include/llvm/Transforms/Scalar/LICM.h

   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/Transforms/Scalar/LoopPassManager.h"
  38
  39 namespace llvm {
  40
  41 extern cl::opt<unsigned> SetLicmMssaOptCap;
  42 extern cl::opt<unsigned> SetLicmMssaNoAccForPromotionCap;
  43
  44 /// Performs Loop Invariant Code Motion Pass.
  45 class LICMPass : public PassInfoMixin<LICMPass> {
  46   unsigned LicmMssaOptCap;
  47   unsigned LicmMssaNoAccForPromotionCap;
  48
  49 public:
  50   LICMPass()
  51       : LicmMssaOptCap(SetLicmMssaOptCap),
  52         LicmMssaNoAccForPromotionCap(SetLicmMssaNoAccForPromotionCap) {}
  53   LICMPass(unsigned LicmMssaOptCap, unsigned LicmMssaNoAccForPromotionCap)
  54       : LicmMssaOptCap(LicmMssaOptCap),
  55         LicmMssaNoAccForPromotionCap(LicmMssaNoAccForPromotionCap) {}
  56   PreservedAnalyses run(Loop &L, LoopAnalysisManager &AM,
  57                         LoopStandardAnalysisResults &AR, LPMUpdater &U);
  58 };
  59 } // end namespace llvm
  60
  61 #endif // LLVM_TRANSFORMS_SCALAR_LICM_H