From 140f4a315b4b1bb5402a78b37756aa6c212ed390 Mon Sep 17 00:00:00 2001
From: Chris Lattner <sabre@nondot.org>
Date: Sun, 19 Dec 2010 03:02:34 +0000
Subject: [PATCH] first step to fixing PR8642: don't fold away empty basic
 blocks which have trapping constant exprs in them due to PHI nodes.
 Eliminating them can cause the constant expr to be evalutated on new paths if
 the input edges are critical.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@122164 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Transforms/Scalar/CodeGenPrepare.cpp | 11 +++++++++
 test/Transforms/CodeGenPrepare/basic.ll  | 31 ++++++++++++++++++++++++
 2 files changed, 42 insertions(+)
 create mode 100644 test/Transforms/CodeGenPrepare/basic.ll

diff --git a/lib/Transforms/Scalar/CodeGenPrepare.cpp b/lib/Transforms/Scalar/CodeGenPrepare.cpp
index 60c7f7565eb..b4a2a12de03 100644
--- a/lib/Transforms/Scalar/CodeGenPrepare.cpp
+++ b/lib/Transforms/Scalar/CodeGenPrepare.cpp
@@ -211,6 +211,17 @@ bool CodeGenPrepare::CanMergeBlocks(const BasicBlock *BB,
   const PHINode *DestBBPN = dyn_cast<PHINode>(DestBB->begin());
   if (!DestBBPN) return true;  // no conflict.
 
+  // Walk all the PHI nodes in DestBB.  If any of the input values to the PHI
+  // are trapping constant exprs, then merging this block would introduce the
+  // possible trap into new control flow if we have any critical predecessor
+  // edges.
+  for (BasicBlock::const_iterator I = DestBB->begin(); isa<PHINode>(I); ++I) {
+    const PHINode *PN = cast<PHINode>(I);
+    if (const Constant *C =dyn_cast<Constant>(PN->getIncomingValueForBlock(BB)))
+      if (C->canTrap())
+        return false;
+  }
+  
   // Collect the preds of BB.
   SmallPtrSet<const BasicBlock*, 16> BBPreds;
   if (const PHINode *BBPN = dyn_cast<PHINode>(BB->begin())) {
diff --git a/test/Transforms/CodeGenPrepare/basic.ll b/test/Transforms/CodeGenPrepare/basic.ll
new file mode 100644
index 00000000000..04b4f34bc33
--- /dev/null
+++ b/test/Transforms/CodeGenPrepare/basic.ll
@@ -0,0 +1,31 @@
+; RUN: opt -codegenprepare %s -S | FileCheck %s
+; PR8642
+
+%0 = type <{ %1, %1 }>
+%1 = type { i8, i8, i8, i8 }
+
+@g_2 = global %0 <{ %1 { i8 1, i8 0, i8 0, i8 undef }, %1 { i8 2, i8 0, i8 0, i8 undef } }>, align 4
+@g_4 = global %1 { i8 3, i8 0, i8 0, i8 undef }, align 4
+
+; CGP shouldn't fold away the empty cond.false.i block, because the constant
+; expr that will get dropped into it could trap.
+define i16 @test1(i8** %argv, i1 %c) nounwind ssp {
+entry:
+  br i1 %c, label %cond.end.i, label %cond.false.i
+
+cond.false.i:                                     ; preds = %entry
+  br label %foo.exit
+
+cond.end.i:                                       ; preds = %entry
+  store i8* null, i8** %argv
+  br label %foo.exit
+
+foo.exit:                                         ; preds = %cond.end.i, %cond.false.i
+  %call1 = phi i16 [ trunc (i32 srem (i32 1, i32 zext (i1 icmp eq (%1* bitcast (i8* getelementptr inbounds (%0* @g_2, i64 0, i32 1, i32 0) to %1*), %1* @g_4) to i32)) to i16), %cond.false.i ], [ 1, %cond.end.i ]
+  ret i16 %call1
+  
+; CHECK: @test1
+; CHECK: cond.false.i:
+; CHECK-NEXT:  br label %foo.exit
+}
+
-- 
2.34.1