GlobalOpt: Be more aggressive about elminating side-effect free static dtors.

GlobalOpt runs early in the pipeline (before inlining) and complex class
hierarchies often introduce bitcasts or GEPs which weren't optimized away.
Teach it to ignore side-effect free instructions instead of depending on
other passes to remove them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@150174 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/IPO/GlobalOpt.cpp b/lib/Transforms/IPO/GlobalOpt.cpp
index 6fa57b4c9b6862cc9d746592fd394fa17e13e6d3..788fee0624fd463302456f48cf63f3cbd6abb387 100644 (file)
--- a/lib/Transforms/IPO/GlobalOpt.cpp
+++ b/lib/Transforms/IPO/GlobalOpt.cpp
@@ -2753,7 +2753,8 @@ static Function *FindCXAAtExit(Module &M) {
 /// destructor and can therefore be eliminated.
 /// Note that we assume that other optimization passes have already simplified
 /// the code so we only look for a function with a single basic block, where
-/// the only allowed instructions are 'ret' or 'call' to empty C++ dtor.
+/// the only allowed instructions side-effect free, 'ret' or 'call' to empty
+/// C++ dtor.
 static bool cxxDtorIsEmpty(const Function &Fn,
                            SmallPtrSet<const Function *, 8> &CalledFunctions) {
   // FIXME: We could eliminate C++ destructors if they're readonly/readnone and
@@ -2786,9 +2787,9 @@ static bool cxxDtorIsEmpty(const Function &Fn,
       if (!cxxDtorIsEmpty(*CalledFn, NewCalledFunctions))
         return false;
     } else if (isa<ReturnInst>(*I))
-      return true;
-    else
-      return false;
+      return true; // We're done.
+    else if (I->mayHaveSideEffects())
+      return false; // Destructor with side effects, bail.
   }
 
   return false;

diff --git a/test/Transforms/GlobalOpt/cxx-dtor.ll b/test/Transforms/GlobalOpt/cxx-dtor.ll
index 22635620baad6036e78c88de52786dc0f31f6bd4..7c6ae78d1bc038e828ae5c92d212766cc2dfaddf 100644 (file)
--- a/test/Transforms/GlobalOpt/cxx-dtor.ll
+++ b/test/Transforms/GlobalOpt/cxx-dtor.ll
@@ -2,6 +2,7 @@
 
 %0 = type { i32, void ()* }
 %struct.A = type { i8 }
+%struct.B = type { }
 
 @a = global %struct.A zeroinitializer, align 1
 @__dso_handle = external global i8*
@@ -15,13 +16,14 @@ define internal void @__cxx_global_var_init() nounwind section "__TEXT,__StaticI
 }
 
 define linkonce_odr void @_ZN1AD1Ev(%struct.A* %this) nounwind align 2 {
-  call void @_ZN1AD2Ev(%struct.A* %this)
+  %t = bitcast %struct.A* %this to %struct.B*
+  call void @_ZN1BD1Ev(%struct.B* %t)
   ret void
 }
 
 declare i32 @__cxa_atexit(void (i8*)*, i8*, i8*)
 
-define linkonce_odr void @_ZN1AD2Ev(%struct.A* %this) nounwind align 2 {
+define linkonce_odr void @_ZN1BD1Ev(%struct.B* %this) nounwind align 2 {
   ret void
 }