GNU bug report logs - #46670
28.0.50; [feature/native-comp] possible miscompilation affecting lsp-mode

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Message #41 received at 46670 <at> debbugs.gnu.org (full text, mbox):

From: Pip Cet <pipcet <at> gmail.com>
To: Andrea Corallo <akrl <at> sdf.org>
Cc: 46670 <at> debbugs.gnu.org, Mauricio Collares <mauricio <at> collares.org>
Subject: Re: bug#46670: 28.0.50; [feature/native-comp] possible miscompilation
 affecting lsp-mode
Date: Tue, 23 Feb 2021 07:59:32 +0000

[Message part 1 (text/plain, inline)]

On Mon, Feb 22, 2021 at 1:12 PM Andrea Corallo <akrl <at> sdf.org> wrote:
> Good catch thanks! :) Should be fixed by d6227f6edc.

I'm also confused by the use of NEGATED in comp-emit-assume: if RHS is
a constraint, it emits the complementary constraint.

However, the code in comp-add-cond-cstrs uses NEGATED to express a
much weaker constraint: that two mvars aren't strictly equal.

If x /= y and y in SET, we can't conclude that x not in SET (unless
SET is a singleton, an important special case).

So it all works right now because emit-assume NEGATED=t RHS=mvar means
"LHS isn't equal to RHS" but NEGATED=t RHS=cstr means "LHS can't
satisfy RHS".

My code changed the call to pass a constraint instead of the mvar, and
then things broke :-)

We should be consistent about what NEGATED means, I think.

But apart from such problems, my code appears to be working. I'm
attaching it for the sake of completeness, not because I expect you to
read it all before it's cleaned up.

[emacs-bug46670-001.diff (text/x-patch, attachment)]

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