From fe70b29cf71283eaa9cc104eeddcc011befcec73 Mon Sep 17 00:00:00 2001
From: Chandler Carruth <chandlerc@gmail.com>
Date: Fri, 22 Jun 2018 02:43:41 +0000
Subject: [PATCH] [LegacyPM] Fix PR37888 by teaching the legacy loop pass
 manager how to clear out deleted loops from the current queue beyond just the
 current loop.

This is important because SimpleLoopUnswitch will now enqueue the same
loop to be re-processed. When it does this with the legacy PM, we don't
have a way of canceling the rest of the pipeline and so we can end up
deleting the loop before we reprocess it. =/

This change also makes it easy to support deleting other loops in the
queue to process, although I don't have any use cases for that.

Differential Revision: https://reviews.llvm.org/D48470

llvm-svn: 335317
---
 llvm/lib/Analysis/LoopPass.cpp                | 11 +++++-
 .../Transforms/SimpleLoopUnswitch/pr37888.ll  | 38 +++++++++++++++++++
 2 files changed, 48 insertions(+), 1 deletion(-)
 create mode 100644 llvm/test/Transforms/SimpleLoopUnswitch/pr37888.ll

diff --git a/llvm/lib/Analysis/LoopPass.cpp b/llvm/lib/Analysis/LoopPass.cpp
index 00d88e8189ee..c5ced03cfa8b 100644
--- a/llvm/lib/Analysis/LoopPass.cpp
+++ b/llvm/lib/Analysis/LoopPass.cpp
@@ -142,8 +142,17 @@ void LPPassManager::getAnalysisUsage(AnalysisUsage &Info) const {
 void LPPassManager::markLoopAsDeleted(Loop &L) {
   assert((&L == CurrentLoop || CurrentLoop->contains(&L)) &&
          "Must not delete loop outside the current loop tree!");
-  if (&L == CurrentLoop)
+  // If this loop appears elsewhere within the queue, we also need to remove it
+  // there. However, we have to be careful to not remove the back of the queue
+  // as that is assumed to match the current loop.
+  assert(LQ.back() == CurrentLoop && "Loop queue back isn't the current loop!");
+  LQ.erase(std::remove(LQ.begin(), LQ.end(), &L), LQ.end());
+
+  if (&L == CurrentLoop) {
     CurrentLoopDeleted = true;
+    // Add this loop back onto the back of the queue to preserve our invariants.
+    LQ.push_back(&L);
+  }
 }
 
 /// run - Execute all of the passes scheduled for execution.  Keep track of
diff --git a/llvm/test/Transforms/SimpleLoopUnswitch/pr37888.ll b/llvm/test/Transforms/SimpleLoopUnswitch/pr37888.ll
new file mode 100644
index 000000000000..307706fc34ab
--- /dev/null
+++ b/llvm/test/Transforms/SimpleLoopUnswitch/pr37888.ll
@@ -0,0 +1,38 @@
+; RUN: opt -simple-loop-unswitch -loop-deletion -S < %s | FileCheck %s
+;
+; Check that when we do unswitching where we re-enqueue the loop to be processed
+; again, but manage to delete the loop before ever getting to iterate on it, it
+; doesn't crash the legacy pass manager.
+
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @pr37888() {
+; CHECK-LABEL: define void @pr37888()
+entry:
+  %tobool = icmp ne i16 undef, 0
+  br label %for.body
+; CHECK:         %[[TOBOOL:.*]] = icmp ne
+; CHECK-NEXT:    br i1 %[[TOBOOL]], label %if.then, label %[[ENTRY_SPLIT:.*]]
+;
+; CHECK:       [[ENTRY_SPLIT]]:
+; CHECK-NEXT:    br label %for.end
+
+for.body:
+  br i1 %tobool, label %if.then, label %if.end
+
+if.then:
+  unreachable
+; CHECK:       if.then:
+; CHECK-NEXT:    unreachable
+
+if.end:
+  br label %for.inc
+
+for.inc:
+  br i1 undef, label %for.body, label %for.end
+
+for.end:
+  ret void
+; CHECK:       for.end:
+; CHECK-NEXT:    ret void
+}