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TailRecursionElimination.h
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1 //===---- TailRecursionElimination.h ----------------------------*- 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 file transforms calls of the current function (self recursion) followed
10 // by a return instruction with a branch to the entry of the function, creating
11 // a loop. This pass also implements the following extensions to the basic
12 // algorithm:
13 //
14 // 1. Trivial instructions between the call and return do not prevent the
15 // transformation from taking place, though currently the analysis cannot
16 // support moving any really useful instructions (only dead ones).
17 // 2. This pass transforms functions that are prevented from being tail
18 // recursive by an associative and commutative expression to use an
19 // accumulator variable, thus compiling the typical naive factorial or
20 // 'fib' implementation into efficient code.
21 // 3. TRE is performed if the function returns void, if the return
22 // returns the result returned by the call, or if the function returns a
23 // run-time constant on all exits from the function. It is possible, though
24 // unlikely, that the return returns something else (like constant 0), and
25 // can still be TRE'd. It can be TRE'd if ALL OTHER return instructions in
26 // the function return the exact same value.
27 // 4. If it can prove that callees do not access their caller stack frame,
28 // they are marked as eligible for tail call elimination (by the code
29 // generator).
30 //
31 // There are several improvements that could be made:
32 //
33 // 1. If the function has any alloca instructions, these instructions will be
34 // moved out of the entry block of the function, causing them to be
35 // evaluated each time through the tail recursion. Safely keeping allocas
36 // in the entry block requires analysis to proves that the tail-called
37 // function does not read or write the stack object.
38 // 2. Tail recursion is only performed if the call immediately precedes the
39 // return instruction. It's possible that there could be a jump between
40 // the call and the return.
41 // 3. There can be intervening operations between the call and the return that
42 // prevent the TRE from occurring. For example, there could be GEP's and
43 // stores to memory that will not be read or written by the call. This
44 // requires some substantial analysis (such as with DSA) to prove safe to
45 // move ahead of the call, but doing so could allow many more TREs to be
46 // performed, for example in TreeAdd/TreeAlloc from the treeadd benchmark.
47 // 4. The algorithm we use to detect if callees access their caller stack
48 // frames is very primitive.
49 //
50 //===----------------------------------------------------------------------===//
51 
52 #ifndef LLVM_TRANSFORMS_SCALAR_TAILRECURSIONELIMINATION_H
53 #define LLVM_TRANSFORMS_SCALAR_TAILRECURSIONELIMINATION_H
54 
55 #include "llvm/IR/Function.h"
56 #include "llvm/IR/PassManager.h"
57 
58 namespace llvm {
59 
60 struct TailCallElimPass : PassInfoMixin<TailCallElimPass> {
62 };
63 }
64 
65 #endif // LLVM_TRANSFORMS_SCALAR_TAILRECURSIONELIMINATION_H
This class represents lattice values for constants.
Definition: AllocatorList.h:23
F(f)
A CRTP mix-in to automatically provide informational APIs needed for passes.
Definition: PassManager.h:372
A set of analyses that are preserved following a run of a transformation pass.
Definition: PassManager.h:153
PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM)
A container for analyses that lazily runs them and caches their results.
This header defines various interfaces for pass management in LLVM.