//===----------------------------------------------------------------------===//
#include "InstCombine.h"
+#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/IR/GetElementPtrTypeIterator.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/PatternMatch.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Debug.h"
#include "llvm/Target/TargetLibraryInfo.h"
+
using namespace llvm;
using namespace PatternMatch;
#define DEBUG_TYPE "instcombine"
+// How many times is a select replaced by one of its operands?
+STATISTIC(NumSel, "Number of select opts");
+
+// Initialization Routines
+
static ConstantInt *getOne(Constant *C) {
return ConstantInt::get(cast<IntegerType>(C->getType()), 1);
}
return GlobalSwapBenefits > 0;
}
+/// \brief Check that one use is in the same block as the definition and all
+/// other uses are in blocks dominated by a given block
+///
+/// \param DI Definition
+/// \param UI Use
+/// \param DB Block that must dominate all uses of \p DI outside
+/// the parent block
+/// \return true when \p UI is the only use of \p DI in the parent block
+/// and all other uses of \p DI are in blocks dominated by \p DB.
+///
+bool InstCombiner::dominatesAllUses(const Instruction *DI,
+ const Instruction *UI,
+ const BasicBlock *DB) const {
+ assert(DI && UI && "Instruction not defined\n");
+ // ignore incomplete definitions
+ if (!DI->getParent())
+ return false;
+ // DI and UI must be in the same block
+ if (DI->getParent() != UI->getParent())
+ return false;
+ // Protect from self-referencing blocks
+ if (DI->getParent() == DB)
+ return false;
+ // DominatorTree available?
+ if (!DT)
+ return false;
+ for (const User *U : DI->users()) {
+ auto *Usr = cast<Instruction>(U);
+ if (Usr != UI && !DT->dominates(DB, Usr->getParent()))
+ return false;
+ }
+ return true;
+}
+
+///
+/// true when the instruction sequence within a block is select-cmp-br.
+///
+static bool isChainSelectCmpBranch(const SelectInst *SI) {
+ const BasicBlock *BB = SI->getParent();
+ if (!BB)
+ return false;
+ auto *BI = dyn_cast_or_null<BranchInst>(BB->getTerminator());
+ if (!BI || BI->getNumSuccessors() != 2)
+ return false;
+ auto *IC = dyn_cast<ICmpInst>(BI->getCondition());
+ if (!IC || (IC->getOperand(0) != SI && IC->getOperand(1) != SI))
+ return false;
+ return true;
+}
+
+///
+/// \brief True when a select result is replaced by one of its operands
+/// in select-icmp sequence. This will eventually result in the elimination
+/// of the select.
+///
+/// \param SI Select instruction
+/// \param Icmp Compare instruction
+/// \param SIOpd Operand that replaces the select
+///
+/// Notes:
+/// - The replacement is global and requires dominator information
+/// - The caller is responsible for the actual replacement
+///
+/// Example:
+///
+/// entry:
+/// %4 = select i1 %3, %C* %0, %C* null
+/// %5 = icmp eq %C* %4, null
+/// br i1 %5, label %9, label %7
+/// ...
+/// ; <label>:7 ; preds = %entry
+/// %8 = getelementptr inbounds %C* %4, i64 0, i32 0
+/// ...
+///
+/// can be transformed to
+///
+/// %5 = icmp eq %C* %0, null
+/// %6 = select i1 %3, i1 %5, i1 true
+/// br i1 %6, label %9, label %7
+/// ...
+/// ; <label>:7 ; preds = %entry
+/// %8 = getelementptr inbounds %C* %0, i64 0, i32 0 // replace by %0!
+///
+/// Similar when the first operand of the select is a constant or/and
+/// the compare is for not equal rather than equal.
+///
+/// NOTE: The function is only called when the select and compare constants
+/// are equal, the optimization can work only for EQ predicates. This is not a
+/// major restriction since a NE compare should be 'normalized' to an equal
+/// compare, which usually happens in the combiner and test case
+/// select-cmp-br.ll
+/// checks for it.
+bool InstCombiner::replacedSelectWithOperand(SelectInst *SI,
+ const ICmpInst *Icmp,
+ const unsigned SIOpd) {
+ assert(SIOpd == 1 || SIOpd == 2 && "Invalid select operand!\n");
+ if (isChainSelectCmpBranch(SI) && Icmp->getPredicate() == ICmpInst::ICMP_EQ) {
+ BasicBlock *Succ = SI->getParent()->getTerminator()->getSuccessor(1);
+ // The check for the unique predecessor is not the best that can be
+ // done. But it protects efficiently against cases like when SI's
+ // home block has two successors, Succ and Succ1, and Succ1 predecessor
+ // of Succ. Then SI can't be replaced by SIOpd because the use that gets
+ // replaced can be reached on either path. So the uniqueness check
+ // guarantees that the path all uses of SI (outside SI's parent) are on
+ // is disjoint from all other paths out of SI. But that information
+ // is more expensive to compute, and the trade-off here is in favor
+ // of compile-time.
+ if (Succ->getUniquePredecessor() && dominatesAllUses(SI, Icmp, Succ)) {
+ NumSel++;
+ SI->replaceUsesOutsideBlock(SI->getOperand(SIOpd), SI->getParent());
+ return true;
+ }
+ }
+ return false;
+}
+
Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
bool Changed = false;
Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
// comparison into the select arms, which will cause one to be
// constant folded and the select turned into a bitwise or.
Value *Op1 = nullptr, *Op2 = nullptr;
- if (Constant *C = dyn_cast<Constant>(LHSI->getOperand(1)))
+ ConstantInt *CI = 0;
+ if (Constant *C = dyn_cast<Constant>(LHSI->getOperand(1))) {
Op1 = ConstantExpr::getICmp(I.getPredicate(), C, RHSC);
- if (Constant *C = dyn_cast<Constant>(LHSI->getOperand(2)))
+ CI = dyn_cast<ConstantInt>(Op1);
+ }
+ if (Constant *C = dyn_cast<Constant>(LHSI->getOperand(2))) {
Op2 = ConstantExpr::getICmp(I.getPredicate(), C, RHSC);
+ CI = dyn_cast<ConstantInt>(Op2);
+ }
// We only want to perform this transformation if it will not lead to
// additional code. This is true if either both sides of the select
// fold to a constant (in which case the icmp is replaced with a select
// which will usually simplify) or this is the only user of the
// select (in which case we are trading a select+icmp for a simpler
- // select+icmp).
- if ((Op1 && Op2) || (LHSI->hasOneUse() && (Op1 || Op2))) {
+ // select+icmp) or all uses of the select can be replaced based on
+ // dominance information ("Global cases").
+ bool Transform = false;
+ if (Op1 && Op2)
+ Transform = true;
+ else if (Op1 || Op2) {
+ // Local case
+ if (LHSI->hasOneUse())
+ Transform = true;
+ // Global cases
+ else if (CI && !CI->isZero())
+ // When Op1 is constant try replacing select with second operand.
+ // Otherwise Op2 is constant and try replacing select with first
+ // operand.
+ Transform = replacedSelectWithOperand(cast<SelectInst>(LHSI), &I,
+ Op1 ? 2 : 1);
+ }
+ if (Transform) {
if (!Op1)
Op1 = Builder->CreateICmp(I.getPredicate(), LHSI->getOperand(1),
RHSC, I.getName());
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