[OpenMP] Show error if VLAs are not supported

Some target devices (e.g. Nvidia GPUs) don't support dynamic stack
allocation and hence no VLAs. Print errors with description instead
of failing in the backend or generating code that doesn't work.

This patch handles explicit uses of VLAs (local variable in target
or declare target region) or implicitly generated (private) VLAs
for reductions on VLAs or on array sections with non-constant size.

Differential Revision: https://reviews.llvm.org/D39505

llvm-svn: 318601

9 files changed, 248 insertions, 13 deletions

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 4542fc91940..def942c1e54 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -141,6 +141,10 @@ def err_vla_decl_has_extern_linkage : Error<
   "variable length array declaration cannot have 'extern' linkage">;
 def ext_vla_folded_to_constant : Extension<
   "variable length array folded to constant array as an extension">, InGroup<GNUFoldingConstant>;
+def err_vla_unsupported : Error<
+  "variable length arrays are not supported for the current target">;
+def note_vla_unsupported : Note<
+  "variable length arrays are not supported for the current target">;
 
 // C99 variably modified types
 def err_variably_modified_template_arg : Error<
@@ -8985,6 +8989,8 @@ def err_omp_reduction_non_addressable_expression : Error<
   "expected addressable reduction item for the task-based directives">;
 def err_omp_reduction_with_nogroup : Error<
   "'reduction' clause cannot be used with 'nogroup' clause">;
+def err_omp_reduction_vla_unsupported : Error<
+  "cannot generate code for reduction on %select{|array section, which requires a }0variable length array">;
 } // end of OpenMP category
 
 let CategoryName = "Related Result Type Issue" in {
diff --git a/clang/include/clang/Basic/TargetInfo.h b/clang/include/clang/Basic/TargetInfo.h
index f80f6b7a6be..da391eedec3 100644
--- a/clang/include/clang/Basic/TargetInfo.h
+++ b/clang/include/clang/Basic/TargetInfo.h
@@ -60,6 +60,7 @@ protected:
   // values are specified by the TargetInfo constructor.
   bool BigEndian;
   bool TLSSupported;
+  bool VLASupported;
   bool NoAsmVariants;  // True if {|} are normal characters.
   bool HasFloat128;
   unsigned char PointerWidth, PointerAlign;
@@ -939,6 +940,9 @@ public:
     return MaxTLSAlign;
   }
 
+  /// \brief Whether target supports variable-length arrays.
+  bool isVLASupported() const { return VLASupported; }
+
   /// \brief Whether the target supports SEH __try.
   bool isSEHTrySupported() const {
     return getTriple().isOSWindows() &&
diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
index 8133995c8df..355e20edc03 100644
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -8653,10 +8653,18 @@ public:
                                     NamedDeclSetType &SameDirectiveDecls);
   /// Check declaration inside target region.
   void checkDeclIsAllowedInOpenMPTarget(Expr *E, Decl *D);
-  /// Return true inside OpenMP target region.
+  /// Return true inside OpenMP declare target region.
   bool isInOpenMPDeclareTargetContext() const {
     return IsInOpenMPDeclareTargetContext;
   }
+  /// Return true inside OpenMP target region.
+  bool isInOpenMPTargetExecutionDirective() const;
+  /// Return true if (un)supported features for the current target should be
+  /// diagnosed if OpenMP (offloading) is enabled.
+  bool shouldDiagnoseTargetSupportFromOpenMP() const {
+    return !getLangOpts().OpenMPIsDevice || isInOpenMPDeclareTargetContext() ||
+      isInOpenMPTargetExecutionDirective();
+  }
 
   /// Return the number of captured regions created for an OpenMP directive.
   static int getOpenMPCaptureLevels(OpenMPDirectiveKind Kind);
diff --git a/clang/lib/Basic/TargetInfo.cpp b/clang/lib/Basic/TargetInfo.cpp
index 9ae466b7869..c58ed1e300e 100644
--- a/clang/lib/Basic/TargetInfo.cpp
+++ b/clang/lib/Basic/TargetInfo.cpp
@@ -31,6 +31,7 @@ TargetInfo::TargetInfo(const llvm::Triple &T) : TargetOpts(), Triple(T) {
   // SPARC.  These should be overridden by concrete targets as needed.
   BigEndian = !T.isLittleEndian();
   TLSSupported = true;
+  VLASupported = true;
   NoAsmVariants = false;
   HasFloat128 = false;
   PointerWidth = PointerAlign = 32;
diff --git a/clang/lib/Basic/Targets/NVPTX.cpp b/clang/lib/Basic/Targets/NVPTX.cpp
index 3889f097350..add3b318aeb 100644
--- a/clang/lib/Basic/Targets/NVPTX.cpp
+++ b/clang/lib/Basic/Targets/NVPTX.cpp
@@ -41,6 +41,7 @@ NVPTXTargetInfo::NVPTXTargetInfo(const llvm::Triple &Triple,
          "NVPTX only supports 32- and 64-bit modes.");
 
   TLSSupported = false;
+  VLASupported = false;
   AddrSpaceMap = &NVPTXAddrSpaceMap;
   UseAddrSpaceMapMangling = true;
 
diff --git a/clang/lib/Basic/Targets/SPIR.h b/clang/lib/Basic/Targets/SPIR.h
index e64ad548290..c384d4260ca 100644
--- a/clang/lib/Basic/Targets/SPIR.h
+++ b/clang/lib/Basic/Targets/SPIR.h
@@ -43,6 +43,7 @@ public:
     assert(getTriple().getEnvironment() == llvm::Triple::UnknownEnvironment &&
            "SPIR target must use unknown environment type");
     TLSSupported = false;
+    VLASupported = false;
     LongWidth = LongAlign = 64;
     AddrSpaceMap = &SPIRAddrSpaceMap;
     UseAddrSpaceMapMangling = true;
diff --git a/clang/lib/Sema/SemaOpenMP.cpp b/clang/lib/Sema/SemaOpenMP.cpp
index 0a48133721d..0f349bb961f 100644
--- a/clang/lib/Sema/SemaOpenMP.cpp
+++ b/clang/lib/Sema/SemaOpenMP.cpp
@@ -1303,6 +1303,17 @@ unsigned Sema::getOpenMPNestingLevel() const {
   return DSAStack->getNestingLevel();
 }
 
+bool Sema::isInOpenMPTargetExecutionDirective() const {
+  return (isOpenMPTargetExecutionDirective(DSAStack->getCurrentDirective()) &&
+          !DSAStack->isClauseParsingMode()) ||
+         DSAStack->hasDirective(
+             [](OpenMPDirectiveKind K, const DeclarationNameInfo &,
+                SourceLocation) -> bool {
+               return isOpenMPTargetExecutionDirective(K);
+             },
+             false);
+}
+
 VarDecl *Sema::IsOpenMPCapturedDecl(ValueDecl *D) {
   assert(LangOpts.OpenMP && "OpenMP is not allowed");
   D = getCanonicalDecl(D);
@@ -1315,18 +1326,8 @@ VarDecl *Sema::IsOpenMPCapturedDecl(ValueDecl *D) {
   // inserted here once support for 'declare target' is added.
   //
   auto *VD = dyn_cast<VarDecl>(D);
-  if (VD && !VD->hasLocalStorage()) {
-    if (isOpenMPTargetExecutionDirective(DSAStack->getCurrentDirective()) &&
-        !DSAStack->isClauseParsingMode())
-      return VD;
-    if (DSAStack->hasDirective(
-            [](OpenMPDirectiveKind K, const DeclarationNameInfo &,
-               SourceLocation) -> bool {
-              return isOpenMPTargetExecutionDirective(K);
-            },
-            false))
-      return VD;
-  }
+  if (VD && !VD->hasLocalStorage() && isInOpenMPTargetExecutionDirective())
+    return VD;
 
   if (DSAStack->getCurrentDirective() != OMPD_unknown &&
       (!DSAStack->isClauseParsingMode() ||
@@ -9812,6 +9813,12 @@ static bool ActOnOMPReductionKindClause(
     if ((OASE && !ConstantLengthOASE) ||
         (!OASE && !ASE &&
          D->getType().getNonReferenceType()->isVariablyModifiedType())) {
+      if (!Context.getTargetInfo().isVLASupported() &&
+          S.shouldDiagnoseTargetSupportFromOpenMP()) {
+        S.Diag(ELoc, diag::err_omp_reduction_vla_unsupported) << !!OASE;
+        S.Diag(ELoc, diag::note_vla_unsupported);
+        continue;
+      }
       // For arrays/array sections only:
       // Create pseudo array type for private copy. The size for this array will
       // be generated during codegen.
diff --git a/clang/lib/Sema/SemaType.cpp b/clang/lib/Sema/SemaType.cpp
index ec0d93cf678..b00cf7311b9 100644
--- a/clang/lib/Sema/SemaType.cpp
+++ b/clang/lib/Sema/SemaType.cpp
@@ -2183,6 +2183,12 @@ QualType Sema::BuildArrayType(QualType T, ArrayType::ArraySizeModifier ASM,
   // CUDA device code doesn't support VLAs.
   if (getLangOpts().CUDA && T->isVariableArrayType())
     CUDADiagIfDeviceCode(Loc, diag::err_cuda_vla) << CurrentCUDATarget();
+  // Some targets don't support VLAs.
+  if (T->isVariableArrayType() && !Context.getTargetInfo().isVLASupported() &&
+      shouldDiagnoseTargetSupportFromOpenMP()) {
+    Diag(Loc, diag::err_vla_unsupported);
+    return QualType();
+  }
 
   // If this is not C99, extwarn about VLA's and C99 array size modifiers.
   if (!getLangOpts().C99) {
diff --git a/clang/test/OpenMP/target_vla_messages.cpp b/clang/test/OpenMP/target_vla_messages.cpp
new file mode 100644
index 00000000000..c970d0def66
--- /dev/null
+++ b/clang/test/OpenMP/target_vla_messages.cpp
@@ -0,0 +1,201 @@
+// PowerPC supports VLAs.
+// RUN: %clang_cc1 -verify -fopenmp -x c++ -triple powerpc64le-unknown-unknown -fopenmp-targets=powerpc64le-unknown-unknown -emit-llvm-bc %s -o %t-ppc-host-ppc.bc
+// RUN: %clang_cc1 -verify -fopenmp -x c++ -triple powerpc64le-unknown-unknown -fopenmp-targets=powerpc64le-unknown-unknown -emit-llvm %s -fopenmp-is-device -fopenmp-host-ir-file-path %t-ppc-host-ppc.bc -o %t-ppc-device.ll
+
+// Nvidia GPUs don't support VLAs.
+// RUN: %clang_cc1 -verify -fopenmp -x c++ -triple powerpc64le-unknown-unknown -fopenmp-targets=nvptx64-nvidia-cuda -emit-llvm-bc %s -o %t-ppc-host-nvptx.bc
+// RUN: %clang_cc1 -verify -DNO_VLA -fopenmp -x c++ -triple nvptx64-unknown-unknown -fopenmp-targets=nvptx64-nvidia-cuda -emit-llvm %s -fopenmp-is-device -fopenmp-host-ir-file-path %t-ppc-host-nvptx.bc -o %t-nvptx-device.ll
+
+#ifndef NO_VLA
+// expected-no-diagnostics
+#endif
+
+#pragma omp declare target
+void declare(int arg) {
+  int a[2];
+#ifdef NO_VLA
+  // expected-error@+2 {{variable length arrays are not supported for the current target}}
+#endif
+  int vla[arg];
+}
+
+void declare_parallel_reduction(int arg) {
+  int a[2];
+
+#pragma omp parallel reduction(+: a)
+  { }
+
+#pragma omp parallel reduction(+: a[0:2])
+  { }
+
+#ifdef NO_VLA
+  // expected-error@+3 {{cannot generate code for reduction on array section, which requires a variable length array}}
+  // expected-note@+2 {{variable length arrays are not supported for the current target}}
+#endif
+#pragma omp parallel reduction(+: a[0:arg])
+  { }
+}
+#pragma omp end declare target
+
+template <typename T>
+void target_template(int arg) {  
+#pragma omp target
+  {
+#ifdef NO_VLA
+    // expected-error@+2 {{variable length arrays are not supported for the current target}}
+#endif
+    T vla[arg];
+  }
+}
+
+void target(int arg) {
+#pragma omp target
+  {
+#ifdef NO_VLA
+    // expected-error@+2 {{variable length arrays are not supported for the current target}}
+#endif
+    int vla[arg];
+  }
+
+#pragma omp target
+  {
+#pragma omp parallel
+    {
+#ifdef NO_VLA
+    // expected-error@+2 {{variable length arrays are not supported for the current target}}
+#endif
+      int vla[arg];
+    }
+  }
+
+  target_template<long>(arg);
+}
+
+void teams_reduction(int arg) {
+  int a[2];
+  int vla[arg];
+
+#pragma omp target map(a)
+#pragma omp teams reduction(+: a)
+  { }
+
+#ifdef NO_VLA
+  // expected-error@+4 {{cannot generate code for reduction on variable length array}}
+  // expected-note@+3 {{variable length arrays are not supported for the current target}}
+#endif
+#pragma omp target map(vla)
+#pragma omp teams reduction(+: vla)
+  { }
+  
+#pragma omp target map(a[0:2])
+#pragma omp teams reduction(+: a[0:2])
+  { }
+
+#pragma omp target map(vla[0:2])
+#pragma omp teams reduction(+: vla[0:2])
+  { }
+
+#ifdef NO_VLA
+  // expected-error@+4 {{cannot generate code for reduction on array section, which requires a variable length array}}
+  // expected-note@+3 {{variable length arrays are not supported for the current target}}
+#endif
+#pragma omp target map(a[0:arg])
+#pragma omp teams reduction(+: a[0:arg])
+  { }
+
+#ifdef NO_VLA
+  // expected-error@+4 {{cannot generate code for reduction on array section, which requires a variable length array}}
+  // expected-note@+3 {{variable length arrays are not supported for the current target}}
+#endif
+#pragma omp target map(vla[0:arg])
+#pragma omp teams reduction(+: vla[0:arg])
+  { }
+}
+
+void parallel_reduction(int arg) {
+  int a[2];
+  int vla[arg];
+
+#pragma omp target map(a)
+#pragma omp parallel reduction(+: a)
+  { }
+
+#ifdef NO_VLA
+  // expected-error@+4 {{cannot generate code for reduction on variable length array}}
+  // expected-note@+3 {{variable length arrays are not supported for the current target}}
+#endif
+#pragma omp target map(vla)
+#pragma omp parallel reduction(+: vla)
+  { }
+
+#pragma omp target map(a[0:2])
+#pragma omp parallel reduction(+: a[0:2])
+  { }
+
+#pragma omp target map(vla[0:2])
+#pragma omp parallel reduction(+: vla[0:2])
+  { }
+
+#ifdef NO_VLA
+  // expected-error@+4 {{cannot generate code for reduction on array section, which requires a variable length array}}
+  // expected-note@+3 {{variable length arrays are not supported for the current target}}
+#endif
+#pragma omp target map(a[0:arg])
+#pragma omp parallel reduction(+: a[0:arg])
+  { }
+
+#ifdef NO_VLA
+  // expected-error@+4 {{cannot generate code for reduction on array section, which requires a variable length array}}
+  // expected-note@+3 {{variable length arrays are not supported for the current target}}
+#endif
+#pragma omp target map(vla[0:arg])
+#pragma omp parallel reduction(+: vla[0:arg])
+  { }
+}
+
+void for_reduction(int arg) {
+  int a[2];
+  int vla[arg];
+
+#pragma omp target map(a)
+#pragma omp parallel
+#pragma omp for reduction(+: a)
+  for (int i = 0; i < arg; i++) ;
+
+#ifdef NO_VLA
+  // expected-error@+5 {{cannot generate code for reduction on variable length array}}
+  // expected-note@+4 {{variable length arrays are not supported for the current target}}
+#endif
+#pragma omp target map(vla)
+#pragma omp parallel
+#pragma omp for reduction(+: vla)
+  for (int i = 0; i < arg; i++) ;
+
+#pragma omp target map(a[0:2])
+#pragma omp parallel
+#pragma omp for reduction(+: a[0:2])
+  for (int i = 0; i < arg; i++) ;
+
+#pragma omp target map(vla[0:2])
+#pragma omp parallel
+#pragma omp for reduction(+: vla[0:2])
+  for (int i = 0; i < arg; i++) ;
+
+#ifdef NO_VLA
+  // expected-error@+5 {{cannot generate code for reduction on array section, which requires a variable length array}}
+  // expected-note@+4 {{variable length arrays are not supported for the current target}}
+#endif
+#pragma omp target map(a[0:arg])
+#pragma omp parallel
+#pragma omp for reduction(+: a[0:arg])
+  for (int i = 0; i < arg; i++) ;
+
+#ifdef NO_VLA
+  // expected-error@+5 {{cannot generate code for reduction on array section, which requires a variable length array}}
+  // expected-note@+4 {{variable length arrays are not supported for the current target}}
+#endif
+#pragma omp target map(vla[0:arg])
+#pragma omp parallel
+#pragma omp for reduction(+: vla[0:arg])
+  for (int i = 0; i < arg; i++) ;
+}