path: root/lib
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authorChandler Carruth <>2014-05-15 09:56:28 +0000
committerChandler Carruth <>2014-05-15 09:56:28 +0000
commitca323cf916fc7e2bb227e253c15d456f3f3ca3eb (patch)
treec639ccfa9b37c30ad695c0d0181070637eaf2218 /lib
parent9cfa5cf7ae1c61a8a1a26d56650ae23746180452 (diff)
Teach the constant folder to look through bitcast constant expressions
much more effectively when trying to constant fold a load of a constant. Previously, we only handled bitcasts by trying to find a totally generic byte representation of the constant and use that. Now, we look through the bitcast to see what constant we might fold the load into, and then try to form a constant expression cast of the found value that would be equivalent to loading the value. You might wonder why on earth this actually matters. Well, turns out that the Itanium ABI causes us to create a single array for a vtable where the first elements are virtual base offsets, followed by the virtual function pointers. Because the array is homogenous the element type is consistently i8* and we inttoptr the virtual base offsets into the initial elements. Then constructors bitcast these pointers to i64 pointers prior to loading them. Boom, no more constant folding of virtual base offsets. This is the first fix to LLVM to address the *insane* performance Eric Niebler discovered with Clang on his range comprehensions[1]. There is more to come though, this doesn't *really* fix the problem fully. [1]: git-svn-id: 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'lib')
1 files changed, 50 insertions, 0 deletions
diff --git a/lib/Analysis/ConstantFolding.cpp b/lib/Analysis/ConstantFolding.cpp
index 4bf172758c9..0ac1cb56aa2 100644
--- a/lib/Analysis/ConstantFolding.cpp
+++ b/lib/Analysis/ConstantFolding.cpp
@@ -466,6 +466,52 @@ static Constant *FoldReinterpretLoadFromConstPtr(Constant *C,
return ConstantInt::get(IntType->getContext(), ResultVal);
+static Constant *ConstantFoldLoadThroughBitcast(ConstantExpr *CE,
+ const DataLayout *DL) {
+ if (!DL)
+ return nullptr;
+ auto *DestPtrTy = dyn_cast<PointerType>(CE->getType());
+ if (!DestPtrTy)
+ return nullptr;
+ Type *DestTy = DestPtrTy->getElementType();
+ Constant *C = ConstantFoldLoadFromConstPtr(CE->getOperand(0), DL);
+ if (!C)
+ return nullptr;
+ do {
+ Type *SrcTy = C->getType();
+ // If the type sizes are the same and a cast is legal, just directly
+ // cast the constant.
+ if (DL->getTypeSizeInBits(DestTy) == DL->getTypeSizeInBits(SrcTy)) {
+ Instruction::CastOps Cast = Instruction::BitCast;
+ // If we are going from a pointer to int or vice versa, we spell the cast
+ // differently.
+ if (SrcTy->isIntegerTy() && DestTy->isPointerTy())
+ Cast = Instruction::IntToPtr;
+ else if (SrcTy->isPointerTy() && DestTy->isIntegerTy())
+ Cast = Instruction::PtrToInt;
+ if (CastInst::castIsValid(Cast, C, DestTy))
+ return ConstantExpr::getCast(Cast, C, DestTy);
+ }
+ // If this isn't an aggregate type, there is nothing we can do to drill down
+ // and find a bitcastable constant.
+ if (!SrcTy->isAggregateType())
+ return nullptr;
+ // We're simulating a load through a pointer that was bitcast to point to
+ // a different type, so we can try to walk down through the initial
+ // elements of an aggregate to see if some part of th e aggregate is
+ // castable to implement the "load" semantic model.
+ C = C->getAggregateElement(0u);
+ } while (C);
+ return nullptr;
/// ConstantFoldLoadFromConstPtr - Return the value that a load from C would
/// produce if it is constant and determinable. If this is not determinable,
/// return null.
@@ -491,6 +537,10 @@ Constant *llvm::ConstantFoldLoadFromConstPtr(Constant *C,
+ if (CE->getOpcode() == Instruction::BitCast)
+ if (Constant *LoadedC = ConstantFoldLoadThroughBitcast(CE, TD))
+ return LoadedC;
// Instead of loading constant c string, use corresponding integer value
// directly if string length is small enough.
StringRef Str;