Re: indexing arrays

[Andrew Haley <aph at redhat dot com>]

On 11/18/2013 02:12 PM, Mischa Baars wrote:
> On 11/18/2013 12:35 PM, Andrew Haley wrote:
>> On 11/18/2013 11:07 AM, Mischa Baars wrote:
>>> On 11/18/2013 10:57 AM, Andrew Haley wrote:
>>>> On 11/18/2013 03:47 AM, Mischa Baars wrote:
>>>>> On 11/17/2013 11:05 PM, Florian Weimer wrote:
>>>>>> * Mischa Baars:
>>>>>>
>>>>>>> Please have a look at the following example, and let me know what
>>>>>>> you think.
>>>>>> The code doesn't contain any comments, so we don't know what you're
>>>>>> trying to do.
>>>>> The assembly file does, if you would just look at it.
>>>>>
>>>>> As you can see, the 'ebx' register is unprotected when the function is
>>>>> called from a location other than 'main'. Normally I shouldn't be able
>>>>> to modify the array index from the assembly.
>>>> You're doing it wrong.  It is the job of the called function to save and
>>>> restore %ebx:
>>>>
>>>> %ebx     As described below, this register serves as the global offset
>>>>            table base register for position-independent code. For
>>>>            absolute code, %ebx serves as a local register and has no
>>>>            specified role in the function calling sequence. In either
>>>>            case, a function must preserve the register value for the
>>>>            caller.
>>>>
>>>> http://www.sco.com/developers/devspecs/abi386-4.pdf
>>> I see what you are pointing at (p.37), but it also says "DRAFT COPY" :)
>> That isn't going to change any time soon.
> 
> As I understand, you're not willing to consider any alternative to the 
> ABI. I understand!

It's not really ours to change.

>>> As you see from the program output, the 'ebx' register IS preserved when
>>> the 'npx_on_double()' function is called from 'main()', but the 'ebx'
>>> registers IS NOT preserved when the 'npx_on_double()' function is called
>>> from another function like 'npx_on_complex()'.
>>>
>>> This makes writing low-level functions very sensitive to errors.
>>
>> No-one can write reliable code by trying something and seeing if it
>> works.  You have know understand what you're doing.  When writing
>> assembly code it's your job to know the ABI, and to follow the rules.
>> Do that, and you'll be fine.
> 
> You seem to have forgotten a couple of my words.
> 
> It took me two or three years to realize there was this minor 
> misconception in code that I was already working with, the construction 
> the compiler is using seems a little sensitive to errors.

It's not that I've forgotten, it's that I disagree.  I think that
you're making a fundamental error.  When you're working with low-level
code, everything is very delicate.  It's delicate because it's very
highly-optimized: the ABI is the way it is so that the compiler can
save a few nanoseconds here and there.  Once you say, in effect,
"trust me, I know what I'm doing" you have to be very careful to get
things right.  If you're working in Python or somesuch the compiler
will protect you from such things, but in assembler it's your job to
look after your registers!

In my opinion, the correct response (by which I mean to say, the
response I'd make) in these circumstances would be "Ooops, I messed
up.  Sorry for the noise."

Andrew.