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      1 //===- LICM.h - Loop Invariant Code Motion Pass -------*- C++ -*-===//
      2 //
      3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
      4 // See https://llvm.org/LICENSE.txt for license information.
      5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
      6 //
      7 //===----------------------------------------------------------------------===//
      8 //
      9 // This pass performs loop invariant code motion, attempting to remove as much
     10 // code from the body of a loop as possible.  It does this by either hoisting
     11 // code into the preheader block, or by sinking code to the exit blocks if it is
     12 // safe.  This pass also promotes must-aliased memory locations in the loop to
     13 // live in registers, thus hoisting and sinking "invariant" loads and stores.
     14 //
     15 // This pass uses alias analysis for two purposes:
     16 //
     17 //  1. Moving loop invariant loads and calls out of loops.  If we can determine
     18 //     that a load or call inside of a loop never aliases anything stored to,
     19 //     we can hoist it or sink it like any other instruction.
     20 //  2. Scalar Promotion of Memory - If there is a store instruction inside of
     21 //     the loop, we try to move the store to happen AFTER the loop instead of
     22 //     inside of the loop.  This can only happen if a few conditions are true:
     23 //       A. The pointer stored through is loop invariant
     24 //       B. There are no stores or loads in the loop which _may_ alias the
     25 //          pointer.  There are no calls in the loop which mod/ref the pointer.
     26 //     If these conditions are true, we can promote the loads and stores in the
     27 //     loop of the pointer to use a temporary alloca'd variable.  We then use
     28 //     the SSAUpdater to construct the appropriate SSA form for the value.
     29 //
     30 //===----------------------------------------------------------------------===//
     31 
     32 #ifndef LLVM_TRANSFORMS_SCALAR_LICM_H
     33 #define LLVM_TRANSFORMS_SCALAR_LICM_H
     34 
     35 #include "llvm/Analysis/LoopInfo.h"
     36 #include "llvm/IR/PassManager.h"
     37 #include "llvm/Support/CommandLine.h"
     38 #include "llvm/Transforms/Scalar/LoopPassManager.h"
     39 
     40 namespace llvm {
     41 
     42 extern cl::opt<unsigned> SetLicmMssaOptCap;
     43 extern cl::opt<unsigned> SetLicmMssaNoAccForPromotionCap;
     44 
     45 /// Performs Loop Invariant Code Motion Pass.
     46 class LICMPass : public PassInfoMixin<LICMPass> {
     47   unsigned LicmMssaOptCap;
     48   unsigned LicmMssaNoAccForPromotionCap;
     49 
     50 public:
     51   LICMPass()
     52       : LicmMssaOptCap(SetLicmMssaOptCap),
     53         LicmMssaNoAccForPromotionCap(SetLicmMssaNoAccForPromotionCap) {}
     54   LICMPass(unsigned LicmMssaOptCap, unsigned LicmMssaNoAccForPromotionCap)
     55       : LicmMssaOptCap(LicmMssaOptCap),
     56         LicmMssaNoAccForPromotionCap(LicmMssaNoAccForPromotionCap) {}
     57   PreservedAnalyses run(Loop &L, LoopAnalysisManager &AM,
     58                         LoopStandardAnalysisResults &AR, LPMUpdater &U);
     59 };
     60 } // end namespace llvm
     61 
     62 #endif // LLVM_TRANSFORMS_SCALAR_LICM_H
     63