Re: [Emacs-diffs] trunk r114593: * lisp.h (eassert): Don't use 'assume'.

[Daniel Colascione <dancol@dancol.org>]

On 10/11/13 2:06 AM, Eli Zaretskii wrote:
>> Date: Fri, 11 Oct 2013 01:19:21 -0700
>> From: Daniel Colascione <dancol@dancol.org>
>> CC: eggert@cs.ucla.edu, emacs-devel@gnu.org
>>
>> On 10/11/13 1:08 AM, Eli Zaretskii wrote:
>>> Let me rephrase: assertions are used in unoptimized code, and compile
>>> to nothing in optimized code.  'assume' is not needed in unoptimized
>>> code, since that code is grossly inefficient anyway.  Thus, it sounds
>>> like the two are almost perfectly orthogonal
>>
>> Say we have this function:
>>
>> void foo (int *a, int *b) { *a = *a / *b; }
>>
>> Suppose it's part of foo's contract that it never be called with *b ==
>> 0.  In checked builds, we add an assertion to catch callers that violate
>> the contract:
>>
>> void foo (int *a, int *b) { eassert (*b != 0); *a = *a / *b; }
>>
>> Now suppose we also want the optimizer to take advantage of the fact
>> that *b != 0.
>
> That is a hypothetical situation.  In Emacs, the code is already
> written on the assumption that *b != 0, so it is already "optimized"
> for that.

While the programmer may have written her C code under the assumption 
that an asserted condition holds, the compiler can't know that the 
asserted condition holds when generating its machine code. The point of 
the assume mechanism is to provide this information to the compiler.

> In most cases, you won't see any code that can be optimized
> out using this assumption, as the programmer already did that --
> that's why she added the assertion in the first place.

At the C level, not the code generation level.

> IOW, assertions in Emacs code are in places where the programmer
> _thinks_ something must be true, and writes code based on that
> assumption, but she isn't really 100% sure that something to be true.
> So the assertion is left behind to catch the cases which the
> programmer missed because of her misconceptions.
>
> You are obviously thinking about a different reason of using
> assertions: to assert certain invariants in the code and enforce
> contracts of certain APIs.  But that is almost never the case in
> Emacs, because (gasp!) almost every Emacs API has no well-defined
> contract at all!

Aren't these two cases actually the same? An API contract violation is 
just a special case of "not 100% sure that something [is] true", where 
"that something" is a function being called correctly. We add assertions 
to check that a program maintains certain invariants. We should take 
maximum advantage of the reasoning we've already done.

>>> , and lumping them
>>> together into a single construct is likely to continue bumping upon
>>> problems that stem from basic incompatibility between the use cases,
>>> which target two different non-overlapping build types.
>>
>> Only when we have side effects.
>
> For now, yes.  I'm afraid that's just the tip of the iceberg, though.

What other problems can you imagine?

>> Looking through the code just now, I only saw a few assertions that
>> weren't obviously free of side effects.
>
> The problem is to make sure an assertion obviously _is_ free of side
> effects.  With Emacs's massive use of macros, which call other macros,
> which call other macros, which... -- that is extremely hard.  And why
> should a programmer who just wants to assert something go to such
> great lengths?  That is just a maintenance burden for which I find no
> good justification.

What great lengths? Most common macros --- things like EQ --- are 
clearly free of side effects. The more exotic assertions probably aren't 
worth assuming anyway.

If GCC had some builtin that allowed us to determine whether an 
expression was free of side effects, we could use that to make the 
decision automatically at compile time. Until we get such a facility, 
though, providing some kind of eassert_and_assume macro helps people 
make the best of simple assertions while avoiding the side effect 
problem for more complicated ones.