST(r)
#define ML_PXBR_TEST(r)
#define LP_AST_TEST(r)
#define LP_MBZ84_TEST(r)
#define LP_MBZ92_TEST(r)

On the 780, they are real tests:

/* Machine specific reserved operand tests */

#define ML_LR_TEST(r)    if ((uint32)((r)&  0xFFFFFF)>  0x200000)
RSVD_OPND_FAULT
#define ML_PXBR_TEST(r)  if ((((r)&  0x80000000) == 0) || \
                           ((r)&  0x00000003)) RSVD_OPND_FAULT
#define ML_SBR_TEST(r)   if ((r)&  0x00000003) RSVD_OPND_FAULT
#define ML_PA_TEST(r)    if ((r)&  0x00000003) RSVD_OPND_FAULT
#define LP_AST_TEST(r)   if ((r)>  AST_MAX) RSVD_OPND_FAULT
#define LP_MBZ84_TEST(r) if ((r)&  0xF8C00000) RSVD_OPND_FAULT
#define LP_MBZ92_TEST(r) if ((r)&  0x7FC00000) RSVD_OPND_FAULT

---

One (or more) of those six tests is the smoking gun, and if I had to put my
money on it, I think it's this one:

#define ML_LR_TEST(r)    if ((uint32)((r)&  0xFFFFFF)>  0x200000)
RSVD_OPND_FAULT

Try adding an extra level of parentheses:

if (((uint32)((r)&  0xFFFFFF))>  0x200000) RSVD_OPND_FAULT


just to be sure that the uint32 cast isn't be applied to the whole
comparison, instead of just the first term.

If that fails to get it running, then try "no-oping" all of the macros to
see if VMS 4.6 will boot.  If it does, then enable each of the macros in
turn, to see where the simulator fails.

/Bob


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Bob,
 
I tried removing the macro definitions one by one. The one that causes the
problem is ML_PXBR_TEST, i.e. if this is defined as nothing and the rest are
left as per vax780_defs.h, then the 780 emulator works on CentOS 5.5. This
is the macro with two comparisons in it. I wonder if that is significant?
 
I tried putting in some extra parentheses, but this did not help. 
 
Of course, it all still works on Fedora Core 14. I will see if I can compare
the assembler that is generated for vax_cpu1.c on the two systems.
 
Peter Allan
 
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