clang-tools-extra/clang-tidy/altera/StructPackAlignCheck.cpp

   1 //===--- StructPackAlignCheck.cpp - clang-tidy ----------------------------===//
   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 #include "StructPackAlignCheck.h"
  10 #include "clang/AST/ASTContext.h"
  11 #include "clang/AST/RecordLayout.h"
  12 #include "clang/ASTMatchers/ASTMatchFinder.h"
  13 #include <cmath>
  14
  15 using namespace clang::ast_matchers;
  16
  17 namespace clang::tidy::altera {
  18
  19 void StructPackAlignCheck::registerMatchers(MatchFinder *Finder) {
  20   Finder->addMatcher(recordDecl(isStruct(), isDefinition(),
  21                                 unless(isExpansionInSystemHeader()))
  22                          .bind("struct"),
  23                      this);
  24 }
  25
  26 CharUnits
  27 StructPackAlignCheck::computeRecommendedAlignment(CharUnits MinByteSize) const {
  28   CharUnits NewAlign = CharUnits::fromQuantity(1);
  29   if (!MinByteSize.isPowerOfTwo()) {
  30     CharUnits::QuantityType MSB = MinByteSize.getQuantity();
  31     for (; MSB > 0; MSB /= 2) {
  32       NewAlign =
  33           NewAlign.alignTo(CharUnits::fromQuantity(NewAlign.getQuantity() * 2));
  34       // Abort if the computed alignment meets the maximum configured alignment.
  35       if (NewAlign.getQuantity() >= MaxConfiguredAlignment)
  36         break;
  37     }
  38   } else {
  39     NewAlign = MinByteSize;
  40   }
  41   return NewAlign;
  42 }
  43
  44 void StructPackAlignCheck::check(const MatchFinder::MatchResult &Result) {
  45   const auto *Struct = Result.Nodes.getNodeAs<RecordDecl>("struct");
  46
  47   // Do not trigger on templated struct declarations because the packing and
  48   // alignment requirements are unknown.
  49   if (Struct->isTemplated())
  50      return;
  51
  52   // Packing and alignment requirements for invalid decls are meaningless.
  53   if (Struct->isInvalidDecl())
  54     return;
  55
  56   // Get sizing info for the struct.
  57   llvm::SmallVector<std::pair<unsigned int, unsigned int>, 10> FieldSizes;
  58   unsigned int TotalBitSize = 0;
  59   for (const FieldDecl *StructField : Struct->fields()) {
  60     // For each StructField, record how big it is (in bits).
  61     // Would be good to use a pair of <offset, size> to advise a better
  62     // packing order.
  63     QualType StructFieldTy = StructField->getType();
  64     if (StructFieldTy->isIncompleteType())
  65       return;
  66     unsigned int StructFieldWidth =
  67         (unsigned int)Result.Context->getTypeInfo(StructFieldTy.getTypePtr())
  68             .Width;
  69     FieldSizes.emplace_back(StructFieldWidth, StructField->getFieldIndex());
  70     // FIXME: Recommend a reorganization of the struct (sort by StructField
  71     // size, largest to smallest).
  72     TotalBitSize += StructFieldWidth;
  73   }
  74
  75   uint64_t CharSize = Result.Context->getCharWidth();
  76   CharUnits CurrSize = Result.Context->getASTRecordLayout(Struct).getSize();
  77   CharUnits MinByteSize =
  78       CharUnits::fromQuantity(std::max<clang::CharUnits::QuantityType>(
  79           ceil(static_cast<float>(TotalBitSize) / CharSize), 1));
  80   CharUnits MaxAlign = CharUnits::fromQuantity(
  81       ceil((float)Struct->getMaxAlignment() / CharSize));
  82   CharUnits CurrAlign =
  83       Result.Context->getASTRecordLayout(Struct).getAlignment();
  84   CharUnits NewAlign = computeRecommendedAlignment(MinByteSize);
  85
  86   bool IsPacked = Struct->hasAttr<PackedAttr>();
  87   bool NeedsPacking = (MinByteSize < CurrSize) && (MaxAlign != NewAlign) &&
  88                       (CurrSize != NewAlign);
  89   bool NeedsAlignment = CurrAlign.getQuantity() != NewAlign.getQuantity();
  90
  91   if (!NeedsAlignment && !NeedsPacking)
  92     return;
  93
  94   // If it's using much more space than it needs, suggest packing.
  95   // (Do not suggest packing if it is currently explicitly aligned to what the
  96   // minimum byte size would suggest as the new alignment.)
  97   if (NeedsPacking && !IsPacked) {
  98     diag(Struct->getLocation(),
  99          "accessing fields in struct %0 is inefficient due to padding; only "
 100          "needs %1 bytes but is using %2 bytes")
 101         << Struct << (int)MinByteSize.getQuantity()
 102         << (int)CurrSize.getQuantity()
 103         << FixItHint::CreateInsertion(Struct->getEndLoc().getLocWithOffset(1),
 104                                       " __attribute__((packed))");
 105     diag(Struct->getLocation(),
 106          "use \"__attribute__((packed))\" to reduce the amount of padding "
 107          "applied to struct %0",
 108          DiagnosticIDs::Note)
 109         << Struct;
 110   }
 111
 112   FixItHint FixIt;
 113   auto *Attribute = Struct->getAttr<AlignedAttr>();
 114   std::string NewAlignQuantity = std::to_string((int)NewAlign.getQuantity());
 115   if (Attribute) {
 116     FixIt = FixItHint::CreateReplacement(
 117         Attribute->getRange(),
 118         (Twine("aligned(") + NewAlignQuantity + ")").str());
 119   } else {
 120     FixIt = FixItHint::CreateInsertion(
 121         Struct->getEndLoc().getLocWithOffset(1),
 122         (Twine(" __attribute__((aligned(") + NewAlignQuantity + ")))").str());
 123   }
 124
 125   // And suggest the minimum power-of-two alignment for the struct as a whole
 126   // (with and without packing).
 127   if (NeedsAlignment) {
 128     diag(Struct->getLocation(),
 129          "accessing fields in struct %0 is inefficient due to poor alignment; "
 130          "currently aligned to %1 bytes, but recommended alignment is %2 bytes")
 131         << Struct << (int)CurrAlign.getQuantity() << NewAlignQuantity << FixIt;
 132
 133     diag(Struct->getLocation(),
 134          "use \"__attribute__((aligned(%0)))\" to align struct %1 to %0 bytes",
 135          DiagnosticIDs::Note)
 136         << NewAlignQuantity << Struct;
 137   }
 138 }
 139
 140 void StructPackAlignCheck::storeOptions(ClangTidyOptions::OptionMap &Opts) {
 141   Options.store(Opts, "MaxConfiguredAlignment", MaxConfiguredAlignment);
 142 }
 143
 144 } // namespace clang::tidy::altera