[PATCH] Remove dangling BlockAddresses in GlobalDCE

[Bruno Cardoso Lopes]

Whenever globaldce delete globals vars, it updates their initializers to nullptr and leave underlying constants to be cleaned up later by its uses. However, initializers containing dangling BlockAddresses are never removed because its uses (Functions and Basic Blocks) may still be alive. The presence of dead BlockAddress after globaldce is a bug - the target basic block reference count is never updated - and also prevents other optimizations; e.g. simplifycfg to merge blocks.

This patch fix this issue by removing dead BlockAddresses.

http://reviews.llvm.org/D4932

Files:
  lib/Transforms/IPO/GlobalDCE.cpp
  test/Transforms/GlobalDCE/deadblockaddr.ll

Index: lib/Transforms/IPO/GlobalDCE.cpp
===================================================================
--- lib/Transforms/IPO/GlobalDCE.cpp
+++ lib/Transforms/IPO/GlobalDCE.cpp
@@ -46,6 +46,7 @@
   private:
     SmallPtrSet<GlobalValue*, 32> AliveGlobals;
     SmallPtrSet<Constant *, 8> SeenConstants;
+    SmallPtrSet<Constant *, 8> SeenBlockAddressFnUsers;
 
     /// GlobalIsNeeded - mark the specific global value as needed, and
     /// recursively mark anything that it uses as also needed.
@@ -192,6 +193,11 @@
     Changed = true;
   }
 
+  // Some global initializers elements may leave dead references to functions,
+  // (e.g. BlockAddresses), clean them up.
+  for (auto F : SeenBlockAddressFnUsers)
+    F->removeDeadConstantUsers();
+
   // Make sure that all memory is released
   AliveGlobals.clear();
   SeenConstants.clear();
@@ -231,6 +237,12 @@
             GlobalIsNeeded(GV);
           else if (Constant *C = dyn_cast<Constant>(*U))
             MarkUsedGlobalsAsNeeded(C);
+
+    // Keep track of functions used by BlockAddresses; global initializers
+    // may leave dangling BlockAddresses referring to those.
+    for (auto UB = G->user_begin(), UE = G->user_end(); UB != UE; ++UB)
+      if (isa<BlockAddress>(*UB))
+        SeenBlockAddressFnUsers.insert(G);
   }
 }
 
Index: test/Transforms/GlobalDCE/deadblockaddr.ll
===================================================================
--- /dev/null
+++ test/Transforms/GlobalDCE/deadblockaddr.ll
@@ -0,0 +1,49 @@
+; RUN: opt -O2 -simplifycfg -inline -early-cse -S < %s | FileCheck %s
+
+; Tests whether globaldce does the right cleanup while removing @foo.addrs
+; so that a dead BlockAddress reference to foo won't prevent other passes
+; to work properly, e.g. simplifycfg and inline in this test.
+
+ at foo.addrs = private unnamed_addr constant [5 x i8*] [i8* blockaddress(@foo, %L1), i8* blockaddress(@foo, %L2), i8* blockaddress(@foo, %L3), i8* blockaddress(@foo, %L4), i8* blockaddress(@foo, %L5)], align 4
+
+; CHECK-LABEL: main
+; CHECK-NOT: call
+; CHECK: ret i32 210
+define i32 @main() #0 {
+entry:
+  %call = tail call i32 @foo(i32 3)
+  ret i32 %call
+}
+
+define internal i32 @foo(i32 %i) #0 {
+entry:
+  %addrs = alloca [5 x i8*], align 4
+  %tmp = bitcast [5 x i8*]* %addrs to i8*
+  call void @llvm.memcpy.p0i8.p0i8.i32(i8* %tmp, i8* bitcast ([5 x i8*]* @foo.addrs to i8*), i32 20, i32 4, i1 false)
+  %arrayidx = getelementptr inbounds [5 x i8*]* %addrs, i32 0, i32 %i
+  %0 = load i8** %arrayidx, align 4
+  indirectbr i8* %0, [label %L1, label %L2, label %L3, label %L4, label %L5]
+
+L5:                                               ; preds = %entry
+  br label %L4
+
+L4:                                               ; preds = %entry, %L5
+  %1 = phi i32 [ 7, %entry ], [ 77, %L5 ]
+  br label %L3
+
+L3:                                               ; preds = %entry, %L4
+  %2 = phi i32 [ 1, %entry ], [ %1, %L4 ]
+  %phitmp = mul i32 %2, 15
+  br label %L2
+
+L2:                                               ; preds = %entry, %L3
+  %3 = phi i32 [ 3, %entry ], [ %phitmp, %L3 ]
+  br label %L1
+
+L1:                                               ; preds = %entry, %L2
+  %4 = phi i32 [ 1, %entry ], [ %3, %L2 ]
+  %mul4 = shl nsw i32 %4, 1
+  ret i32 %mul4
+}
+
+declare void @llvm.memcpy.p0i8.p0i8.i32(i8* nocapture, i8* nocapture readonly, i32, i32, i1) #1
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