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Date: Thu, 22 Apr 2021 18:26:16 -0500
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Gabriel Paubert <paubert@iram.es>
Subject: Re: [PATCH 1/2] powerpc/sstep: =?utf-8?Q?A?=
 =?utf-8?Q?dd_emulation_support_for_=E2=80=98setb=E2=80=99?= instruction
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Cc: Sathvika Vasireddy <sathvika@linux.vnet.ibm.com>,
 naveen.n.rao@linux.ibm.com, linuxppc-dev@lists.ozlabs.org,
 Daniel Axtens <dja@axtens.net>
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Hi!

On Fri, Apr 23, 2021 at 12:16:18AM +0200, Gabriel Paubert wrote:
> On Thu, Apr 22, 2021 at 02:13:34PM -0500, Segher Boessenkool wrote:
> > On Fri, Apr 16, 2021 at 05:44:52PM +1000, Daniel Axtens wrote:
> > > Sathvika Vasireddy <sathvika@linux.vnet.ibm.com> writes:
> > > > +			if ((regs->ccr) & (1 << (31 - ra)))
> > > > +				op->val = -1;
> > > > +			else if ((regs->ccr) & (1 << (30 - ra)))
> > > > +				op->val = 1;
> > > > +			else
> > > > +				op->val = 0;
> > 
> > It imo is clearer if written
> > 
> > 			if ((regs->ccr << ra) & 0x80000000)
> > 				op->val = -1;
> > 			else if ((regs->ccr << ra) & 0x40000000)
> > 				op->val = 1;
> > 			else
> > 				op->val = 0;
> > 
> > but I guess not everyone agrees :-)
> 
> But this can be made jump free :-):
> 
> 	int tmp = regs->ccr << ra;
> 	op->val = (tmp >> 31) | ((tmp >> 30) & 1);

The compiler will do so automatically (or think of some better way to
get the same result); in source code, what matters most is readability,
or clarity in general (also clarity to the compiler).

(Right shifts of negative numbers are implementation-defined in C,
fwiw -- but work like you expect in GCC).

> (IIRC the srawi instruction sign-extends its result to 64 bits).

If you consider it to work on 32-bit inputs, yeah, that is a simple way
to express it.

> > > CR field:      7    6    5    4    3    2    1    0
> > > bit:          0123 0123 0123 0123 0123 0123 0123 0123
> > > normal bit #: 0.....................................31
> > > ibm bit #:   31.....................................0
> > 
> > The bit numbers in CR fields are *always* numbered left-to-right.  I
> > have never seen anyone use LE for it, anyway.
> > 
> > Also, even people who write LE have the bigger end on the left normally
> > (they just write some things right-to-left, and other things
> > left-to-right).
> 
> Around 1985, I had a documentation for the the National's 32032
> (little-endian) processor family, and all the instruction encodings were
> presented with the LSB on the left and MSB on the right.

Ouch!  Did they write "regular" numbers with the least significant digit
on the left as well?

> BTW on these processors, the immediate operands and the offsets (1, 2 or
> 4 bytes) for the addressing modes were encoded in big-endian byte order,
> but I digress. Consistency is overrated ;-)

Inconsistency is the spice of life, yeah :-)


Segher