//===---- TailRecursionElimination.h ----------------------------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file transforms calls of the current function (self recursion) followed
// by a return instruction with a branch to the entry of the function, creating
// a loop. This pass also implements the following extensions to the basic
// algorithm:
//
// 1. Trivial instructions between the call and return do not prevent the
// transformation from taking place, though currently the analysis cannot
// support moving any really useful instructions (only dead ones).
// 2. This pass transforms functions that are prevented from being tail
// recursive by an associative and commutative expression to use an
// accumulator variable, thus compiling the typical naive factorial or
// 'fib' implementation into efficient code.
// 3. TRE is performed if the function returns void, if the return
// returns the result returned by the call, or if the function returns a
// run-time constant on all exits from the function. It is possible, though
// unlikely, that the return returns something else (like constant 0), and
// can still be TRE'd. It can be TRE'd if ALL OTHER return instructions in
// the function return the exact same value.
// 4. If it can prove that callees do not access their caller stack frame,
// they are marked as eligible for tail call elimination (by the code
// generator).
//
// There are several improvements that could be made:
//
// 1. If the function has any alloca instructions, these instructions will be
// moved out of the entry block of the function, causing them to be
// evaluated each time through the tail recursion. Safely keeping allocas
// in the entry block requires analysis to proves that the tail-called
// function does not read or write the stack object.
// 2. Tail recursion is only performed if the call immediately precedes the
// return instruction. It's possible that there could be a jump between
// the call and the return.
// 3. There can be intervening operations between the call and the return that
// prevent the TRE from occurring. For example, there could be GEP's and
// stores to memory that will not be read or written by the call. This
// requires some substantial analysis (such as with DSA) to prove safe to
// move ahead of the call, but doing so could allow many more TREs to be
// performed, for example in TreeAdd/TreeAlloc from the treeadd benchmark.
// 4. The algorithm we use to detect if callees access their caller stack
// frames is very primitive.
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_TRANSFORMS_SCALAR_TAILRECURSIONELIMINATION_H
#define LLVM_TRANSFORMS_SCALAR_TAILRECURSIONELIMINATION_H
#include "llvm/IR/PassManager.h"
namespace llvm {
class Function;
struct TailCallElimPass : PassInfoMixin<TailCallElimPass> {
PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
};
}
#endif // LLVM_TRANSFORMS_SCALAR_TAILRECURSIONELIMINATION_H