[llvm] [Coroutines][LazyCallGraph] resumes are not really SCC (PR #116285)

[Tyler Nowicki via llvm-commits]

https://github.com/TylerNowicki created https://github.com/llvm/llvm-project/pull/116285

In this debug code the check assumes the lookup find an object. If it does not it tries to dereference the nullptr. This patch adds a check, however, notice it does not require the object to exist.

This method (addSplitRefRecursiveFunctions) was added for coroutines with continuations. The split coroutine transform generates the continuations by cloning the entire function then changing the entry block and a few other things. This results in a lot of dead code that is kept around only to satisfy a few assumptions then is removed.

With the dead code the original function and its continuations appear to be SCC (all functions reference all other functions). However, when the dead code is removed they are not SCC. Consider a simple coroutine { begin ... suspend ... suspend ... end }. After dead code is eliminated the ramp only references resume0, resume0 only references resume1, etc... I found the original PR [1] for addSplitRefRecursiveFunctions, but it seems to assume that the new resume functions will be a SCC. 

In our use case the dead code can be a particular problem due to its size. For compile-time/size it is necessary to split the coroutine without generating dead code (e.g. CloneAndPrune). However, this also removes the dead references (on a phi's edges) to the other resume functions. The resulting resume functions are not an SCC, but this contradicts what has been written about addSplitRefRecursiveFunctions "To keep things simple, this only supports the case where all new edges are ref edges, and **every new function references every other new function**." [1]. The comments on are a little more relaxed saying "All new functions must reference (not call) each other." Does this mean that each resume function doesn't need to reference every other resume function? 

>From my testing it seems that this condition can be relaxed, and it is only this debug code that checks that "every other new function" is referenced!

Can we safely relax this such that the set of new (resume) functions is not necessarily an SCC?

[1] https://reviews.llvm.org/D93828#change-Xmlqywc9evLH

>From 3075d2d7f8ff511d1ea16fe60ea06f151c16d39e Mon Sep 17 00:00:00 2001
From: tnowicki <tnowicki.nowicki at amd.com>
Date: Thu, 14 Nov 2024 16:02:39 -0500
Subject: [PATCH] [Coroutines][LazyCallGraph] resumes are not really SCC

In this debug code the check assumes the lookup find an object. If it
does not it tries to dereference the nullptr. This patch adds a check,
however, notice it does not require the object to exist.

This method (addSplitRefRecursiveFunctions) was added for coroutines
with continuations. The split coroutine transform generates the
continuations by cloning the entire function then changing the entry
block and a few other things. This results in a lot of dead code that is
kept around only to satisfy a few assumptions then is removed.

With the deadcode the original function and its continuations appear to
be SCC. However, when the deadcode is removed they are not SCC. Consider
a simple coroutine { begin ... suspend ... suspend ... end }. After
deadcode is eliminated the ramp references resume0, resume0 references
resume1, etc...

To be efficient it is necessary to split coroutines without generating
deadcode and this removese the unnecessary references (on a phi's edges).
This does not satisfy one of the conditions of addSplitRefRecursiveFunctions:
"All new functions must reference (not call) each other.".

>From my testing so far it seems that this condition is not necessary and
it is only this debug code that checks for it.

Can we safely remove the "All new functions must reference (not call) each
other." requirement of addSplitRefRecursiveFunctions?

If not, what alternative do we have so we avoid cloning deadcode? (In
our use case the deadcode can be a particular problem due to its size).
---
 llvm/lib/Analysis/LazyCallGraph.cpp | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Analysis/LazyCallGraph.cpp b/llvm/lib/Analysis/LazyCallGraph.cpp
index 5aa36bfc36d468..724bd0ff416c8a 100644
--- a/llvm/lib/Analysis/LazyCallGraph.cpp
+++ b/llvm/lib/Analysis/LazyCallGraph.cpp
@@ -1775,8 +1775,9 @@ void LazyCallGraph::addSplitRefRecursiveFunctions(
       if (F1 == F2)
         continue;
       Node &N2 = get(*F2);
-      assert(!N1->lookup(N2)->isCall() &&
-             "Edges between new functions must be ref edges");
+      assert(!N1->lookup(N2) ||
+             (!N1->lookup(N2)->isCall() &&
+              "Edges between new functions must be ref edges"));
     }
   }
 #endif