llvm/polly/include/polly/CodeGen/IslExprBuilder.h

//===-IslExprBuilder.h - Helper to generate code for isl AST expressions --===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
//===----------------------------------------------------------------------===//

#ifndef POLLY_ISL_EXPR_BUILDER_H
#define POLLY_ISL_EXPR_BUILDER_H

#include "polly/CodeGen/IRBuilder.h"
#include "polly/Support/ScopHelper.h"
#include "isl/isl-noexceptions.h"

namespace llvm {
// Provide PointerLikeTypeTraits for isl_id.
template <> struct PointerLikeTypeTraits<isl_id *> { … };
} // namespace llvm

namespace polly {
class ScopArrayInfo;

/// LLVM-IR generator for isl_ast_expr[essions]
///
/// This generator generates LLVM-IR that performs the computation described by
/// an isl_ast_expr[ession].
///
/// Example:
///
///   An isl_ast_expr[ession] can look like this:
///
///     (N + M) + 10
///
///   The IslExprBuilder could create the following LLVM-IR:
///
///     %tmp1 = add nsw i64 %N
///     %tmp2 = add nsw i64 %tmp1, %M
///     %tmp3 = add nsw i64 %tmp2, 10
///
/// The implementation of this class is mostly a mapping from isl_ast_expr
/// constructs to the corresponding LLVM-IR constructs.
///
/// The following decisions may need some explanation:
///
/// 1) Which data-type to choose
///
/// isl_ast_expr[essions] are untyped expressions that assume arbitrary
/// precision integer computations. LLVM-IR instead has fixed size integers.
/// When lowering to LLVM-IR we need to chose both the size of the data type and
/// the sign of the operations we use.
///
/// At the moment, we hardcode i64 bit signed computations. Our experience has
/// shown that 64 bit are generally large enough for the loop bounds that appear
/// in the wild. Signed computations are needed, as loop bounds may become
/// negative.
///
/// It is possible to track overflows that occurred in the generated IR. See the
/// description of @see OverflowState for more information.
///
/// FIXME: Hardcoding sizes can cause issues:
///
///   -  On embedded systems and especially for high-level-synthesis 64 bit
///      computations are very costly.
///
///   The right approach is to compute the minimal necessary bitwidth and
///   signedness for each subexpression during in the isl AST generation and
///   to use this information in our IslAstGenerator. Preliminary patches are
///   available, but have not been committed yet.
///
class IslExprBuilder final { … };
} // namespace polly

#endif