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

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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
This commit is contained in:
Benjamin Kramer 2012-02-09 14:26:06 +00:00
parent 2c6ae095b8
commit d469274743
2 changed files with 9 additions and 6 deletions
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9
lib/Transforms/IPO/GlobalOpt.cpp
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@ -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;

6
test/Transforms/GlobalOpt/cxx-dtor.ll
View File

@ -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
}