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Implement a bunch of symbolic constant folding opportunities. This implements
testcase test/Regression/Assembler/ConstantExprFold.llx Note that these kinds of things only rarely show up in source code, but are exceedingly common in the intermediate stages of algorithms like SCCP. By folding things (especially relational operators) that use symbolic constants, we are able to speculatively fold more conditional branches, which can lead to some big simplifications. It would be easy to add a lot more special cases here, so if you notice SCCP missing anything "obvious", you know what to make smarter. :) git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@10812 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner
parent
75d1412cc2
commit
504e8fb74e
@ -21,6 +21,7 @@
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#include "ConstantFolding.h"
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#include "llvm/Constants.h"
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#include "llvm/iPHINode.h"
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#include "llvm/iOperators.h"
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#include "llvm/InstrTypes.h"
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#include "llvm/DerivedTypes.h"
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#include "llvm/Support/GetElementPtrTypeIterator.h"
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@ -573,42 +574,344 @@ Constant *llvm::ConstantFoldCastInstruction(const Constant *V,
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}
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}
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/// IdxCompare - Compare the two constants as though they were getelementptr
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/// indices. This allows coersion of the types to be the same thing.
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///
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/// If the two constants are the "same" (after coersion), return 0. If the
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/// first is less than the second, return -1, if the second is less than the
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/// first, return 1. If the constants are not integral, return -2.
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///
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static int IdxCompare(Constant *C1, Constant *C2) {
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if (C1 == C2) return 0;
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// Ok, we found a different index. Are either of the operands
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// ConstantExprs? If so, we can't do anything with them.
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if (!isa<ConstantInt>(C1) || !isa<ConstantInt>(C2))
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return -2; // don't know!
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// Ok, we have two differing integer indices. Convert them to
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// be the same type. Long is always big enough, so we use it.
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C1 = ConstantExpr::getCast(C1, Type::LongTy);
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C2 = ConstantExpr::getCast(C2, Type::LongTy);
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if (C1 == C2) return 0; // Are they just differing types?
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// If they are really different, now that they are the same type, then we
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// found a difference!
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if (cast<ConstantSInt>(C1)->getValue() < cast<ConstantSInt>(C2)->getValue())
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return -1;
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else
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return 1;
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}
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/// evaluateRelation - This function determines if there is anything we can
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/// decide about the two constants provided. This doesn't need to handle simple
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/// things like integer comparisons, but should instead handle ConstantExpr's
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/// and ConstantPointerRef's. If we can determine that the two constants have a
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/// particular relation to each other, we should return the corresponding SetCC
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/// code, otherwise return Instruction::BinaryOpsEnd.
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///
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/// To simplify this code we canonicalize the relation so that the first
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/// operand is always the most "complex" of the two. We consider simple
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/// constants (like ConstantInt) to be the simplest, followed by
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/// ConstantPointerRef's, followed by ConstantExpr's (the most complex).
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///
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static Instruction::BinaryOps evaluateRelation(const Constant *V1,
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const Constant *V2) {
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assert(V1->getType() == V2->getType() &&
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"Cannot compare different types of values!");
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if (V1 == V2) return Instruction::SetEQ;
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if (!isa<ConstantExpr>(V1) && !isa<ConstantPointerRef>(V1)) {
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// If the first operand is simple, swap operands.
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assert((isa<ConstantPointerRef>(V2) || isa<ConstantExpr>(V2)) &&
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"Simple cases should have been handled by caller!");
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return SetCondInst::getSwappedCondition(evaluateRelation(V2, V1));
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} else if (const ConstantPointerRef *CPR1 = dyn_cast<ConstantPointerRef>(V1)){
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if (isa<ConstantExpr>(V2)) // Swap as necessary.
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return SetCondInst::getSwappedCondition(evaluateRelation(V2, V1));
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// Now we know that the RHS is a ConstantPointerRef or simple constant,
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// which (since the types must match) means that it's a ConstantPointerNull.
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if (const ConstantPointerRef *CPR2 = dyn_cast<ConstantPointerRef>(V2)) {
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assert(CPR1->getValue() != CPR2->getValue() &&
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"CPRs for the same value exist at different addresses??");
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// FIXME: If both globals are external weak, they might both be null!
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return Instruction::SetNE;
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} else {
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assert(isa<ConstantPointerNull>(V2) && "Canonicalization guarantee!");
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// Global can never be null. FIXME: if we implement external weak
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// linkage, this is not necessarily true!
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return Instruction::SetNE;
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}
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} else {
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// Ok, the LHS is known to be a constantexpr. The RHS can be any of a
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// constantexpr, a CPR, or a simple constant.
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const ConstantExpr *CE1 = cast<ConstantExpr>(V1);
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Constant *CE1Op0 = CE1->getOperand(0);
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switch (CE1->getOpcode()) {
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case Instruction::Cast:
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// If the cast is not actually changing bits, and the second operand is a
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// null pointer, do the comparison with the pre-casted value.
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if (V2->isNullValue() &&
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CE1->getType()->isLosslesslyConvertibleTo(CE1Op0->getType()))
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return evaluateRelation(CE1Op0,
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Constant::getNullValue(CE1Op0->getType()));
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case Instruction::GetElementPtr:
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// Ok, since this is a getelementptr, we know that the constant has a
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// pointer type. Check the various cases.
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if (isa<ConstantPointerNull>(V2)) {
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// If we are comparing a GEP to a null pointer, check to see if the base
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// of the GEP equals the null pointer.
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if (isa<ConstantPointerRef>(CE1Op0)) {
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// FIXME: this is not true when we have external weak references!
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// No offset can go from a global to a null pointer.
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return Instruction::SetGT;
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} else if (isa<ConstantPointerNull>(CE1Op0)) {
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// If we are indexing from a null pointer, check to see if we have any
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// non-zero indices.
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for (unsigned i = 1, e = CE1->getNumOperands(); i != e; ++i)
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if (!CE1->getOperand(i)->isNullValue())
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// Offsetting from null, must not be equal.
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return Instruction::SetGT;
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// Only zero indexes from null, must still be zero.
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return Instruction::SetEQ;
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}
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// Otherwise, we can't really say if the first operand is null or not.
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} else if (const ConstantPointerRef *CPR2 =
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dyn_cast<ConstantPointerRef>(V2)) {
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if (isa<ConstantPointerNull>(CE1Op0)) {
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// FIXME: This is not true with external weak references.
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return Instruction::SetLT;
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} else if (const ConstantPointerRef *CPR1 =
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dyn_cast<ConstantPointerRef>(CE1Op0)) {
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if (CPR1 == CPR2) {
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// If this is a getelementptr of the same global, then it must be
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// different. Because the types must match, the getelementptr could
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// only have at most one index, and because we fold getelementptr's
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// with a single zero index, it must be nonzero.
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assert(CE1->getNumOperands() == 2 &&
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!CE1->getOperand(1)->isNullValue() &&
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"Suprising getelementptr!");
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return Instruction::SetGT;
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} else {
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// If they are different globals, we don't know what the value is,
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// but they can't be equal.
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return Instruction::SetNE;
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}
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}
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} else {
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const ConstantExpr *CE2 = cast<ConstantExpr>(V2);
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const Constant *CE2Op0 = CE2->getOperand(0);
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// There are MANY other foldings that we could perform here. They will
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// probably be added on demand, as they seem needed.
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switch (CE2->getOpcode()) {
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default: break;
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case Instruction::GetElementPtr:
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// By far the most common case to handle is when the base pointers are
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// obviously to the same or different globals.
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if (isa<ConstantPointerRef>(CE1Op0) &&
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isa<ConstantPointerRef>(CE2Op0)) {
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if (CE1Op0 != CE2Op0) // Don't know relative ordering, but not equal
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return Instruction::SetNE;
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// Ok, we know that both getelementptr instructions are based on the
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// same global. From this, we can precisely determine the relative
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// ordering of the resultant pointers.
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unsigned i = 1;
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// Compare all of the operands the GEP's have in common.
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for (;i != CE1->getNumOperands() && i != CE2->getNumOperands(); ++i)
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switch (IdxCompare(CE1->getOperand(i), CE2->getOperand(i))) {
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case -1: return Instruction::SetLT;
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case 1: return Instruction::SetGT;
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case -2: return Instruction::BinaryOpsEnd;
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}
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// Ok, we ran out of things they have in common. If any leftovers
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// are non-zero then we have a difference, otherwise we are equal.
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for (; i < CE1->getNumOperands(); ++i)
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if (!CE1->getOperand(i)->isNullValue())
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return Instruction::SetGT;
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for (; i < CE2->getNumOperands(); ++i)
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if (!CE2->getOperand(i)->isNullValue())
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return Instruction::SetLT;
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return Instruction::SetEQ;
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}
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}
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}
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default:
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break;
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}
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}
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return Instruction::BinaryOpsEnd;
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}
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Constant *llvm::ConstantFoldBinaryInstruction(unsigned Opcode,
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const Constant *V1,
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const Constant *V2) {
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Constant *C;
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Constant *C = 0;
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switch (Opcode) {
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default: return 0;
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case Instruction::Add: return ConstRules::get(V1, V2).add(V1, V2);
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case Instruction::Sub: return ConstRules::get(V1, V2).sub(V1, V2);
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case Instruction::Mul: return ConstRules::get(V1, V2).mul(V1, V2);
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case Instruction::Div: return ConstRules::get(V1, V2).div(V1, V2);
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case Instruction::Rem: return ConstRules::get(V1, V2).rem(V1, V2);
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case Instruction::And: return ConstRules::get(V1, V2).op_and(V1, V2);
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case Instruction::Or: return ConstRules::get(V1, V2).op_or (V1, V2);
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case Instruction::Xor: return ConstRules::get(V1, V2).op_xor(V1, V2);
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case Instruction::Shl: return ConstRules::get(V1, V2).shl(V1, V2);
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case Instruction::Shr: return ConstRules::get(V1, V2).shr(V1, V2);
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case Instruction::SetEQ: return ConstRules::get(V1, V2).equalto(V1, V2);
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case Instruction::SetLT: return ConstRules::get(V1, V2).lessthan(V1, V2);
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case Instruction::SetGT: return ConstRules::get(V1, V2).lessthan(V2, V1);
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default: break;
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case Instruction::Add: C = ConstRules::get(V1, V2).add(V1, V2); break;
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case Instruction::Sub: C = ConstRules::get(V1, V2).sub(V1, V2); break;
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case Instruction::Mul: C = ConstRules::get(V1, V2).mul(V1, V2); break;
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case Instruction::Div: C = ConstRules::get(V1, V2).div(V1, V2); break;
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case Instruction::Rem: C = ConstRules::get(V1, V2).rem(V1, V2); break;
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case Instruction::And: C = ConstRules::get(V1, V2).op_and(V1, V2); break;
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case Instruction::Or: C = ConstRules::get(V1, V2).op_or (V1, V2); break;
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case Instruction::Xor: C = ConstRules::get(V1, V2).op_xor(V1, V2); break;
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case Instruction::Shl: C = ConstRules::get(V1, V2).shl(V1, V2); break;
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case Instruction::Shr: C = ConstRules::get(V1, V2).shr(V1, V2); break;
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case Instruction::SetEQ: C = ConstRules::get(V1, V2).equalto(V1, V2); break;
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case Instruction::SetLT: C = ConstRules::get(V1, V2).lessthan(V1, V2);break;
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case Instruction::SetGT: C = ConstRules::get(V1, V2).lessthan(V2, V1);break;
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case Instruction::SetNE: // V1 != V2 === !(V1 == V2)
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C = ConstRules::get(V1, V2).equalto(V1, V2);
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if (C) return ConstantExpr::get(Instruction::Xor, C, ConstantBool::True);
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break;
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case Instruction::SetLE: // V1 <= V2 === !(V2 < V1)
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C = ConstRules::get(V1, V2).lessthan(V2, V1);
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if (C) return ConstantExpr::get(Instruction::Xor, C, ConstantBool::True);
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break;
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case Instruction::SetGE: // V1 >= V2 === !(V1 < V2)
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C = ConstRules::get(V1, V2).lessthan(V1, V2);
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if (C) return ConstantExpr::get(Instruction::Xor, C, ConstantBool::True);
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break;
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}
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// If the folder broke out of the switch statement, invert the boolean
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// constant value, if it exists, and return it.
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if (!C) return 0;
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return ConstantExpr::get(Instruction::Xor, ConstantBool::True, C);
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// If we successfully folded the expression, return it now.
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if (C) return C;
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if (SetCondInst::isRelational(Opcode))
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switch (evaluateRelation(V1, V2)) {
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default: assert(0 && "Unknown relational!");
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case Instruction::BinaryOpsEnd:
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break; // Couldn't determine anything about these constants.
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case Instruction::SetEQ: // We know the constants are equal!
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// If we know the constants are equal, we can decide the result of this
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// computation precisely.
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return ConstantBool::get(Opcode == Instruction::SetEQ ||
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Opcode == Instruction::SetLE ||
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Opcode == Instruction::SetGE);
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case Instruction::SetLT:
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// If we know that V1 < V2, we can decide the result of this computation
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// precisely.
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return ConstantBool::get(Opcode == Instruction::SetLT ||
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Opcode == Instruction::SetNE ||
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Opcode == Instruction::SetLE);
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case Instruction::SetGT:
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// If we know that V1 > V2, we can decide the result of this computation
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// precisely.
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return ConstantBool::get(Opcode == Instruction::SetGT ||
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Opcode == Instruction::SetNE ||
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Opcode == Instruction::SetGE);
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case Instruction::SetLE:
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// If we know that V1 <= V2, we can only partially decide this relation.
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if (Opcode == Instruction::SetGT) return ConstantBool::False;
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if (Opcode == Instruction::SetLT) return ConstantBool::True;
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break;
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case Instruction::SetGE:
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// If we know that V1 >= V2, we can only partially decide this relation.
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if (Opcode == Instruction::SetLT) return ConstantBool::False;
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if (Opcode == Instruction::SetGT) return ConstantBool::True;
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break;
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case Instruction::SetNE:
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// If we know that V1 != V2, we can only partially decide this relation.
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if (Opcode == Instruction::SetEQ) return ConstantBool::False;
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if (Opcode == Instruction::SetNE) return ConstantBool::True;
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break;
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}
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if (const ConstantExpr *CE1 = dyn_cast<ConstantExpr>(V1)) {
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if (const ConstantExpr *CE2 = dyn_cast<ConstantExpr>(V2)) {
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// There are many possible foldings we could do here. We should probably
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// at least fold add of a pointer with an integer into the appropriate
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// getelementptr. This will improve alias analysis a bit.
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} else {
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// Just implement a couple of simple identities.
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switch (Opcode) {
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case Instruction::Add:
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if (V2->isNullValue()) return const_cast<Constant*>(V1); // X + 0 == X
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break;
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case Instruction::Sub:
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if (V2->isNullValue()) return const_cast<Constant*>(V1); // X - 0 == X
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break;
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case Instruction::Mul:
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if (V2->isNullValue()) return const_cast<Constant*>(V2); // X * 0 == 0
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if (const ConstantInt *CI = dyn_cast<ConstantInt>(V2))
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if (CI->getRawValue() == 1)
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return const_cast<Constant*>(V1); // X * 1 == X
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break;
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case Instruction::Div:
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if (const ConstantInt *CI = dyn_cast<ConstantInt>(V2))
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if (CI->getRawValue() == 1)
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return const_cast<Constant*>(V1); // X / 1 == X
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break;
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case Instruction::Rem:
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if (const ConstantInt *CI = dyn_cast<ConstantInt>(V2))
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if (CI->getRawValue() == 1)
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return Constant::getNullValue(CI->getType()); // X % 1 == 0
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break;
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case Instruction::And:
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if (cast<ConstantIntegral>(V2)->isAllOnesValue())
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return const_cast<Constant*>(V1); // X & -1 == X
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if (V2->isNullValue()) return const_cast<Constant*>(V2); // X & 0 == 0
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break;
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case Instruction::Or:
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if (V2->isNullValue()) return const_cast<Constant*>(V1); // X | 0 == X
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if (cast<ConstantIntegral>(V2)->isAllOnesValue())
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return const_cast<Constant*>(V2); // X | -1 == -1
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break;
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case Instruction::Xor:
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if (V2->isNullValue()) return const_cast<Constant*>(V1); // X ^ 0 == X
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break;
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}
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}
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} else if (const ConstantExpr *CE2 = dyn_cast<ConstantExpr>(V2)) {
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// If V2 is a constant expr and V1 isn't, flop them around and fold the
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// other way if possible.
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switch (Opcode) {
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case Instruction::Add:
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case Instruction::Mul:
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case Instruction::And:
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case Instruction::Or:
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case Instruction::Xor:
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case Instruction::SetEQ:
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case Instruction::SetNE:
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// No change of opcode required.
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return ConstantFoldBinaryInstruction(Opcode, V2, V1);
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case Instruction::SetLT:
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case Instruction::SetGT:
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case Instruction::SetLE:
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case Instruction::SetGE:
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// Change the opcode as necessary to swap the operands.
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Opcode = SetCondInst::getSwappedCondition((Instruction::BinaryOps)Opcode);
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return ConstantFoldBinaryInstruction(Opcode, V2, V1);
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case Instruction::Shl:
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case Instruction::Shr:
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case Instruction::Sub:
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case Instruction::Div:
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case Instruction::Rem:
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default: // These instructions cannot be flopped around.
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break;
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}
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}
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return 0;
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}
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Constant *llvm::ConstantFoldGetElementPtr(const Constant *C,
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