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* Implement minor performance optimization for the getelementptr case

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* Implement SCCP of load instructions, implementing Transforms/SCCP/loadtest.ll
  This allows us to fold expressions like "foo"[2], even if the pointer is only
  a conditional constant.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@10767 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2004-01-12 04:29:41 +00:00
parent debcbd0a46
commit c6a4d6a066
1 changed files with 69 additions and 3 deletions
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72
lib/Transforms/Scalar/SCCP.cpp
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@ -24,6 +24,7 @@
#include "llvm/Transforms/Scalar.h"
#include "llvm/ConstantHandling.h"
#include "llvm/Function.h"
#include "llvm/GlobalVariable.h"
#include "llvm/Instructions.h"
#include "llvm/Pass.h"
#include "llvm/Support/InstVisitor.h"
@ -215,7 +216,7 @@ private:
// Instructions that cannot be folded away...
void visitStoreInst (Instruction &I) { /*returns void*/ }
void visitLoadInst (Instruction &I) { markOverdefined(&I); }
void visitLoadInst (LoadInst &I);
void visitGetElementPtrInst(GetElementPtrInst &I);
void visitCallInst (Instruction &I) { markOverdefined(&I); }
void visitInvokeInst (TerminatorInst &I) {
@ -666,6 +667,9 @@ void SCCP::visitBinaryOperator(Instruction &I) {
// can turn this into a getelementptr ConstantExpr.
//
void SCCP::visitGetElementPtrInst(GetElementPtrInst &I) {
InstVal &IV = ValueState[&I];
if (IV.isOverdefined()) return;
std::vector<Constant*> Operands;
Operands.reserve(I.getNumOperands());
@ -674,7 +678,7 @@ void SCCP::visitGetElementPtrInst(GetElementPtrInst &I) {
if (State.isUndefined())
return; // Operands are not resolved yet...
else if (State.isOverdefined()) {
markOverdefined(&I);
markOverdefined(IV, &I);
return;
}
assert(State.isConstant() && "Unknown state!");
@ -684,6 +688,68 @@ void SCCP::visitGetElementPtrInst(GetElementPtrInst &I) {
Constant *Ptr = Operands[0];
Operands.erase(Operands.begin()); // Erase the pointer from idx list...
markConstant(&I, ConstantExpr::getGetElementPtr(Ptr, Operands));
markConstant(IV, &I, ConstantExpr::getGetElementPtr(Ptr, Operands));
}
/// GetGEPGlobalInitializer - Given a constant and a getelementptr constantexpr,
/// return the constant value being addressed by the constant expression, or
/// null if something is funny.
///
static Constant *GetGEPGlobalInitializer(Constant *C, ConstantExpr *CE) {
if (CE->getOperand(1) != Constant::getNullValue(Type::LongTy))
return 0; // Do not allow stepping over the value!
// Loop over all of the operands, tracking down which value we are
// addressing...
for (unsigned i = 2, e = CE->getNumOperands(); i != e; ++i)
if (ConstantUInt *CU = dyn_cast<ConstantUInt>(CE->getOperand(i))) {
ConstantStruct *CS = cast<ConstantStruct>(C);
if (CU->getValue() >= CS->getValues().size()) return 0;
C = cast<Constant>(CS->getValues()[CU->getValue()]);
} else if (ConstantSInt *CS = dyn_cast<ConstantSInt>(CE->getOperand(i))) {
ConstantArray *CA = cast<ConstantArray>(C);
if ((uint64_t)CS->getValue() >= CA->getValues().size()) return 0;
C = cast<Constant>(CA->getValues()[CS->getValue()]);
} else
return 0;
return C;
}
// Handle load instructions. If the operand is a constant pointer to a constant
// global, we can replace the load with the loaded constant value!
void SCCP::visitLoadInst(LoadInst &I) {
InstVal &IV = ValueState[&I];
if (IV.isOverdefined()) return;
InstVal &PtrVal = getValueState(I.getOperand(0));
if (PtrVal.isUndefined()) return; // The pointer is not resolved yet!
if (PtrVal.isConstant() && !I.isVolatile()) {
Value *Ptr = PtrVal.getConstant();
if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(Ptr))
Ptr = CPR->getValue();
// Transform load (constant global) into the value loaded.
if (GlobalVariable *GV = dyn_cast<GlobalVariable>(Ptr))
if (GV->isConstant() && !GV->isExternal()) {
markConstant(IV, &I, GV->getInitializer());
return;
}
// Transform load (constantexpr_GEP global, 0, ...) into the value loaded.
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Ptr))
if (CE->getOpcode() == Instruction::GetElementPtr)
if (ConstantPointerRef *G
= dyn_cast<ConstantPointerRef>(CE->getOperand(0)))
if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G->getValue()))
if (GV->isConstant() && !GV->isExternal())
if (Constant *V =
GetGEPGlobalInitializer(GV->getInitializer(), CE)) {
markConstant(IV, &I, V);
return;
}
}
// Otherwise we cannot say for certain what value this load will produce.
// Bail out.
markOverdefined(IV, &I);
}