Date:   Wed, 15 Jun 2022 11:43:29 +0000
From:   Paul Heidekrüger <paul.heidekrueger@...tum.de>
To:     llvm@...ts.linux.dev, linux-toolchains@...r.kernel.org,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>,
        Paul Heidekrüger <paul.heidekrueger@...tum.de>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Cc:     Marco Elver <elver@...gle.com>,
        Charalampos Mainas <charalampos.mainas@...il.com>,
        Pramod Bhatotia <pramod.bhatotia@...tum.de>,
        Soham Chakraborty <s.s.chakraborty@...elft.nl>,
        Martin Fink <martin.fink@...tum.de>
Subject: [PATCH RFC] tools/memory-model: Adjust ctrl dependency definition

Hi all,

I have been confused by explanation.txt's definition of control
dependencies:

> Finally, a read event and another memory access event are linked by a
> control dependency if the value obtained by the read affects whether
> the second event is executed at all.

I'll go into the following:

====
1. "At all", to me, is misleading
  1.1 The code which confused me
  1.2 The traditional definition via post-dominance doesn't work either
2. Solution
====

1. "At all", to me, is misleading:

"At all" to me suggests a question for which we require a definitive
"yes" or "no" answer: given a programme and an input, can a certain
piece of code be executed? Can we always answer this this question?
Doesn't this sound similar to the halting problem?

1.1 The Example which confused me:

For the dependency checker project [1], I've been thinking about
tracking dependency chains in code, and I stumbled upon the following
edge case, which made me question the "at all" part of the current
definition. The below C-code is derived from some optimised kernel code
in LLVM intermediate representation (IR) I encountered:

> int *x, *y;
>
> int foo()
> {
> /* More code */
>
> 	 loop:
> 		/* More code */
>
> 	 	if(READ_ONCE(x)) {
> 	 		WRITE_ONCE(y, 42);
> 	 		return 0;
> 	 	}
>
> 		/* More code */
>
> 	 	goto loop;
>
>       /* More code */
> }

Assuming that foo() will return, the READ_ONCE() does not determine
whether the WRITE_ONCE() will be executed __at all__, as it will be
executed exactly when the function returns, instead, it determines
__when__ the WRITE_ONCE() will be executed.

1.2. The definition via post-dominance doesn't work either:

I have seen control dependencies being defined in terms of the first
basic block that post-dominates the basic block of the if-condition,
that is the first basic block control flow must take to reach the
function return regardless of what the if condition returned.

E.g. [2] defines control dependencies as follows:

> A statement y is said to be control dependent on another statement x
> if (1) there exists a nontrivial path from x to y such that every
> statement z != x in the path is post-dominated by y, and (2) x is not
> post-dominated by y.

Again, this definition doesn't work for the example above. As the basic
block of the if branch trivially post-dominates any other basic block,
because it contains the function return.

2. Solution:

The definition I came up with instead is the following:

> A basic block B is control-dependent on a basic block A if
> B is reachable from A, but control flow can take a path through A
> which avoids B. The scope of a control dependency ends at the first
> basic block where all control flow paths running through A meet.

Note that this allows control dependencies to remain "unresolved".

I'm happy to submit a patch which covers more of what I mentioned above
as part of explanation.txt, but figured that in the spirit of keeping
things simple, leaving out "at all" might be enough?

What do you think?

Many thanks,
Paul

[1]: https://lore.kernel.org/all/Yk7%2FT8BJITwz+Og1@Pauls-MacBook-Pro.local/T/#u
[2]: Optimizing Compilers for Modern Architectures: A Dependence-Based
Approach, Randy Allen, Ken Kennedy, 2002, p. 350

Signed-off-by: Paul Heidekrüger <paul.heidekrueger@...tum.de>
Cc: Marco Elver <elver@...gle.com>
Cc: Charalampos Mainas <charalampos.mainas@...il.com>
Cc: Pramod Bhatotia <pramod.bhatotia@...tum.de>
Cc: Soham Chakraborty <s.s.chakraborty@...elft.nl>
Cc: Martin Fink <martin.fink@...tum.de>
---
 tools/memory-model/Documentation/explanation.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/memory-model/Documentation/explanation.txt b/tools/memory-model/Documentation/explanation.txt
index ee819a402b69..42af7ed91313 100644
--- a/tools/memory-model/Documentation/explanation.txt
+++ b/tools/memory-model/Documentation/explanation.txt
@@ -466,7 +466,7 @@ pointer.
 
 Finally, a read event and another memory access event are linked by a
 control dependency if the value obtained by the read affects whether
-the second event is executed at all.  Simple example:
+the second event is executed.  Simple example:
 
 	int x, y;
 
-- 
2.35.1

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