Unaligned Access Warning Added

Added warning for potential cases of
unaligned access when option
-mno-unaligned-access has been specified

Author: mub-at-arm

clang/include/clang/Basic/DiagnosticASTKinds.td

@@ -590,4 +590,9 @@ def warn_padded_struct_size : Warning<
   InGroup<Padded>, DefaultIgnore;
 def warn_unnecessary_packed : Warning<
   "packed attribute is unnecessary for %0">, InGroup<Packed>, DefaultIgnore;
+
+// -Wunaligned-access
+def warn_unaligned_access : Warning<
+  "field %1 within its parent %0 has an alignment greater than its parent "
+  "this may be caused by %0 being packed and can lead to unaligned accesses">, InGroup<UnalignedAccess>, DefaultIgnore;
 }

clang/include/clang/Basic/DiagnosticGroups.td

@@ -542,6 +542,7 @@ def ExplicitInitializeCall : DiagGroup<"explicit-initialize-call">;
 def OrderedCompareFunctionPointers : DiagGroup<"ordered-compare-function-pointers">;
 def Packed : DiagGroup<"packed">;
 def Padded : DiagGroup<"padded">;
+def UnalignedAccess : DiagGroup<"unaligned-access">;
 
 def PessimizingMove : DiagGroup<"pessimizing-move">;
 def ReturnStdMove : DiagGroup<"return-std-move">;

clang/lib/AST/RecordLayoutBuilder.cpp

@@ -2021,6 +2021,7 @@ void ItaniumRecordLayoutBuilder::LayoutField(const FieldDecl *D,
   CharUnits UnpackedFieldAlign =
       !DefaultsToAIXPowerAlignment ? FieldAlign : PreferredAlign;
   CharUnits UnpackedFieldOffset = FieldOffset;
+  CharUnits OriginalFieldAlign = UnpackedFieldAlign;
 
   if (FieldPacked) {
     FieldAlign = CharUnits::One();
@@ -2105,6 +2106,22 @@ void ItaniumRecordLayoutBuilder::LayoutField(const FieldDecl *D,
   // Remember max struct/class ABI-specified alignment.
   UnadjustedAlignment = std::max(UnadjustedAlignment, FieldAlign);
   UpdateAlignment(FieldAlign, UnpackedFieldAlign, PreferredAlign);
+
+  // For checking the alignment of inner fields against
+  // the alignment of its parent record.
+  if (const RecordDecl *RD = D->getParent()) {
+    // Check if packed attribute or pragma pack is present.
+    if (RD->hasAttr<PackedAttr>() || !MaxFieldAlignment.isZero())
+      if (FieldAlign < OriginalFieldAlign)
+        if (D->getType()->isRecordType()) {
+          // If the offset is a multiple of the alignment of
+          // the type, raise the warning.
+          // TODO: Takes no account the alignment of the outer struct
+          if (FieldOffset % OriginalFieldAlign != 0)
+            Diag(D->getLocation(), diag::warn_unaligned_access)
+                << Context.getTypeDeclType(RD) << D->getName();
+        }
+  }
 }
 
 void ItaniumRecordLayoutBuilder::FinishLayout(const NamedDecl *D) {

clang/lib/Driver/ToolChains/Arch/AArch64.cpp

@@ -221,6 +221,7 @@ getAArch64MicroArchFeaturesFromMcpu(const Driver &D, StringRef Mcpu,
 void aarch64::getAArch64TargetFeatures(const Driver &D,
                                        const llvm::Triple &Triple,
                                        const ArgList &Args,
+                                       llvm::opt::ArgStringList &CmdArgs,
                                        std::vector<StringRef> &Features,
                                        bool ForAS) {
   Arg *A;
@@ -464,10 +465,14 @@ void aarch64::getAArch64TargetFeatures(const Driver &D,
 
   if (Arg *A = Args.getLastArg(options::OPT_mno_unaligned_access,
                                options::OPT_munaligned_access)) {
-    if (A->getOption().matches(options::OPT_mno_unaligned_access))
+    if (A->getOption().matches(options::OPT_mno_unaligned_access)) {
       Features.push_back("+strict-align");
-  } else if (Triple.isOSOpenBSD())
+      CmdArgs.push_back("-Wunaligned-access");
+    }
+  } else if (Triple.isOSOpenBSD()) {
     Features.push_back("+strict-align");
+    CmdArgs.push_back("-Wunaligned-access");
+  }
 
   if (Args.hasArg(options::OPT_ffixed_x1))
     Features.push_back("+reserve-x1");

clang/lib/Driver/ToolChains/Arch/AArch64.h

@@ -22,6 +22,7 @@ namespace aarch64 {
 
 void getAArch64TargetFeatures(const Driver &D, const llvm::Triple &Triple,
                               const llvm::opt::ArgList &Args,
+                              llvm::opt::ArgStringList &CmdArgs,
                               std::vector<llvm::StringRef> &Features,
                               bool ForAS);
 

clang/lib/Driver/ToolChains/Arch/ARM.cpp

@@ -769,10 +769,11 @@ void arm::getARMTargetFeatures(const Driver &D, const llvm::Triple &Triple,
   }
 
   // Kernel code has more strict alignment requirements.
-  if (KernelOrKext)
+  if (KernelOrKext) {
     Features.push_back("+strict-align");
-  else if (Arg *A = Args.getLastArg(options::OPT_mno_unaligned_access,
-                                    options::OPT_munaligned_access)) {
+    CmdArgs.push_back("-Wunaligned-access");
+  } else if (Arg *A = Args.getLastArg(options::OPT_mno_unaligned_access,
+                                      options::OPT_munaligned_access)) {
     if (A->getOption().matches(options::OPT_munaligned_access)) {
       // No v6M core supports unaligned memory access (v6M ARM ARM A3.2).
       if (Triple.getSubArch() == llvm::Triple::SubArchType::ARMSubArch_v6m)
@@ -781,8 +782,10 @@ void arm::getARMTargetFeatures(const Driver &D, const llvm::Triple &Triple,
       // access either.
       else if (Triple.getSubArch() == llvm::Triple::SubArchType::ARMSubArch_v8m_baseline)
         D.Diag(diag::err_target_unsupported_unaligned) << "v8m.base";
-    } else
+    } else {
       Features.push_back("+strict-align");
+      CmdArgs.push_back("-Wunaligned-access");
+    }
   } else {
     // Assume pre-ARMv6 doesn't support unaligned accesses.
     //
@@ -801,14 +804,20 @@ void arm::getARMTargetFeatures(const Driver &D, const llvm::Triple &Triple,
     int VersionNum = getARMSubArchVersionNumber(Triple);
     if (Triple.isOSDarwin() || Triple.isOSNetBSD()) {
       if (VersionNum < 6 ||
-          Triple.getSubArch() == llvm::Triple::SubArchType::ARMSubArch_v6m)
+          Triple.getSubArch() == llvm::Triple::SubArchType::ARMSubArch_v6m) {
         Features.push_back("+strict-align");
+        CmdArgs.push_back("-Wunaligned-access");
+      }
     } else if (Triple.isOSLinux() || Triple.isOSNaCl() ||
                Triple.isOSWindows()) {
-      if (VersionNum < 7)
+      if (VersionNum < 7) {
         Features.push_back("+strict-align");
-    } else
+        CmdArgs.push_back("-Wunaligned-access");
+      }
+    } else {
       Features.push_back("+strict-align");
+      CmdArgs.push_back("-Wunaligned-access");
+    }
   }
 
   // llvm does not support reserving registers in general. There is support

clang/lib/Driver/ToolChains/Clang.cpp

@@ -346,7 +346,8 @@ static void getTargetFeatures(const Driver &D, const llvm::Triple &Triple,
   case llvm::Triple::aarch64:
   case llvm::Triple::aarch64_32:
   case llvm::Triple::aarch64_be:
-    aarch64::getAArch64TargetFeatures(D, Triple, Args, Features, ForAS);
+    aarch64::getAArch64TargetFeatures(D, Triple, Args, CmdArgs, Features,
+                                      ForAS);
     break;
   case llvm::Triple::x86:
   case llvm::Triple::x86_64: