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give form-memset a significantly more sane heuristic, enable it by default.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@48937 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2008-03-29 04:36:18 +00:00
parent c7c00361ce
commit 9f8a6a7498
1 changed files with 50 additions and 8 deletions
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58
lib/Transforms/Scalar/GVN.cpp
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@ -34,6 +34,7 @@
#include "llvm/Support/CFG.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/GetElementPtrTypeIterator.h"
#include "llvm/Target/TargetData.h"
#include <list>
@ -47,7 +48,7 @@ namespace {
cl::opt<bool>
FormMemSet("form-memset-from-stores",
cl::desc("Transform straight-line stores to memsets"),
cl::init(false), cl::Hidden);
cl::init(true), cl::Hidden);
}
//===----------------------------------------------------------------------===//
@ -1131,9 +1132,44 @@ struct MemsetRange {
/// TheStores - The actual stores that make up this range.
SmallVector<StoreInst*, 16> TheStores;
};
bool isProfitableToUseMemset(const TargetData &TD) const;
};
} // end anon namespace
bool MemsetRange::isProfitableToUseMemset(const TargetData &TD) const {
// If we found more than 8 stores to merge or 64 bytes, use memset.
if (TheStores.size() >= 8 || End-Start >= 64) return true;
// Assume that the code generator is capable of merging pairs of stores
// together if it wants to.
if (TheStores.size() <= 2) return false;
// If we have fewer than 8 stores, it can still be worthwhile to do this.
// For example, merging 4 i8 stores into an i32 store is useful almost always.
// However, merging 2 32-bit stores isn't useful on a 32-bit architecture (the
// memset will be split into 2 32-bit stores anyway) and doing so can
// pessimize the llvm optimizer.
//
// Since we don't have perfect knowledge here, make some assumptions: assume
// the maximum GPR width is the same size as the pointer size and assume that
// this width can be stored. If so, check to see whether we will end up
// actually reducing the number of stores used.
unsigned Bytes = unsigned(End-Start);
unsigned NumPointerStores = Bytes/TD.getPointerSize();
// Assume the remaining bytes if any are done a byte at a time.
unsigned NumByteStores = Bytes - NumPointerStores*TD.getPointerSize();
// If we will reduce the # stores (according to this heuristic), do the
// transformation. This encourages merging 4 x i8 -> i32 and 2 x i16 -> i32
// etc.
return TheStores.size() > NumPointerStores+NumByteStores;
}
namespace {
class MemsetRanges {
/// Ranges - A sorted list of the memset ranges. We use std::list here
/// because each element is relatively large and expensive to copy.
@ -1151,7 +1187,8 @@ public:
void addStore(int64_t OffsetFromFirst, StoreInst *SI);
};
}
} // end anon namespace
/// addStore - Add a new store to the MemsetRanges data structure. This adds a
/// new range for the specified store at the specified offset, merging into
@ -1292,9 +1329,10 @@ bool GVN::processStore(StoreInst *SI, SmallVectorImpl<Instruction*> &toErase) {
I != E; ++I) {
const MemsetRange &Range = *I;
// If we found less than 4 stores to merge, ignore the subrange: it isn't
// worth losing type information in llvm IR to do the transformation.
if (Range.TheStores.size() < 4)
if (Range.TheStores.size() == 1) continue;
// If it is profitable to lower this range to memset, do so now.
if (!Range.isProfitableToUseMemset(TD))
continue;
// Otherwise, we do want to transform this! Create a new memset. We put
@ -1328,7 +1366,11 @@ bool GVN::processStore(StoreInst *SI, SmallVectorImpl<Instruction*> &toErase) {
ConstantInt::get(Type::Int64Ty, Range.End-Range.Start), // size
ConstantInt::get(Type::Int32Ty, Range.Alignment) // align
};
new CallInst(MemSetF, Ops, Ops+4, "", InsertPt);
Value *C = new CallInst(MemSetF, Ops, Ops+4, "", InsertPt);
DEBUG(cerr << "Replace stores:\n";
for (unsigned i = 0, e = Range.TheStores.size(); i != e; ++i)
cerr << *Range.TheStores[i];
cerr << "With: " << *C); C=C;
// Zap all the stores.
toErase.append(Range.TheStores.begin(), Range.TheStores.end());