[SYCL] Add support for new FPGA function attribute stall_enable (#2734)

This patch adds support a new FPGA function attribute, stall_enable,
to be sent through to the backend (and ignored by the emulator).

Syntax:
[[intel::stall_enable]]

This function attribute applies to a lambda function, or function definition.
Requests, to the extent possible, that statically-scheduled clusters handle
stalls using a stall-enable signal to freeze computation within the cluster.

Stall_enable attribute can be applied to device function/kernel 
and it takes no arguments.  

LLVM IR function metadata should simply be i32 1.

Signed-off-by: Soumi Manna <[email protected]>

Author: smanna12

clang/include/clang/Basic/Attr.td

@@ -1226,6 +1226,19 @@ def SYCLIntelNumSimdWorkItems : InheritableAttr {
   let PragmaAttributeSupport = 0;
 }
 
+def SYCLIntelStallEnable : InheritableAttr {
+  let Spellings = [CXX11<"intel","stall_enable">];
+  let LangOpts = [SYCLIsHost, SYCLIsDevice];
+  let Subjects = SubjectList<[Function], ErrorDiag>;
+  let AdditionalMembers = [{
+    static const char *getName() {
+      return "stall_enable";
+    }
+  }];
+  let Documentation = [SYCLIntelStallEnableAttrDocs];
+  let PragmaAttributeSupport = 0;
+}
+
 def SYCLIntelSchedulerTargetFmaxMhz : InheritableAttr {
   let Spellings = [CXX11<"intelfpga","scheduler_target_fmax_mhz">,
                    CXX11<"intel","scheduler_target_fmax_mhz">];

clang/include/clang/Basic/AttrDocs.td

@@ -2212,6 +2212,39 @@ device kernel, the attribute is ignored and it is not propagated to a kernel.
   }];
 }
 
+def SYCLIntelStallEnableAttrDocs : Documentation {
+  let Category = DocCatFunction;
+  let Heading = "intel::stall_enable";
+  let Content = [{
+When applied to a lambda or function call operator (of a function object)
+on device, this requests, to the extent possible, that statically-scheduled
+clusters handle stalls using a stall-enable signal to freeze computation
+within the cluster. This attribute is ignored on the host.
+
+If ``intel::stall_enable`` is applied to a function called from a device
+kernel, the attribute is ignored and it is not propagated to a kernel.
+
+The ``intel::stall_enable`` attribute takes no argument and has an effect
+when applied to a function, and no effect otherwise.
+
+.. code-block:: c++
+
+  class Functor
+  {
+      [[intel::stall_enable]] void operator()(item<1> item)
+      {
+          /* kernel code */
+      }
+  }
+
+  kernel<class kernel_name>(
+  []() [[intel::stall_enable]] {
+       /* kernel code */
+  });
+
+  }];
+}
+
 def ReqdWorkGroupSizeAttrDocs : Documentation {
   let Category = DocCatFunction;
   let Heading = "reqd_work_group_size";

clang/include/clang/Basic/AttributeCommonInfo.h

@@ -165,7 +165,8 @@ class AttributeCommonInfo {
         ParsedAttr == AT_SYCLIntelSchedulerTargetFmaxMhz ||
         ParsedAttr == AT_SYCLIntelMaxWorkGroupSize ||
         ParsedAttr == AT_SYCLIntelMaxGlobalWorkDim ||
-        ParsedAttr == AT_SYCLIntelNoGlobalWorkOffset)
+        ParsedAttr == AT_SYCLIntelNoGlobalWorkOffset ||
+        ParsedAttr == AT_SYCLIntelStallEnable)
       return true;
 
     return false;

clang/lib/CodeGen/CodeGenFunction.cpp

@@ -676,6 +676,12 @@ void CodeGenFunction::EmitOpenCLKernelMetadata(const FunctionDecl *FD,
     if (A->getEnabled())
       Fn->setMetadata("no_global_work_offset", llvm::MDNode::get(Context, {}));
   }
+
+  if (FD->hasAttr<SYCLIntelStallEnableAttr>()) {
+    llvm::Metadata *AttrMDArgs[] = {
+        llvm::ConstantAsMetadata::get(Builder.getInt32(1))};
+    Fn->setMetadata("stall_enable", llvm::MDNode::get(Context, AttrMDArgs));
+  }
 }
 
 /// Determine whether the function F ends with a return stmt.

clang/lib/Sema/SemaDeclAttr.cpp

@@ -3029,6 +3029,20 @@ static void handleNumSimdWorkItemsAttr(Sema &S, Decl *D,
                                                                      E);
 }
 
+// Handles stall_enable
+static void handleStallEnableAttr(Sema &S, Decl *D, const ParsedAttr &Attr) {
+  if (D->isInvalidDecl())
+    return;
+
+  unsigned NumArgs = Attr.getNumArgs();
+  if (NumArgs > 0) {
+    S.Diag(Attr.getLoc(), diag::warn_attribute_too_many_arguments) << Attr << 0;
+    return;
+  }
+
+  handleSimpleAttribute<SYCLIntelStallEnableAttr>(S, D, Attr);
+}
+
 // Add scheduler_target_fmax_mhz
 void Sema::addSYCLIntelSchedulerTargetFmaxMhzAttr(
     Decl *D, const AttributeCommonInfo &Attr, Expr *E) {
@@ -8388,6 +8402,9 @@ static void ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D,
   case ParsedAttr::AT_SYCLIntelNoGlobalWorkOffset:
     handleNoGlobalWorkOffsetAttr(S, D, AL);
     break;
+  case ParsedAttr::AT_SYCLIntelStallEnable:
+    handleStallEnableAttr(S, D, AL);
+    break;
   case ParsedAttr::AT_VecTypeHint:
     handleVecTypeHint(S, D, AL);
     break;

clang/lib/Sema/SemaSYCL.cpp

@@ -545,6 +545,20 @@ class MarkDeviceFunction : public RecursiveASTVisitor<MarkDeviceFunction> {
 
       if (auto *A = FD->getAttr<SYCLSimdAttr>())
         Attrs.insert(A);
+
+      // Allow the kernel attribute "stall_enable" only on lambda functions
+      // and function objects that are called directly from a kernel
+      // (i.e. the one passed to the single_task or parallel_for functions).
+      // For all other cases, emit a warning and ignore.
+      if (auto *A = FD->getAttr<SYCLIntelStallEnableAttr>()) {
+        if (ParentFD == SYCLKernel) {
+          Attrs.insert(A);
+        } else {
+          SemaRef.Diag(A->getLocation(), diag::warn_attribute_ignored) << A;
+          FD->dropAttr<SYCLIntelStallEnableAttr>();
+        }
+      }
+
       // Propagate the explicit SIMD attribute through call graph - it is used
       // to distinguish ESIMD code in ESIMD LLVM passes.
       if (KernelBody && KernelBody->hasAttr<SYCLSimdAttr>() &&
@@ -3222,6 +3236,7 @@ void Sema::MarkDevice(void) {
         case attr::Kind::SYCLIntelSchedulerTargetFmaxMhz:
         case attr::Kind::SYCLIntelMaxGlobalWorkDim:
         case attr::Kind::SYCLIntelNoGlobalWorkOffset:
+        case attr::Kind::SYCLIntelStallEnable:
         case attr::Kind::SYCLSimd: {
           if ((A->getKind() == attr::Kind::SYCLSimd) && KernelBody &&
               !KernelBody->getAttr<SYCLSimdAttr>()) {

clang/test/CodeGenSYCL/stall_enable.cpp

@@ -0,0 +1,26 @@
+// RUN: %clang_cc1 -fsycl -fsycl-is-device -internal-isystem %S/Inputs -triple spir64-unknown-unknown-sycldevice -disable-llvm-passes -emit-llvm -o - %s | FileCheck %s
+
+#include "sycl.hpp"
+
+using namespace cl::sycl;
+queue q;
+
+class Foo {
+public:
+  [[intel::stall_enable]] void operator()() const {}
+};
+
+int main() {
+  q.submit([&](handler &h) {
+    Foo f;
+    h.single_task<class test_kernel1>(f);
+
+    h.single_task<class test_kernel2>(
+        []() [[intel::stall_enable]]{});
+  });
+  return 0;
+}
+
+// CHECK: define spir_kernel void @"{{.*}}test_kernel1"() #0 {{.*}} !stall_enable ![[NUM5:[0-9]+]]
+// CHECK: define spir_kernel void @"{{.*}}test_kernel2"() #0 {{.*}} !stall_enable ![[NUM5]]
+// CHECK: ![[NUM5]] = !{i32 1}

clang/test/SemaSYCL/stall_enable.cpp

@@ -0,0 +1,38 @@
+// RUN: %clang_cc1 %s -fsyntax-only -fsycl -internal-isystem %S/Inputs -fsycl-is-device -Wno-sycl-2017-compat -DTRIGGER_ERROR -verify
+// RUN: %clang_cc1 -fsycl -fsycl-is-device -internal-isystem %S/Inputs -fsyntax-only -ast-dump -Wno-sycl-2017-compat %s | FileCheck %s
+
+#include "sycl.hpp"
+
+using namespace cl::sycl;
+queue q;
+
+[[intel::stall_enable]] void test() {} //expected-warning{{'stall_enable' attribute ignored}}
+
+#ifdef TRIGGER_ERROR
+[[intel::stall_enable(1)]] void bar1() {} // expected-error{{'stall_enable' attribute takes no arguments}}
+[[intel::stall_enable]] int N;            // expected-error{{'stall_enable' attribute only applies to functions}}
+#endif
+
+struct FuncObj {
+  [[intel::stall_enable]] void operator()() const {}
+};
+
+int main() {
+  q.submit([&](handler &h) {
+    // CHECK-LABEL: FunctionDecl {{.*}}test_kernel1
+    // CHECK:       SYCLIntelStallEnableAttr {{.*}}
+    h.single_task<class test_kernel1>(
+        FuncObj());
+
+    // CHECK-LABEL: FunctionDecl {{.*}}test_kernel2
+    // CHECK:       SYCLIntelStallEnableAttr {{.*}}
+    h.single_task<class test_kernel2>(
+        []() [[intel::stall_enable]]{});
+
+    // CHECK-LABEL: FunctionDecl {{.*}}test_kernel3
+    // CHECK-NOT:   SYCLIntelStallEnableAttr {{.*}}
+    h.single_task<class test_kernel3>(
+        []() { test(); });
+  });
+  return 0;
+}