r177211 - Exploit this-return of a callsite in a this-return function.
Richard Smith
richard at metafoo.co.uk
Fri Mar 15 17:28:42 PDT 2013
On Fri, Mar 15, 2013 at 5:11 PM, Manman Ren <mren at apple.com> wrote:
> Author: mren
> Date: Fri Mar 15 19:11:09 2013
> New Revision: 177211
>
> URL: http://llvm.org/viewvc/llvm-project?rev=177211&view=rev
> Log:
> Exploit this-return of a callsite in a this-return function.
>
> For constructors/desctructors that return 'this', if there exists a
> callsite
> that returns 'this' and is immediately before the return instruction, make
> sure we are using the return value from the callsite.
>
> We don't need to keep 'this' alive through the callsite. It also enables
> optimizations in the backend, such as tail call optimization.
>
> rdar://12818789
>
> Modified:
> cfe/trunk/lib/CodeGen/CGCXXABI.h
> cfe/trunk/lib/CodeGen/CGCall.cpp
> cfe/trunk/lib/CodeGen/CGClass.cpp
> cfe/trunk/lib/CodeGen/CodeGenFunction.cpp
> cfe/trunk/lib/CodeGen/CodeGenFunction.h
> cfe/trunk/lib/CodeGen/ItaniumCXXABI.cpp
> cfe/trunk/lib/CodeGen/MicrosoftCXXABI.cpp
> cfe/trunk/test/CodeGenCXX/arm.cpp
>
> Modified: cfe/trunk/lib/CodeGen/CGCXXABI.h
> URL:
> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/CodeGen/CGCXXABI.h?rev=177211&r1=177210&r2=177211&view=diff
>
> ==============================================================================
> --- cfe/trunk/lib/CodeGen/CGCXXABI.h (original)
> +++ cfe/trunk/lib/CodeGen/CGCXXABI.h Fri Mar 15 19:11:09 2013
> @@ -91,6 +91,10 @@ public:
> return *MangleCtx;
> }
>
> + /// Returns true if the given instance method is one of the
> + /// kinds that the ABI says returns 'this'.
> + virtual bool HasThisReturn(GlobalDecl GD) const { return false; }
> +
> /// Find the LLVM type used to represent the given member pointer
> /// type.
> virtual llvm::Type *
> @@ -209,7 +213,8 @@ public:
> /// Emit the ABI-specific prolog for the function.
> virtual void EmitInstanceFunctionProlog(CodeGenFunction &CGF) = 0;
>
> - virtual void EmitConstructorCall(CodeGenFunction &CGF,
> + /// Emit the constructor call. Return the function that is called.
> + virtual llvm::Value *EmitConstructorCall(CodeGenFunction &CGF,
> const CXXConstructorDecl *D,
> CXXCtorType Type, bool ForVirtualBase,
> bool Delegating,
>
> Modified: cfe/trunk/lib/CodeGen/CGCall.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/CodeGen/CGCall.cpp?rev=177211&r1=177210&r2=177211&view=diff
>
> ==============================================================================
> --- cfe/trunk/lib/CodeGen/CGCall.cpp (original)
> +++ cfe/trunk/lib/CodeGen/CGCall.cpp Fri Mar 15 19:11:09 2013
> @@ -1705,6 +1705,18 @@ void CodeGenFunction::EmitFunctionEpilog
> llvm_unreachable("Invalid ABI kind for return argument");
> }
>
> + // If this function returns 'this' and the last instruction is a
> CallInst
> + // that returns 'this', use the return value from the CallInst. We will
> not
> + // need to keep 'this' alive through the callsite. It also enables
> + // optimizations in the backend, such as tail call optimization.
> + if (CalleeWithThisReturn && CGM.getCXXABI().HasThisReturn(CurGD)) {
> + llvm::BasicBlock *IP = Builder.GetInsertBlock();
> + llvm::CallInst *Callsite;
> + if (!IP->empty() && (Callsite =
> dyn_cast<llvm::CallInst>(&IP->back())) &&
> + Callsite->getCalledFunction() == CalleeWithThisReturn)
>
This doesn't look right. Suppose we have:
struct A { A(); /* trivial dtor */ };
struct B : A {
B() : A() { A(); }
};
It looks like we'll return the 'this' from the temporary. Why not compare
the llvm::Value* corresponding to the CallInst itself, rather than
comparing the callee function? (Or maybe check that the 'this' argument to
the callee matches the caller's 'this'? That'd also solve the problems
below.)
> + // Create a bitcast of Callsite.
> + RV = Builder.CreateBitCast(Callsite, RetAI.getCoerceToType());
> + }
> llvm::Instruction *Ret = RV ? Builder.CreateRet(RV) :
> Builder.CreateRetVoid();
> if (!RetDbgLoc.isUnknown())
> Ret->setDebugLoc(RetDbgLoc);
>
> Modified: cfe/trunk/lib/CodeGen/CGClass.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/CodeGen/CGClass.cpp?rev=177211&r1=177210&r2=177211&view=diff
>
> ==============================================================================
> --- cfe/trunk/lib/CodeGen/CGClass.cpp (original)
> +++ cfe/trunk/lib/CodeGen/CGClass.cpp Fri Mar 15 19:11:09 2013
> @@ -1666,8 +1666,11 @@ CodeGenFunction::EmitCXXConstructorCall(
> }
>
> // Non-trivial constructors are handled in an ABI-specific manner.
> - CGM.getCXXABI().EmitConstructorCall(*this, D, Type, ForVirtualBase,
> - Delegating, This, ArgBeg, ArgEnd);
> + llvm::Value *Callee = CGM.getCXXABI().EmitConstructorCall(*this, D,
> Type,
> + ForVirtualBase, Delegating, This, ArgBeg,
> ArgEnd);
> + if (CGM.getCXXABI().HasThisReturn(CurGD) &&
> + CGM.getCXXABI().HasThisReturn(GlobalDecl(D, Type)))
> + CalleeWithThisReturn = Callee;
>
There's no guarantee that the caller and the callee are constructing the
same object here.
> }
>
> void
> @@ -1756,9 +1759,12 @@ CodeGenFunction::EmitDelegateCXXConstruc
> EmitDelegateCallArg(DelegateArgs, param);
> }
>
> + llvm::Value *Callee = CGM.GetAddrOfCXXConstructor(Ctor, CtorType);
> EmitCall(CGM.getTypes().arrangeCXXConstructorDeclaration(Ctor,
> CtorType),
> - CGM.GetAddrOfCXXConstructor(Ctor, CtorType),
> - ReturnValueSlot(), DelegateArgs, Ctor);
> + Callee, ReturnValueSlot(), DelegateArgs, Ctor);
> + if (CGM.getCXXABI().HasThisReturn(CurGD) &&
> + CGM.getCXXABI().HasThisReturn(GlobalDecl(Ctor, CtorType)))
> + CalleeWithThisReturn = Callee;
> }
>
> namespace {
> @@ -1825,6 +1831,9 @@ void CodeGenFunction::EmitCXXDestructorC
> EmitCXXMemberCall(DD, SourceLocation(), Callee, ReturnValueSlot(), This,
> VTT,
> getContext().getPointerType(getContext().VoidPtrTy),
> 0, 0);
> + if (CGM.getCXXABI().HasThisReturn(CurGD) &&
> + CGM.getCXXABI().HasThisReturn(GlobalDecl(DD, Type)))
> + CalleeWithThisReturn = Callee;
>
There's no guarantee that the caller and callee are destroying the same
object here.
> }
>
> namespace {
>
> Modified: cfe/trunk/lib/CodeGen/CodeGenFunction.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/CodeGen/CodeGenFunction.cpp?rev=177211&r1=177210&r2=177211&view=diff
>
> ==============================================================================
> --- cfe/trunk/lib/CodeGen/CodeGenFunction.cpp (original)
> +++ cfe/trunk/lib/CodeGen/CodeGenFunction.cpp Fri Mar 15 19:11:09 2013
> @@ -564,6 +564,9 @@ void CodeGenFunction::GenerateCode(Globa
> SourceRange BodyRange;
> if (Stmt *Body = FD->getBody()) BodyRange = Body->getSourceRange();
>
> + // Reset CalleeWithThisReturn.
>
Well, obviously. It would be better to explain why this is being done, not
what is being done.
> + CalleeWithThisReturn = 0;
> +
> // Emit the standard function prologue.
> StartFunction(GD, ResTy, Fn, FnInfo, Args, BodyRange.getBegin());
>
> @@ -615,6 +618,8 @@ void CodeGenFunction::GenerateCode(Globa
>
> // Emit the standard function epilogue.
> FinishFunction(BodyRange.getEnd());
> + // Reset CalleeWithThisReturn.
> + CalleeWithThisReturn = 0;
>
> // If we haven't marked the function nothrow through other means, do
> // a quick pass now to see if we can.
>
> Modified: cfe/trunk/lib/CodeGen/CodeGenFunction.h
> URL:
> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/CodeGen/CodeGenFunction.h?rev=177211&r1=177210&r2=177211&view=diff
>
> ==============================================================================
> --- cfe/trunk/lib/CodeGen/CodeGenFunction.h (original)
> +++ cfe/trunk/lib/CodeGen/CodeGenFunction.h Fri Mar 15 19:11:09 2013
> @@ -1131,6 +1131,10 @@ private:
> CGDebugInfo *DebugInfo;
> bool DisableDebugInfo;
>
> + /// If the current function returns 'this', use the field to keep track
> of
> + /// the callee that returns 'this'.
> + llvm::Value *CalleeWithThisReturn;
> +
> /// DidCallStackSave - Whether llvm.stacksave has been called. Used to
> avoid
> /// calling llvm.stacksave for multiple VLAs in the same scope.
> bool DidCallStackSave;
>
> Modified: cfe/trunk/lib/CodeGen/ItaniumCXXABI.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/CodeGen/ItaniumCXXABI.cpp?rev=177211&r1=177210&r2=177211&view=diff
>
> ==============================================================================
> --- cfe/trunk/lib/CodeGen/ItaniumCXXABI.cpp (original)
> +++ cfe/trunk/lib/CodeGen/ItaniumCXXABI.cpp Fri Mar 15 19:11:09 2013
> @@ -112,7 +112,7 @@ public:
>
> void EmitInstanceFunctionProlog(CodeGenFunction &CGF);
>
> - void EmitConstructorCall(CodeGenFunction &CGF,
> + llvm::Value *EmitConstructorCall(CodeGenFunction &CGF,
> const CXXConstructorDecl *D,
> CXXCtorType Type, bool ForVirtualBase,
> bool Delegating,
> @@ -177,11 +177,11 @@ public:
> llvm::Value *readArrayCookieImpl(CodeGenFunction &CGF, llvm::Value
> *allocPtr,
> CharUnits cookieSize);
>
> -private:
> /// \brief Returns true if the given instance method is one of the
> /// kinds that the ARM ABI says returns 'this'.
> - static bool HasThisReturn(GlobalDecl GD) {
> - const CXXMethodDecl *MD = cast<CXXMethodDecl>(GD.getDecl());
> + bool HasThisReturn(GlobalDecl GD) const {
> + const CXXMethodDecl *MD =
> dyn_cast_or_null<CXXMethodDecl>(GD.getDecl());
> + if (!MD) return false;
> return ((isa<CXXDestructorDecl>(MD) && GD.getDtorType() !=
> Dtor_Deleting) ||
> (isa<CXXConstructorDecl>(MD)));
> }
> @@ -834,7 +834,7 @@ void ARMCXXABI::EmitInstanceFunctionProl
> CGF.Builder.CreateStore(getThisValue(CGF), CGF.ReturnValue);
> }
>
> -void ItaniumCXXABI::EmitConstructorCall(CodeGenFunction &CGF,
> +llvm::Value *ItaniumCXXABI::EmitConstructorCall(CodeGenFunction &CGF,
> const CXXConstructorDecl *D,
> CXXCtorType Type, bool
> ForVirtualBase,
> bool Delegating,
> @@ -849,6 +849,7 @@ void ItaniumCXXABI::EmitConstructorCall(
> // FIXME: Provide a source location here.
> CGF.EmitCXXMemberCall(D, SourceLocation(), Callee, ReturnValueSlot(),
> This,
> VTT, VTTTy, ArgBeg, ArgEnd);
> + return Callee;
> }
>
> RValue ItaniumCXXABI::EmitVirtualDestructorCall(CodeGenFunction &CGF,
>
> Modified: cfe/trunk/lib/CodeGen/MicrosoftCXXABI.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/CodeGen/MicrosoftCXXABI.cpp?rev=177211&r1=177210&r2=177211&view=diff
>
> ==============================================================================
> --- cfe/trunk/lib/CodeGen/MicrosoftCXXABI.cpp (original)
> +++ cfe/trunk/lib/CodeGen/MicrosoftCXXABI.cpp Fri Mar 15 19:11:09 2013
> @@ -55,7 +55,7 @@ public:
>
> void EmitInstanceFunctionProlog(CodeGenFunction &CGF);
>
> - void EmitConstructorCall(CodeGenFunction &CGF,
> + llvm::Value *EmitConstructorCall(CodeGenFunction &CGF,
> const CXXConstructorDecl *D,
> CXXCtorType Type, bool ForVirtualBase,
> bool Delegating,
> @@ -238,7 +238,7 @@ void MicrosoftCXXABI::EmitInstanceFuncti
> }
> }
>
> -void MicrosoftCXXABI::EmitConstructorCall(CodeGenFunction &CGF,
> +llvm::Value *MicrosoftCXXABI::EmitConstructorCall(CodeGenFunction &CGF,
> const CXXConstructorDecl *D,
> CXXCtorType Type, bool
> ForVirtualBase,
> bool Delegating,
> @@ -259,6 +259,7 @@ void MicrosoftCXXABI::EmitConstructorCal
> CGF.EmitCXXMemberCall(D, SourceLocation(), Callee, ReturnValueSlot(),
> This,
> ImplicitParam, ImplicitParamTy,
> ArgBeg, ArgEnd);
> + return Callee;
> }
>
> RValue MicrosoftCXXABI::EmitVirtualDestructorCall(CodeGenFunction &CGF,
>
> Modified: cfe/trunk/test/CodeGenCXX/arm.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/cfe/trunk/test/CodeGenCXX/arm.cpp?rev=177211&r1=177210&r2=177211&view=diff
>
> ==============================================================================
> --- cfe/trunk/test/CodeGenCXX/arm.cpp (original)
> +++ cfe/trunk/test/CodeGenCXX/arm.cpp Fri Mar 15 19:11:09 2013
> @@ -56,15 +56,15 @@ namespace test1 {
> // CHECK: [[THIS:%.*]] = alloca [[A]]*, align 4
> // CHECK: store [[A]]* {{.*}}, [[A]]** [[THIS]]
> // CHECK: [[THIS1:%.*]] = load [[A]]** [[THIS]]
> - // CHECK: call [[A]]* @_ZN5test11AC2Ei(
> - // CHECK: ret [[A]]* [[THIS1]]
> + // CHECK: [[THIS2:%.*]] = call [[A]]* @_ZN5test11AC2Ei(
> + // CHECK: ret [[A]]* [[THIS2]]
>
> // CHECK: define linkonce_odr [[A]]* @_ZN5test11AD1Ev([[A]]* %this)
> unnamed_addr
> // CHECK: [[THIS:%.*]] = alloca [[A]]*, align 4
> // CHECK: store [[A]]* {{.*}}, [[A]]** [[THIS]]
> // CHECK: [[THIS1:%.*]] = load [[A]]** [[THIS]]
> - // CHECK: call [[A]]* @_ZN5test11AD2Ev(
> - // CHECK: ret [[A]]* [[THIS1]]
> + // CHECK: [[THIS2:%.*]] = call [[A]]* @_ZN5test11AD2Ev(
> + // CHECK: ret [[A]]* [[THIS2]]
> }
>
> // Awkward virtual cases.
>
>
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