From df9db45c944a7f21cf4483a1c1b8dd1eaead2f47 Mon Sep 17 00:00:00 2001 From: Igor Laevsky Date: Fri, 27 May 2016 13:13:59 +0000 Subject: [RewriteStatepointsForGC] All constant should have null base pointer Currently we consider that each constant has itself as a base value. I.e "base(const) = const". This introduces couple of problems when we are trying to avoid reporting constants in statepoint live sets: 1. When querying "base( phi(const1, const2) )" we will get "phi(const1, const2)" as a base pointer. Since it's not a constant we will record it in a stack map. However on practice we don't want this to happen (constant are never relocated). 2. base( phi(const, gc ptr) ) = phi( const, base(gc ptr) ). This particular case imposes challenge on our runtime - we don't expect to see constant base pointers other than null. This problems can be avoided by treating all constant as if they were derived from null pointer base. I.e in a first case we will not include constant pointer in a stack map at all. In a second case we will get "phi(null, base(gc ptr))" as a base pointer which is a lot more convenient. Differential Revision: http://reviews.llvm.org/D20584 llvm-svn: 270993 --- .../Transforms/Scalar/RewriteStatepointsForGC.cpp | 24 ++++++++++++++-------- 1 file changed, 16 insertions(+), 8 deletions(-) (limited to 'llvm/lib/Transforms') diff --git a/llvm/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp b/llvm/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp index 991147aeb04..218a52bb3b1 100644 --- a/llvm/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp +++ b/llvm/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp @@ -343,8 +343,10 @@ findBaseDefiningValueOfVector(Value *I) { return BaseDefiningValueResult(I, true); if (isa(I)) - // Constant vectors consist only of constant pointers. - return BaseDefiningValueResult(I, true); + // Base of constant vector consists only of constant null pointers. + // For reasoning see similar case inside 'findBaseDefiningValue' function. + return BaseDefiningValueResult(ConstantAggregateZero::get(I->getType()), + true); if (isa(I)) return BaseDefiningValueResult(I, true); @@ -386,14 +388,20 @@ static BaseDefiningValueResult findBaseDefiningValue(Value *I) { // We should have never reached here if this argument isn't an gc value return BaseDefiningValueResult(I, true); - if (isa(I)) + if (isa(I)) { // We assume that objects with a constant base (e.g. a global) can't move // and don't need to be reported to the collector because they are always - // live. All constants have constant bases. Besides global references, all - // kinds of constants (e.g. undef, constant expressions, null pointers) can - // be introduced by the inliner or the optimizer, especially on dynamically - // dead paths. See e.g. test4 in constants.ll. - return BaseDefiningValueResult(I, true); + // live. Besides global references, all kinds of constants (e.g. undef, + // constant expressions, null pointers) can be introduced by the inliner or + // the optimizer, especially on dynamically dead paths. + // Here we treat all of them as having single null base. By doing this we + // trying to avoid problems reporting various conflicts in a form of + // "phi (const1, const2)" or "phi (const, regular gc ptr)". + // See constant.ll file for relevant test cases. + + return BaseDefiningValueResult( + ConstantPointerNull::get(cast(I->getType())), true); + } if (CastInst *CI = dyn_cast(I)) { Value *Def = CI->stripPointerCasts(); -- cgit v1.2.3