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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 02:18:51 -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 2:06 AM, Eli Zaretskii wrote:
Date: Fri, 11 Oct 2013 01:19:21 -0700
From: Daniel Colascione <address@hidden>
CC: address@hidden, address@hidden

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.

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