[Simh] O2 optimization on Linux

[Jan-Benedict Glaw]

On Thu, 2011-01-06 02:48:30 -0700, Tim Riker <Tim at Rikers.org> wrote:
> On 01/03/2011 12:57 PM, Jason Stevens wrote:
> > wait, it should have been more like this
> > 
> > 
> > > #define ML_SBR_TEST(r) if (((uint32)(r)) & 0xC000003u) != 0) RSVD_OPND_FAULT
> > > #define ML_PXBR_TEST(r) if (((((uint32)(r)) & 0x80000000u) == 0) || \
> > >                            (((uint32)(r)) & 0x40000003u) != 0))
> > > RSVD_OPND_FAULT
> > 
> > into this:
> > 
> > #define ML_SBR_TEST(r)  if (((uint32)(r) & 0xC0000003u)!=0) RSVD_OPND_FAULT
> > 
> > #define ML_PXBR_TEST(r) if (((((uint32)r) & 0x80000000u) == 0) || \
> >                             (((uint32)r) & 0x40000003u)!=0) RSVD_OPND_FAULT
> You want the typecast to apply after any math that might be happening in
> the expression r.
> 
> You should include r in parentheses in the expansion. ie: not (uint32)r
> ever, but instead (uint32)(r) so that r is evaluated first before
> typecast is applied.

In terms of correctness, there are several things that could be made
better:

First of all, `r' is used twice here. The current uses seem sane, but
what, if `r' contained side effects (pre/post inc/dec etc.)? That
could probably be written as:

#define ML_PXBR_TEST(r)	{						\
				uint32 ____t = (r);			\
				if ((____t & 0x80000000u) == 0) ||	\
				    ____t & 0x40000003u)		\
					RSVD_OPND_FAULT;		\
			}

That would ensure `r' being evaluated only once. And it would
introduce a block around the if () ...;  construct. Think of:

	if (foo)
		ML_PXBR_TEST (x);
	else
		do_something ();

Looks sane, but would work totally irrational with the original macro.
To accept a `;' after the closing brace of the block, maybe a do/while
should be added (as eg. the Linux kernel code does it):

#define ML_PXBR_TEST(r)	do {						\
				uint32 ____t = (r);			\
				if ((____t & 0x80000000u) == 0) ||	\
				    ____t & 0x40000003u)		\
					RSVD_OPND_FAULT;		\
			} while (0)

Comments?

MfG, JBG

-- 
      Jan-Benedict Glaw      jbglaw at lug-owl.de              +49-172-7608481
  Signature of:                          Zensur im Internet? Nein danke!
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