[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
diff --git a/llvm/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp b/llvm/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp
index 991147a..218a52b 100644
--- a/llvm/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp
+++ b/llvm/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp
@@ -343,8 +343,10 @@
return BaseDefiningValueResult(I, true);
if (isa<Constant>(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<LoadInst>(I))
return BaseDefiningValueResult(I, true);
@@ -386,14 +388,20 @@
// We should have never reached here if this argument isn't an gc value
return BaseDefiningValueResult(I, true);
- if (isa<Constant>(I))
+ if (isa<Constant>(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<PointerType>(I->getType())), true);
+ }
if (CastInst *CI = dyn_cast<CastInst>(I)) {
Value *Def = CI->stripPointerCasts();