include/llvm/CodeGen/AtomicExpandUtils.h

   1 //===- AtomicExpandUtils.h - Utilities for expanding atomic instructions --===//
   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 #ifndef LLVM_CODEGEN_ATOMICEXPANDUTILS_H
  10 #define LLVM_CODEGEN_ATOMICEXPANDUTILS_H
  11
  12 #include "llvm/ADT/STLExtras.h"
  13 #include "llvm/IR/IRBuilder.h"
  14 #include "llvm/Support/AtomicOrdering.h"
  15
  16 namespace llvm {
  17
  18 class AtomicRMWInst;
  19 class Value;
  20
  21 /// Parameters (see the expansion example below):
  22 /// (the builder, %addr, %loaded, %new_val, ordering,
  23 ///  /* OUT */ %success, /* OUT */ %new_loaded)
  24 using CreateCmpXchgInstFun =
  25     function_ref<void(IRBuilder<> &, Value *, Value *, Value *, AtomicOrdering,
  26                       Value *&, Value *&)>;
  27
  28 /// Expand an atomic RMW instruction into a loop utilizing
  29 /// cmpxchg. You'll want to make sure your target machine likes cmpxchg
  30 /// instructions in the first place and that there isn't another, better,
  31 /// transformation available (for example AArch32/AArch64 have linked loads).
  32 ///
  33 /// This is useful in passes which can't rewrite the more exotic RMW
  34 /// instructions directly into a platform specific intrinsics (because, say,
  35 /// those intrinsics don't exist). If such a pass is able to expand cmpxchg
  36 /// instructions directly however, then, with this function, it could avoid two
  37 /// extra module passes (avoiding passes by `-atomic-expand` and itself). A
  38 /// specific example would be PNaCl's `RewriteAtomics` pass.
  39 ///
  40 /// Given: atomicrmw some_op iN* %addr, iN %incr ordering
  41 ///
  42 /// The standard expansion we produce is:
  43 ///     [...]
  44 ///     %init_loaded = load atomic iN* %addr
  45 ///     br label %loop
  46 /// loop:
  47 ///     %loaded = phi iN [ %init_loaded, %entry ], [ %new_loaded, %loop ]
  48 ///     %new = some_op iN %loaded, %incr
  49 /// ; This is what -atomic-expand will produce using this function on i686
  50 /// targets:
  51 ///     %pair = cmpxchg iN* %addr, iN %loaded, iN %new_val
  52 ///     %new_loaded = extractvalue { iN, i1 } %pair, 0
  53 ///     %success = extractvalue { iN, i1 } %pair, 1
  54 /// ; End callback produced IR
  55 ///     br i1 %success, label %atomicrmw.end, label %loop
  56 /// atomicrmw.end:
  57 ///     [...]
  58 ///
  59 /// Returns true if the containing function was modified.
  60 bool expandAtomicRMWToCmpXchg(AtomicRMWInst *AI, CreateCmpXchgInstFun CreateCmpXchg);
  61
  62 } // end namespace llvm
  63
  64 #endif // LLVM_CODEGEN_ATOMICEXPANDUTILS_H