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 
61 /// Performs LoopNest Invariant Code Motion Pass.
62 class LNICMPass : public PassInfoMixin<LNICMPass> {
63   unsigned LicmMssaOptCap;
64   unsigned LicmMssaNoAccForPromotionCap;
65 
66 public:
67   LNICMPass()
68       : LicmMssaOptCap(SetLicmMssaOptCap),
69         LicmMssaNoAccForPromotionCap(SetLicmMssaNoAccForPromotionCap) {}
70   LNICMPass(unsigned LicmMssaOptCap, unsigned LicmMssaNoAccForPromotionCap)
71       : LicmMssaOptCap(LicmMssaOptCap),
72         LicmMssaNoAccForPromotionCap(LicmMssaNoAccForPromotionCap) {}
73   PreservedAnalyses run(LoopNest &L, LoopAnalysisManager &AM,
74                         LoopStandardAnalysisResults &AR, LPMUpdater &U);
75 };
76 } // end namespace llvm
77 
78 #endif // LLVM_TRANSFORMS_SCALAR_LICM_H
79