[clang] Add support for new loop attribute [[clang::code_align()]] (#70762)

This patch adds support for new loop attribute:
[[clang::code_align(N)]].
This attribute applies to a loop and specifies the byte alignment for a
loop.
The attribute accepts a positive integer constant initialization
expression
indicating the number of bytes for the minimum alignment boundary.
Its value must be a power of 2, between 1 and 4096 (inclusive).

Author: smanna12

clang/docs/ReleaseNotes.rst

@@ -317,6 +317,23 @@ Attribute Changes in Clang
   should be a coroutine. A non-coroutine function marked with ``[[clang::coro_wrapper]]``
   is still allowed to return the such a type. This is helpful for analyzers to recognize coroutines from the function signatures.
 
+- Clang now supports ``[[clang::code_align(N)]]`` as an attribute which can be
+  applied to a loop and specifies the byte alignment for a loop. This attribute
+  accepts a positive integer constant initialization expression indicating the
+  number of bytes for the minimum alignment boundary. Its value must be a power
+  of 2, between 1 and 4096(inclusive).
+
+  .. code-block:: c++
+
+      void Array(int *array, size_t n) {
+        [[clang::code_align(64)]] for (int i = 0; i < n; ++i) array[i] = 0;
+      }
+
+      template<int A>
+      void func() {
+        [[clang::code_align(A)]] for(;;) { }
+      }
+
 Improvements to Clang's diagnostics
 -----------------------------------
 - Clang constexpr evaluator now prints template arguments when displaying

clang/include/clang/Basic/Attr.td

@@ -4313,3 +4313,15 @@ def PreferredType: InheritableAttr {
   let Args = [TypeArgument<"Type", 1>];
   let Documentation = [PreferredTypeDocumentation];
 }
+
+def CodeAlign: StmtAttr {
+  let Spellings = [Clang<"code_align">];
+  let Subjects = SubjectList<[ForStmt, CXXForRangeStmt, WhileStmt, DoStmt],
+                              ErrorDiag, "'for', 'while', and 'do' statements">;
+  let Args = [ExprArgument<"Alignment">];
+  let Documentation = [CodeAlignAttrDocs];
+  let AdditionalMembers = [{
+    static constexpr int MinimumAlignment = 1;
+    static constexpr int MaximumAlignment = 4096;
+  }];
+}

clang/include/clang/Basic/AttrDocs.td

@@ -7540,3 +7540,44 @@ Note: ``a_promise_type::get_return_object`` is exempted from this analysis as it
 implementation detail of any coroutine library.
 }];
 }
+
+def CodeAlignAttrDocs : Documentation {
+  let Category = DocCatVariable;
+  let Heading = "clang::code_align";
+  let Content = [{
+The ``clang::code_align(N)`` attribute applies to a loop and specifies the byte
+alignment for a loop. The attribute accepts a positive integer constant
+initialization expression indicating the number of bytes for the minimum
+alignment boundary. Its value must be a power of 2, between 1 and 4096
+(inclusive).
+
+.. code-block:: c++
+
+  void foo() {
+    int var = 0;
+    [[clang::code_align(16)]] for (int i = 0; i < 10; ++i) var++;
+  }
+
+  void Array(int *array, size_t n) {
+    [[clang::code_align(64)]] for (int i = 0; i < n; ++i) array[i] = 0;
+  }
+
+  void count () {
+    int a1[10], int i = 0;
+    [[clang::code_align(32)]] while (i < 10) { a1[i] += 3; }
+  }
+
+  void check() {
+    int a = 10;
+    [[clang::code_align(8)]] do {
+      a = a + 1;
+    } while (a < 20);
+  }
+
+  template<int A>
+  void func() {
+    [[clang::code_align(A)]] for(;;) { }
+  }
+
+  }];
+}

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -10025,6 +10025,11 @@ def err_duplicate_case_differing_expr : Error<
 def warn_case_empty_range : Warning<"empty case range specified">;
 def warn_missing_case_for_condition :
   Warning<"no case matching constant switch condition '%0'">;
+def err_loop_attr_conflict : Error<
+  "conflicting loop attribute %0">;
+def err_attribute_power_of_two_in_range : Error<
+  "%0 attribute requires an integer argument which is a constant power of two "
+  "between %1 and %2 inclusive; provided argument was %3">;
 
 def warn_def_missing_case : Warning<"%plural{"
   "1:enumeration value %1 not explicitly handled in switch|"

clang/include/clang/Sema/Sema.h

@@ -2099,6 +2099,9 @@ class Sema final {
   QualType BuildAddressSpaceAttr(QualType &T, Expr *AddrSpace,
                                  SourceLocation AttrLoc);
 
+  CodeAlignAttr *BuildCodeAlignAttr(const AttributeCommonInfo &CI, Expr *E);
+  bool CheckRebuiltCodeAlignStmtAttributes(ArrayRef<const Attr *> Attrs);
+
   bool CheckQualifiedFunctionForTypeId(QualType T, SourceLocation Loc);
 
   bool CheckFunctionReturnType(QualType T, SourceLocation Loc);

clang/lib/CodeGen/CGLoopInfo.cpp

@@ -440,6 +440,14 @@ MDNode *LoopInfo::createMetadata(
         Ctx, {MDString::get(Ctx, "llvm.loop.parallel_accesses"), AccGroup}));
   }
 
+  // Setting clang::code_align attribute.
+  if (Attrs.CodeAlign > 0) {
+    Metadata *Vals[] = {MDString::get(Ctx, "llvm.loop.align"),
+                        ConstantAsMetadata::get(ConstantInt::get(
+                            llvm::Type::getInt32Ty(Ctx), Attrs.CodeAlign))};
+    LoopProperties.push_back(MDNode::get(Ctx, Vals));
+  }
+
   LoopProperties.insert(LoopProperties.end(), AdditionalLoopProperties.begin(),
                         AdditionalLoopProperties.end());
   return createFullUnrollMetadata(Attrs, LoopProperties, HasUserTransforms);
@@ -453,7 +461,7 @@ LoopAttributes::LoopAttributes(bool IsParallel)
       VectorizeScalable(LoopAttributes::Unspecified), InterleaveCount(0),
       UnrollCount(0), UnrollAndJamCount(0),
       DistributeEnable(LoopAttributes::Unspecified), PipelineDisabled(false),
-      PipelineInitiationInterval(0), MustProgress(false) {}
+      PipelineInitiationInterval(0), CodeAlign(0), MustProgress(false) {}
 
 void LoopAttributes::clear() {
   IsParallel = false;
@@ -469,6 +477,7 @@ void LoopAttributes::clear() {
   DistributeEnable = LoopAttributes::Unspecified;
   PipelineDisabled = false;
   PipelineInitiationInterval = 0;
+  CodeAlign = 0;
   MustProgress = false;
 }
 
@@ -493,8 +502,8 @@ LoopInfo::LoopInfo(BasicBlock *Header, const LoopAttributes &Attrs,
       Attrs.VectorizeEnable == LoopAttributes::Unspecified &&
       Attrs.UnrollEnable == LoopAttributes::Unspecified &&
       Attrs.UnrollAndJamEnable == LoopAttributes::Unspecified &&
-      Attrs.DistributeEnable == LoopAttributes::Unspecified && !StartLoc &&
-      !EndLoc && !Attrs.MustProgress)
+      Attrs.DistributeEnable == LoopAttributes::Unspecified &&
+      Attrs.CodeAlign == 0 && !StartLoc && !EndLoc && !Attrs.MustProgress)
     return;
 
   TempLoopID = MDNode::getTemporary(Header->getContext(), std::nullopt);
@@ -788,6 +797,15 @@ void LoopInfoStack::push(BasicBlock *Header, clang::ASTContext &Ctx,
     }
   }
 
+  // Identify loop attribute 'code_align' from Attrs.
+  // For attribute code_align:
+  // n - 'llvm.loop.align i32 n' metadata will be emitted.
+  if (const auto *CodeAlign = getSpecificAttr<const CodeAlignAttr>(Attrs)) {
+    const auto *CE = cast<ConstantExpr>(CodeAlign->getAlignment());
+    llvm::APSInt ArgVal = CE->getResultAsAPSInt();
+    setCodeAlign(ArgVal.getSExtValue());
+  }
+
   setMustProgress(MustProgress);
 
   if (CGOpts.OptimizationLevel > 0)

clang/lib/CodeGen/CGLoopInfo.h

@@ -79,6 +79,9 @@ struct LoopAttributes {
   /// Value for llvm.loop.pipeline.iicount metadata.
   unsigned PipelineInitiationInterval;
 
+  /// Value for 'llvm.loop.align' metadata.
+  unsigned CodeAlign;
+
   /// Value for whether the loop is required to make progress.
   bool MustProgress;
 };
@@ -282,6 +285,9 @@ class LoopInfoStack {
     StagedAttrs.PipelineInitiationInterval = C;
   }
 
+  /// Set value of code align for the next loop pushed.
+  void setCodeAlign(unsigned C) { StagedAttrs.CodeAlign = C; }
+
   /// Set no progress for the next loop pushed.
   void setMustProgress(bool P) { StagedAttrs.MustProgress = P; }
 

clang/lib/Sema/SemaStmtAttr.cpp

@@ -322,6 +322,81 @@ static Attr *handleUnlikely(Sema &S, Stmt *St, const ParsedAttr &A,
   return ::new (S.Context) UnlikelyAttr(S.Context, A);
 }
 
+CodeAlignAttr *Sema::BuildCodeAlignAttr(const AttributeCommonInfo &CI,
+                                        Expr *E) {
+  if (!E->isValueDependent()) {
+    llvm::APSInt ArgVal;
+    ExprResult Res = VerifyIntegerConstantExpression(E, &ArgVal);
+    if (Res.isInvalid())
+      return nullptr;
+    E = Res.get();
+
+    // This attribute requires an integer argument which is a constant power of
+    // two between 1 and 4096 inclusive.
+    if (ArgVal < CodeAlignAttr::MinimumAlignment ||
+        ArgVal > CodeAlignAttr::MaximumAlignment || !ArgVal.isPowerOf2()) {
+      if (std::optional<int64_t> Value = ArgVal.trySExtValue())
+        Diag(CI.getLoc(), diag::err_attribute_power_of_two_in_range)
+            << CI << CodeAlignAttr::MinimumAlignment
+            << CodeAlignAttr::MaximumAlignment << Value.value();
+      else
+        Diag(CI.getLoc(), diag::err_attribute_power_of_two_in_range)
+            << CI << CodeAlignAttr::MinimumAlignment
+            << CodeAlignAttr::MaximumAlignment << E;
+      return nullptr;
+    }
+  }
+  return new (Context) CodeAlignAttr(Context, CI, E);
+}
+
+static Attr *handleCodeAlignAttr(Sema &S, Stmt *St, const ParsedAttr &A) {
+
+  Expr *E = A.getArgAsExpr(0);
+  return S.BuildCodeAlignAttr(A, E);
+}
+
+// Diagnose non-identical duplicates as a 'conflicting' loop attributes
+// and suppress duplicate errors in cases where the two match for
+// [[clang::code_align()]] attribute.
+static void CheckForDuplicateCodeAlignAttrs(Sema &S,
+                                            ArrayRef<const Attr *> Attrs) {
+  auto FindFunc = [](const Attr *A) { return isa<const CodeAlignAttr>(A); };
+  const auto *FirstItr = std::find_if(Attrs.begin(), Attrs.end(), FindFunc);
+
+  if (FirstItr == Attrs.end()) // no attributes found
+    return;
+
+  const auto *LastFoundItr = FirstItr;
+  std::optional<llvm::APSInt> FirstValue;
+
+  const auto *CAFA =
+      dyn_cast<ConstantExpr>(cast<CodeAlignAttr>(*FirstItr)->getAlignment());
+  // Return early if first alignment expression is dependent (since we don't
+  // know what the effective size will be), and skip the loop entirely.
+  if (!CAFA)
+    return;
+
+  while (Attrs.end() != (LastFoundItr = std::find_if(LastFoundItr + 1,
+                                                     Attrs.end(), FindFunc))) {
+    const auto *CASA = dyn_cast<ConstantExpr>(
+        cast<CodeAlignAttr>(*LastFoundItr)->getAlignment());
+    // If the value is dependent, we can not test anything.
+    if (!CASA)
+      return;
+    // Test the attribute values.
+    llvm::APSInt SecondValue = CASA->getResultAsAPSInt();
+    if (!FirstValue)
+      FirstValue = CAFA->getResultAsAPSInt();
+
+    if (FirstValue != SecondValue) {
+      S.Diag((*LastFoundItr)->getLocation(), diag::err_loop_attr_conflict)
+          << *FirstItr;
+      S.Diag((*FirstItr)->getLocation(), diag::note_previous_attribute);
+    }
+    return;
+  }
+}
+
 #define WANT_STMT_MERGE_LOGIC
 #include "clang/Sema/AttrParsedAttrImpl.inc"
 #undef WANT_STMT_MERGE_LOGIC
@@ -523,6 +598,8 @@ static Attr *ProcessStmtAttribute(Sema &S, Stmt *St, const ParsedAttr &A,
     return handleLikely(S, St, A, Range);
   case ParsedAttr::AT_Unlikely:
     return handleUnlikely(S, St, A, Range);
+  case ParsedAttr::AT_CodeAlign:
+    return handleCodeAlignAttr(S, St, A);
   default:
     // N.B., ClangAttrEmitter.cpp emits a diagnostic helper that ensures a
     // declaration attribute is not written on a statement, but this code is
@@ -541,4 +618,10 @@ void Sema::ProcessStmtAttributes(Stmt *S, const ParsedAttributes &InAttrs,
   }
 
   CheckForIncompatibleAttributes(*this, OutAttrs);
+  CheckForDuplicateCodeAlignAttrs(*this, OutAttrs);
+}
+
+bool Sema::CheckRebuiltCodeAlignStmtAttributes(ArrayRef<const Attr *> Attrs) {
+  CheckForDuplicateCodeAlignAttrs(*this, Attrs);
+  return false;
 }

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -1374,7 +1374,7 @@ namespace {
     const AlwaysInlineAttr *
     TransformStmtAlwaysInlineAttr(const Stmt *OrigS, const Stmt *InstS,
                                   const AlwaysInlineAttr *A);
-
+    const CodeAlignAttr *TransformCodeAlignAttr(const CodeAlignAttr *CA);
     ExprResult TransformPredefinedExpr(PredefinedExpr *E);
     ExprResult TransformDeclRefExpr(DeclRefExpr *E);
     ExprResult TransformCXXDefaultArgExpr(CXXDefaultArgExpr *E);
@@ -1906,6 +1906,12 @@ const AlwaysInlineAttr *TemplateInstantiator::TransformStmtAlwaysInlineAttr(
   return A;
 }
 
+const CodeAlignAttr *
+TemplateInstantiator::TransformCodeAlignAttr(const CodeAlignAttr *CA) {
+  Expr *TransformedExpr = getDerived().TransformExpr(CA->getAlignment()).get();
+  return getSema().BuildCodeAlignAttr(*CA, TransformedExpr);
+}
+
 ExprResult TemplateInstantiator::transformNonTypeTemplateParmRef(
     Decl *AssociatedDecl, const NonTypeTemplateParmDecl *parm,
     SourceLocation loc, TemplateArgument arg,

clang/lib/Sema/TreeTransform.h

@@ -1378,6 +1378,8 @@ class TreeTransform {
   StmtResult RebuildAttributedStmt(SourceLocation AttrLoc,
                                    ArrayRef<const Attr *> Attrs,
                                    Stmt *SubStmt) {
+    if (SemaRef.CheckRebuiltCodeAlignStmtAttributes(Attrs))
+      return StmtError();
     return SemaRef.BuildAttributedStmt(AttrLoc, Attrs, SubStmt);
   }
 

clang/test/CodeGen/code_align.c

@@ -0,0 +1,59 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -emit-llvm -x c %s %s -o - | FileCheck -check-prefix=CHECK-C %s
+// RUN: %clang_cc1 -fsyntax-only -emit-llvm -x c++ -std=c++11 %s -o - | FileCheck %s --check-prefixes CHECK-C,CHECK-CPP
+
+// CHECK-C: br label %for.cond, !llvm.loop ![[MD_FP:[0-9]+]]
+// CHECK-C: br label %while.cond, !llvm.loop ![[MD_FP_1:[0-9]+]]
+// CHECK-C: br i1 %cmp3, label %do.body, label %do.end, !llvm.loop ![[MD_FP_2:[0-9]+]]
+// CHECK-C: br label %for.cond5, !llvm.loop ![[MD_FP_3:[0-9]+]]
+
+// CHECK-CPP: br label %for.cond, !llvm.loop ![[MD_FP_4:[0-9]+]]
+// CHECK-CPP: br label %for.cond2, !llvm.loop ![[MD_FP_5:[0-9]+]]
+
+void bar(int);
+void code_align() {
+  int a[10];
+  // CHECK-C: ![[MD_FP]] = distinct !{![[MD_FP]], ![[MP:[0-9]+]], ![[MD_code_align:[0-9]+]]}
+  // CHECK-C-NEXT: ![[MP]] = !{!"llvm.loop.mustprogress"}
+  // CHECK-C-NEXT: ![[MD_code_align]] = !{!"llvm.loop.align", i32 4}
+  [[clang::code_align(4)]]
+  for(int I=0; I<128; ++I) { bar(I); }
+
+  // CHECK-C: ![[MD_FP_1]] = distinct !{![[MD_FP_1]], ![[MP]], ![[MD_code_align_1:[0-9]+]]}
+  // CHECK-C-NEXT: ![[MD_code_align_1]] = !{!"llvm.loop.align", i32 16}
+  int i = 0;
+  [[clang::code_align(16)]] while (i < 60) {
+    a[i] += 3;
+  }
+
+  // CHECK-C: ![[MD_FP_2]] = distinct !{![[MD_FP_2]], ![[MP]], ![[MD_code_align_2:[0-9]+]]}
+  // CHECK-C-NEXT: ![[MD_code_align_2]] = !{!"llvm.loop.align", i32 8}
+  int b = 10;
+  [[clang::code_align(8)]] do {
+    b = b + 1;
+  } while (b < 20);
+
+  // CHECK-C: ![[MD_FP_3]] = distinct !{![[MD_FP_3]], ![[MP]], ![[MD_code_align_3:[0-9]+]]}
+  // CHECK-C-NEXT: ![[MD_code_align_3]] = !{!"llvm.loop.align", i32 64}
+  [[clang::code_align(64)]]
+  for(int I=0; I<128; ++I) { bar(I); }
+}
+
+#if __cplusplus >= 201103L
+template <int A, int B>
+void code_align_cpp() {
+  int a[10];	
+  // CHECK-CPP: ![[MD_FP_4]] = distinct !{![[MD_FP_4]], ![[MP]], ![[MD_code_align_4:[0-9]+]]}
+  // CHECK-CPP-NEXT: ![[MD_code_align_4]] = !{!"llvm.loop.align", i32 32}
+  [[clang::code_align(A)]] for (int i = 0; i != 10; ++i)
+    a[i] = 0;
+
+  // CHECK-CPP: ![[MD_FP_5]] = distinct !{![[MD_FP_5]], ![[MD_code_align]]}
+  int c[] = {0, 1, 2, 3, 4, 5};
+  [[clang::code_align(B)]] for (int n : c) { n *= 2; }
+}
+
+int main() {
+  code_align_cpp<32, 4>();
+  return 0;
+}
+#endif