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

From: Daniel Colascione
Subject: Re: [Emacs-diffs] trunk r114593: * lisp.h (eassert): Don't use 'assume'.
Date: Fri, 11 Oct 2013 02:55:44 -0700
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:24.0) Gecko/20100101 Thunderbird/24.0

On 10/11/13 2:36 AM, Eli Zaretskii wrote:
Date: Fri, 11 Oct 2013 02:18:51 -0700
From: Daniel Colascione <address@hidden>
CC: address@hidden, address@hidden

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.

The compiler will be unable to take advantage of that information,
because there's no source code to apply that information to.

I don't understand this argument. In my example, the assume would inform the compiler that it didn't have to emit code to handle division in the case that *b is zero. Similar principles apply to assertions about numeric ranges, pointer non-NULL-ness, etc. The C99 restrict keyword is similar, in a sense.

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.

Code is generated from the C code, not out of thin air.

And we're talking about giving the compiler more information about the C code.

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?

No, they are not.  E.g., if the programmer's assumption was wrong, and
there are in fact valid use cases where her assertion doesn't hold,
then in production code (where the assertions are defined away) your
'assume' will degrade the code quality for legitimate use cases.

Any assertion that might not actually hold is a bug. That a progammer might, after an assertion, insert code to act sanely in the case that an assertion does not hold is irrelevant: it's reacting to an error, not dealing with a situation that ought to occur in practice. If the programmer really expects an assertion not to hold, he can use a variant that doesn't assume the condition holds. But these cases should be rare.

, 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?

How should I know?  Does anyone know under which conditions, exactly,
a badly engineered bridge will collapse?

So this point boils down to "I have a bad feeling about this"?

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.

There are a lot of macros much more complex than EQ.

So don't use the assume-and-assert macros for questionable cases.

The more exotic assertions probably aren't worth assuming anyway.

Not sure I understand what you are saying here.

I'm speculating that the optimization value to be gained from assuming very complex conditions is smaller than the value gained for assuming relatively simple conditions.

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.

I think you are wrong, but I guess I'm unable to convince you.

AFAICT, your opposition here boils down to the idea that there's some fundamental difference between, on one hand, statements we make about program behavior when we're debugging, and on the other hand, statements we make about program behavior when we're optimizing. Except for some cases involving deficiencies in the language-level mechanisms with which we make these statements, I don't see why we should regard these statements as different at all. To the exent these deficiencies force us to care about the difference --- the side effect issue --- we can get around the problem by using one macro to express "assert and assume" and to express "just assert", since it's always safe to assert a statement we're assuming to be true, but not necessarily the other way around.

You could argue that having two macros instead of one imposes a maintenance burden and that there isn't a payoff sufficient to justify this burden, but I don't think the maintenance cost of having another macro is very large, especially if we leave existing assertions as they are and use the assume-and-assert macro only for cases that are clearly free of side effects.

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