[CUDA] Make unattributed constexpr functions implicitly host+device.

With this patch, by a constexpr function is implicitly host+device
unless:

 a) it's a variadic function (variadic functions are not allowed on the
    device side), or
 b) it's preceeded by a __device__ overload in a system header.

The restriction on overloading __host__ __device__ functions on the
basis of their CUDA attributes remains in place, but we use (b) to allow
us to define __device__ overloads for constexpr functions in cmath,
which would otherwise be __host__ __device__ and thus not overloadable.

You can disable this behavior with -fno-cuda-host-device-constexpr.

Reviewers: tra, rnk, rsmith

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D18380

llvm-svn: 264964

3 files changed, 55 insertions, 3 deletions

diff --git a/clang/lib/Sema/SemaCUDA.cpp b/clang/lib/Sema/SemaCUDA.cpp
index 07a5ac32807..fee1ccf22b2 100644
--- a/clang/lib/Sema/SemaCUDA.cpp
+++ b/clang/lib/Sema/SemaCUDA.cpp
@@ -11,12 +11,14 @@
 ///
 //===----------------------------------------------------------------------===//
 
-#include "clang/Sema/Sema.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/Decl.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/Lex/Preprocessor.h"
+#include "clang/Sema/Lookup.h"
+#include "clang/Sema/Sema.h"
 #include "clang/Sema/SemaDiagnostic.h"
+#include "clang/Sema/Template.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SmallVector.h"
 using namespace clang;
@@ -381,3 +383,50 @@ bool Sema::isEmptyCudaConstructor(SourceLocation Loc, CXXConstructorDecl *CD) {
 
   return true;
 }
+
+// With -fcuda-host-device-constexpr, an unattributed constexpr function is
+// treated as implicitly __host__ __device__, unless:
+//  * it is a variadic function (device-side variadic functions are not
+//    allowed), or
+//  * a __device__ function with this signature was already declared, in which
+//    case in which case we output an error, unless the __device__ decl is in a
+//    system header, in which case we leave the constexpr function unattributed.
+void Sema::maybeAddCUDAHostDeviceAttrs(Scope *S, FunctionDecl *NewD,
+                                       const LookupResult &Previous) {
+  assert(getLangOpts().CUDA && "May be called only for CUDA compilations.");
+  if (!getLangOpts().CUDAHostDeviceConstexpr || !NewD->isConstexpr() ||
+      NewD->isVariadic() || NewD->hasAttr<CUDAHostAttr>() ||
+      NewD->hasAttr<CUDADeviceAttr>() || NewD->hasAttr<CUDAGlobalAttr>())
+    return;
+
+  // Is D a __device__ function with the same signature as NewD, ignoring CUDA
+  // attributes?
+  auto IsMatchingDeviceFn = [&](NamedDecl *D) {
+    if (UsingShadowDecl *Using = dyn_cast<UsingShadowDecl>(D))
+      D = Using->getTargetDecl();
+    FunctionDecl *OldD = D->getAsFunction();
+    return OldD && OldD->hasAttr<CUDADeviceAttr>() &&
+           !OldD->hasAttr<CUDAHostAttr>() &&
+           !IsOverload(NewD, OldD, /* UseMemberUsingDeclRules = */ false,
+                       /* ConsiderCudaAttrs = */ false);
+  };
+  auto It = llvm::find_if(Previous, IsMatchingDeviceFn);
+  if (It != Previous.end()) {
+    // We found a __device__ function with the same name and signature as NewD
+    // (ignoring CUDA attrs).  This is an error unless that function is defined
+    // in a system header, in which case we simply return without making NewD
+    // host+device.
+    NamedDecl *Match = *It;
+    if (!getSourceManager().isInSystemHeader(Match->getLocation())) {
+      Diag(NewD->getLocation(),
+           diag::err_cuda_unattributed_constexpr_cannot_overload_device)
+          << NewD->getName();
+      Diag(Match->getLocation(),
+           diag::note_cuda_conflicting_device_function_declared_here);
+    }
+    return;
+  }
+
+  NewD->addAttr(CUDAHostAttr::CreateImplicit(Context));
+  NewD->addAttr(CUDADeviceAttr::CreateImplicit(Context));
+}
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 66abb655bbc..0d91e976d47 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -8009,6 +8009,9 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
   // Handle attributes.
   ProcessDeclAttributes(S, NewFD, D);
 
+  if (getLangOpts().CUDA)
+    maybeAddCUDAHostDeviceAttrs(S, NewFD, Previous);
+
   if (getLangOpts().OpenCL) {
     // OpenCL v1.1 s6.5: Using an address space qualifier in a function return
     // type declaration will generate a compilation error.
diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index 11f6286438d..5b2ed0d8e43 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -992,7 +992,7 @@ Sema::CheckOverload(Scope *S, FunctionDecl *New, const LookupResult &Old,
 }
 
 bool Sema::IsOverload(FunctionDecl *New, FunctionDecl *Old,
-                      bool UseMemberUsingDeclRules) {
+                      bool UseMemberUsingDeclRules, bool ConsiderCudaAttrs) {
   // C++ [basic.start.main]p2: This function shall not be overloaded.
   if (New->isMain())
     return false;
@@ -1125,7 +1125,7 @@ bool Sema::IsOverload(FunctionDecl *New, FunctionDecl *Old,
       return true;
   }
 
-  if (getLangOpts().CUDA) {
+  if (getLangOpts().CUDA && ConsiderCudaAttrs) {
     CUDAFunctionTarget NewTarget = IdentifyCUDATarget(New),
                        OldTarget = IdentifyCUDATarget(Old);
     if (NewTarget == CFT_InvalidTarget || NewTarget == CFT_Global)