ARM as ... possible bug

[Dave Pawson]

Thanks for the explanation Matt.

On 12 December 2011 09:27, Matthew Gretton-Dann
<matthew.gretton-dann@arm.com> wrote:
> On 11/12/11 10:19, Dave Pawson wrote:
>>
>> strexd r0, r1,r2,[r4]
>>
>> produces
>> arm.asm:275: Error: even register required -- `strexd r0,r1,r2,[r4]'
>> failed, quitting

> r1 in your example is the register producing the error.  In ARM state[1] the
> first register transferred must be an even numbered register, and the second
> register transferred must be the register numbered one higher.
>
> So   strexd r0, r2, r3, [r4] is valid
> But  strexd r0, r2, r6, [r4] is not.
>
> The ARMARM describes this in the ARM state encoding specific operations (the
> pseudo-code that goes with the ARM state encoding).

<admission>I'm not very good with the p-code</admission>

if ConditionPassed() then
EncodingSpecificOperations(); NullCheckIfThumbEE(n);
address = R[n];
// Create doubleword to store such that R[t] will be stored at address
and R[t2] at address+4.
value = if BigEndian() then R[t]:R[t2] else R[t2]:R[t];
if ExclusiveMonitorsPass(address,8) then
MemA[address,8] = value; R[d] = 0;
else
R[d] = 1;

Sorry Matt, I see nothing there about Rt having to be even numbered?
It's the sort of error that leaves me scratching my head and moaning
about the documentation?
It's quite confusing, appears to me unreasonable.
I can deduce that the two registers must be consecutive (Rt2 not stored in
the instruction) but I find the documentation sadly lacking on this
(distinctly odd) restriction of use?




>
> Gas is correct - although admittedly unclear.

I'd appreciate this error message being made clear for gas.



regards


-- 
Dave Pawson
XSLT XSL-FO FAQ.
Docbook FAQ.
http://www.dpawson.co.uk