[HLSL] add loop unroll

- `Attr.td` - Define the HLSL loop attribute hints (unroll and loop)
- `AttrDocs.td` - Add documentation for unroll and loop
- `CGLoopInfo.cpp` - Add codegen for HLSL unroll that maps to clang
  unroll expectations
- `ParseStmt.cpp` - For statements if HLSL define DeclSpecAttrs via MaybeParseMicrosoftAttributes
- `SemaStmtAttr.cpp` - Add the HLSL loop unroll handeling

Author: farzonl

clang/include/clang/Basic/Attr.td

@@ -4114,6 +4114,19 @@ def LoopHint : Attr {
   let HasCustomParsing = 1;
 }
 
+/// The HLSL loop attributes
+def HLSLLoopHint: StmtAttr {
+  /// [unroll(directive)]
+  /// [loop]
+  let Spellings = [Microsoft<"unroll">, Microsoft<"loop">];
+  let Args = [UnsignedArgument<"directive">];
+  let Subjects = SubjectList<[ForStmt, WhileStmt, DoStmt],
+                              ErrorDiag, "'for', 'while', and 'do' statements">;
+  let LangOpts = [HLSL];
+  let Documentation = [HLSLLoopHintDocs, HLSLUnrollHintDocs];
+  let HasCustomParsing = 1;
+}
+
 def CapturedRecord : InheritableAttr {
   // This attribute has no spellings as it is only ever created implicitly.
   let Spellings = [];

clang/include/clang/Basic/AttrDocs.td

@@ -7342,6 +7342,53 @@ where shaders must be compiled into a library and linked at runtime.
   }];
 }
 
+def HLSLLoopHintDocs : Documentation {
+  let Category = DocCatStmt;
+  let Heading = "#[loop]";
+  let Content = [{
+The ``[loop]`` directive allows loop optimization hints to be
+specified for the subsequent loop. The directive allows unrolling to
+be disabled and is not compatible with [unroll(x)]. 
+
+Specifying the parameter, ``[loop]``, directs the
+unroller to not unroll the loop. 
+
+.. code-block:: hlsl
+
+  [loop]
+  for (...) {
+    ...
+  }
+
+See `hlsl loop extensions
+<https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl-for>`_
+for details.
+  }];
+}
+
+def HLSLUnrollHintDocs : Documentation {
+  let Category = DocCatStmt;
+  let Heading = "[unroll(x)]";
+  let Content = [{
+Loop unrolling optimization hints can be specified with ``[unroll(x)]``
+. The attribute is placed immediately before a for, while,
+or do-while.
+Specifying the parameter, ``[unroll(_value_)]``, directs the
+unroller to unroll the loop ``_value_`` times. Note: [unroll(x)] is not compatible with [loop].
+
+.. code-block:: hlsl
+
+  [unroll(4)]
+  for (...) {
+    ...
+  }
+See
+`hlsl loop extensions
+<https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl-for>`_
+for details.
+  }];
+}
+
 def ClangRandomizeLayoutDocs : Documentation {
   let Category = DocCatDecl;
   let Heading = "randomize_layout, no_randomize_layout";

clang/lib/CodeGen/CGLoopInfo.cpp

@@ -612,9 +612,9 @@ void LoopInfoStack::push(BasicBlock *Header, clang::ASTContext &Ctx,
     const LoopHintAttr *LH = dyn_cast<LoopHintAttr>(Attr);
     const OpenCLUnrollHintAttr *OpenCLHint =
         dyn_cast<OpenCLUnrollHintAttr>(Attr);
-
+    const HLSLLoopHintAttr *HLSLLoopHint = dyn_cast<HLSLLoopHintAttr>(Attr);
     // Skip non loop hint attributes
-    if (!LH && !OpenCLHint) {
+    if (!LH && !OpenCLHint && !HLSLLoopHint) {
       continue;
     }
 
@@ -635,6 +635,17 @@ void LoopInfoStack::push(BasicBlock *Header, clang::ASTContext &Ctx,
         Option = LoopHintAttr::UnrollCount;
         State = LoopHintAttr::Numeric;
       }
+    } else if (HLSLLoopHint) {
+      ValueInt = HLSLLoopHint->getDirective();
+      if (HLSLLoopHint->getSemanticSpelling() ==
+          HLSLLoopHintAttr::Spelling::Microsoft_unroll) {
+        if (ValueInt == 0)
+          State = LoopHintAttr::Enable;
+        if (ValueInt > 0) {
+          Option = LoopHintAttr::UnrollCount;
+          State = LoopHintAttr::Numeric;
+        }
+      }
     } else if (LH) {
       auto *ValueExpr = LH->getValue();
       if (ValueExpr) {

clang/lib/Parse/ParseStmt.cpp

@@ -114,18 +114,21 @@ Parser::ParseStatementOrDeclaration(StmtVector &Stmts,
   // here because we don't want to allow arbitrary orderings.
   ParsedAttributes CXX11Attrs(AttrFactory);
   MaybeParseCXX11Attributes(CXX11Attrs, /*MightBeObjCMessageSend*/ true);
-  ParsedAttributes GNUAttrs(AttrFactory);
+  ParsedAttributes DeclSpecAttrs(AttrFactory);
   if (getLangOpts().OpenCL)
-    MaybeParseGNUAttributes(GNUAttrs);
+    MaybeParseGNUAttributes(DeclSpecAttrs);
+
+  if (getLangOpts().HLSL)
+    MaybeParseMicrosoftAttributes(DeclSpecAttrs);
 
   StmtResult Res = ParseStatementOrDeclarationAfterAttributes(
-      Stmts, StmtCtx, TrailingElseLoc, CXX11Attrs, GNUAttrs);
+      Stmts, StmtCtx, TrailingElseLoc, CXX11Attrs, DeclSpecAttrs);
   MaybeDestroyTemplateIds();
 
   // Attributes that are left should all go on the statement, so concatenate the
   // two lists.
   ParsedAttributes Attrs(AttrFactory);
-  takeAndConcatenateAttrs(CXX11Attrs, GNUAttrs, Attrs);
+  takeAndConcatenateAttrs(CXX11Attrs, DeclSpecAttrs, Attrs);
 
   assert((Attrs.empty() || Res.isInvalid() || Res.isUsable()) &&
          "attributes on empty statement");

clang/lib/Sema/SemaStmtAttr.cpp

@@ -16,6 +16,7 @@
 #include "clang/Basic/TargetInfo.h"
 #include "clang/Sema/DelayedDiagnostic.h"
 #include "clang/Sema/Lookup.h"
+#include "clang/Sema/ParsedAttr.h"
 #include "clang/Sema/ScopeInfo.h"
 #include "clang/Sema/SemaInternal.h"
 #include "llvm/ADT/StringExtras.h"
@@ -584,6 +585,32 @@ static Attr *handleOpenCLUnrollHint(Sema &S, Stmt *St, const ParsedAttr &A,
   return ::new (S.Context) OpenCLUnrollHintAttr(S.Context, A, UnrollFactor);
 }
 
+static Attr *handleHLSLLoopHintAttr(Sema &S, Stmt *St, const ParsedAttr &A,
+                                    SourceRange Range) {
+  unsigned UnrollFactor = 0;
+  if (A.getNumArgs() == 1) {
+    Expr *E = A.getArgAsExpr(0);
+    std::optional<llvm::APSInt> ArgVal;
+
+    if (!(ArgVal = E->getIntegerConstantExpr(S.Context))) {
+      S.Diag(A.getLoc(), diag::err_attribute_argument_type)
+          << A << AANT_ArgumentIntegerConstant << E->getSourceRange();
+      return nullptr;
+    }
+
+    int Val = ArgVal->getSExtValue();
+    if (Val <= 0) {
+      S.Diag(A.getRange().getBegin(),
+             diag::err_attribute_requires_positive_integer)
+          << A << /* positive */ 0;
+      return nullptr;
+    }
+    UnrollFactor = static_cast<unsigned>(Val);
+  }
+
+  return ::new (S.Context) HLSLLoopHintAttr(S.Context, A, UnrollFactor);
+}
+
 static Attr *ProcessStmtAttribute(Sema &S, Stmt *St, const ParsedAttr &A,
                                   SourceRange Range) {
   if (A.isInvalid() || A.getKind() == ParsedAttr::IgnoredAttribute)
@@ -618,6 +645,8 @@ static Attr *ProcessStmtAttribute(Sema &S, Stmt *St, const ParsedAttr &A,
     return handleFallThroughAttr(S, St, A, Range);
   case ParsedAttr::AT_LoopHint:
     return handleLoopHintAttr(S, St, A, Range);
+  case ParsedAttr::AT_HLSLLoopHint:
+    return handleHLSLLoopHintAttr(S, St, A, Range);
   case ParsedAttr::AT_OpenCLUnrollHint:
     return handleOpenCLUnrollHint(S, St, A, Range);
   case ParsedAttr::AT_Suppress:

clang/test/CodeGenHLSL/loops/unroll.hlsl

@@ -0,0 +1,99 @@
+// RUN: %clang_cc1 -std=hlsl2021 -finclude-default-header -x hlsl -triple \
+// RUN: dxil-pc-shadermodel6.3-library %s -emit-llvm -o - | FileCheck %s
+
+/*** for ***/
+void for_count()
+{
+// CHECK-LABEL: for_count
+    [unroll(8)]
+    for( int i = 0; i < 1000; ++i);
+// CHECK: br label %{{.*}}, !llvm.loop ![[FOR_DISTINCT:.*]]
+}
+
+void for_disable()
+{
+// CHECK-LABEL: for_disable
+    [loop]
+    for( int i = 0; i < 1000; ++i);
+// CHECK: br label %{{.*}}, !llvm.loop ![[FOR_DISABLE:.*]]
+}
+
+void for_enable()
+{
+// CHECK-LABEL: for_enable
+    [unroll]
+    for( int i = 0; i < 1000; ++i);
+// CHECK: br label %{{.*}}, !llvm.loop ![[FOR_ENABLE:.*]]
+}
+
+/*** while ***/
+void while_count()
+{
+// CHECK-LABEL: while_count
+    int i = 1000;
+    [unroll(4)]
+    while(i-->0);
+// CHECK: br label %{{.*}}, !llvm.loop ![[WHILE_DISTINCT:.*]]
+}
+
+void while_disable()
+{
+// CHECK-LABEL: while_disable
+    int i = 1000;
+    [loop]
+    while(i-->0);
+// CHECK: br label %{{.*}}, !llvm.loop ![[WHILE_DISABLE:.*]]
+}
+
+void while_enable()
+{
+// CHECK-LABEL: while_enable
+    int i = 1000;
+    [unroll]
+    while(i-->0);
+// CHECK: br label %{{.*}}, !llvm.loop ![[WHILE_ENABLE:.*]]
+}
+
+/*** do ***/
+void do_count()
+{
+// CHECK-LABEL: do_count
+    int i = 1000;
+    [unroll(16)]
+    do {} while(i--> 0);
+// CHECK: br i1 %{{.*}}, label %{{.*}}, label %{{.*}}, !llvm.loop ![[DO_DISTINCT:.*]]
+}
+
+void do_disable()
+{
+// CHECK-LABEL: do_disable
+    int i = 1000;
+    [loop]
+    do {} while(i--> 0);
+// CHECK: br i1 %{{.*}}, label %{{.*}}, label %{{.*}}, !llvm.loop ![[DO_DISABLE:.*]]
+}
+
+void do_enable()
+{
+// CHECK-LABEL: do_enable
+    int i = 1000;
+    [unroll]
+    do {} while(i--> 0);
+// CHECK: br i1 %{{.*}}, label %{{.*}}, label %{{.*}}, !llvm.loop ![[DO_ENABLE:.*]]
+}
+
+
+// CHECK: ![[FOR_DISTINCT]]     =  distinct !{![[FOR_DISTINCT]],  ![[FOR_COUNT:.*]]}
+// CHECK: ![[FOR_COUNT]]         =  !{!"llvm.loop.unroll.count", i32 8}
+// CHECK: ![[FOR_DISABLE]]   =  distinct !{![[FOR_DISABLE]],  ![[DISABLE:.*]]}
+// CHECK: ![[DISABLE]]       =  !{!"llvm.loop.unroll.disable"}
+// CHECK: ![[FOR_ENABLE]]      =  distinct !{![[FOR_ENABLE]],  ![[ENABLE:.*]]}
+// CHECK: ![[ENABLE]]          =  !{!"llvm.loop.unroll.enable"}
+// CHECK: ![[WHILE_DISTINCT]]   =  distinct !{![[WHILE_DISTINCT]],    ![[WHILE_COUNT:.*]]}
+// CHECK: ![[WHILE_COUNT]]         =  !{!"llvm.loop.unroll.count", i32 4}
+// CHECK: ![[WHILE_DISABLE]] =  distinct !{![[WHILE_DISABLE]],  ![[DISABLE]]}
+// CHECK: ![[WHILE_ENABLE]]    =  distinct !{![[WHILE_ENABLE]],     ![[ENABLE]]}
+// CHECK: ![[DO_DISTINCT]]      =  distinct !{![[DO_DISTINCT]],       ![[DO_COUNT:.*]]}
+// CHECK: ![[DO_COUNT]]         =  !{!"llvm.loop.unroll.count", i32 16}
+// CHECK: ![[DO_DISABLE]]    =  distinct !{![[DO_DISABLE]],     ![[DISABLE]]}
+// CHECK: ![[DO_ENABLE]]       =  distinct !{![[DO_ENABLE]],        ![[ENABLE]]}