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> { 61 PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM); 62 }; 63 } 64 65 #endif // LLVM_TRANSFORMS_SCALAR_TAILRECURSIONELIMINATION_H 66
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