[OpenMP] Add flag for preventing the extension to 64 bits for the collapse loop counter

[gtbercea]

Summary

Introduce a compiler flag for cases when the user knows that the collapsed loop counter can be safely represented using at most 32 bits. This will prevent the emission of expensive mathematical operations (such as the div operation) on the iteration variable using 64 bits where 32 bit operations are sufficient.

[ABataev]

docs/OpenMPSupport.rst

118

-fopenmp-optimistic-collapse

include/clang/Basic/LangOptions.def

210

Fix the description and the option in accordance with the option name

include/clang/Driver/Options.td

1577

Missed -fno... option

lib/Driver/ToolChains/Clang.cpp

4437

You need to change the processing taking into account -fno... option.

lib/Frontend/CompilerInvocation.cpp

2851

Also, need to check for -fno... option

[kkwli0]

docs/OpenMPSupport.rst

118

How about -fopenmp-use-32bit-collapse-parameter?

[gtbercea]

• Address comments.

[gtbercea]

include/clang/Basic/LangOptions.def

210

The description accurately describes what Sema is doing. We allow an extension to up to 32 bits but no further.

[ABataev]

lib/Driver/ToolChains/Clang.cpp

4437

fno_openmp_optimistic_collapse is not a frontend option. better to check for

if (Args.hasFlag(options::OPT_fopenmp_optimistic_collapse,
                   options::OPT_fno_openmp_optimistic_collapse,
                   /*Default=*/false))
  CmdArgs.push_back("-fopenmp-optimistic-collapse");

here rather than in the frontend. In the frontend, you can just check for the presence of the fopenmp_optimistic_collapse flag

[gtbercea]

• Address comments.

[ABataev]

LG

[gtbercea]

docs/OpenMPSupport.rst

118

Alexey suggested we have no numbers in the flag name so we tried to find something that avoids that. I'm happy to give it a different name than optimistic collapse.

[hfinkel]

docs/OpenMPSupport.rst

119

Can you please clarify here what happens when the loop induction variables are already 64 bits. If any of them are already 64 bits, then we still use 64 bits overall?

[gtbercea]

docs/OpenMPSupport.rst

119

This flag is for the user to guarantee that the total size of the collapsed loops can be represented using at most 32 bits (regardless of the actual width of the individual loop induction variables).

docs/OpenMPSupport.rst

 Clang supports two data-sharing models for Cuda devices: `Generic` and `Cuda`
 modes. The default mode is `Generic`. `Cuda` mode can give an additional
 performance and can be activated using the `-fopenmp-cuda-mode` flag. In
 `Generic` mode all local variables that can be shared in the parallel regions
 are stored in the global memory. In `Cuda` mode local variables are not shared
 between the threads and it is user responsibility to share the required data
 between the threads in the parallel regions.
 
+Collapsed loop nest counter
+---------------------------
+
+When using the collapse clause on a loop nest the default behaviour is to
+automatically extend the representation of the loop counter to 64 bits for
+the cases where the sizes of the collapsed loops are not known at compile
+time. To prevent this conservative choice and use at most 32 bits,
+compile your program with the `-fopenmp-optimistic-collapse`.
+
+
 Features not supported or with limited support for Cuda devices
 ---------------------------------------------------------------
 
 - Cancellation constructs are not supported.
 
 - Doacross loop nest is not supported.
 
 - User-defined reductions are supported only for trivial types.

include/clang/Basic/LangOptions.def

 LANGOPT(OpenMPSimd        , 1, 0, "Use SIMD only OpenMP support.")
 LANGOPT(OpenMPUseTLS      , 1, 0, "Use TLS for threadprivates or runtime calls")
 LANGOPT(OpenMPIsDevice    , 1, 0, "Generate code only for OpenMP target device")
 LANGOPT(OpenMPCUDAMode    , 1, 0, "Generate code for OpenMP pragmas in SIMT/SPMD mode")
 LANGOPT(OpenMPCUDAForceFullRuntime , 1, 0, "Force to use full runtime in all constructs when offloading to CUDA devices")
 LANGOPT(OpenMPHostCXXExceptions    , 1, 0, "C++ exceptions handling in the host code.")
 LANGOPT(OpenMPCUDANumSMs  , 32, 0, "Number of SMs for CUDA devices.")
 LANGOPT(OpenMPCUDABlocksPerSM  , 32, 0, "Number of blocks per SM for CUDA devices.")
+LANGOPT(OpenMPOptimisticCollapse  , 1, 0, "Use at most 32 bits to represent the collapsed loop nest counter.")
 LANGOPT(RenderScript      , 1, 0, "RenderScript")
 
 LANGOPT(CUDAIsDevice      , 1, 0, "compiling for CUDA device")
 LANGOPT(CUDAAllowVariadicFunctions, 1, 0, "allowing variadic functions in CUDA device code")
 LANGOPT(CUDAHostDeviceConstexpr, 1, 1, "treating unattributed constexpr functions as __host__ __device__")
 LANGOPT(CUDADeviceApproxTranscendentals, 1, 0, "using approximate transcendental functions")
 LANGOPT(GPURelocatableDeviceCode, 1, 0, "generate relocatable device code")
 

include/clang/Driver/Options.td

 def fopenmp_cuda_force_full_runtime : Flag<["-"], "fopenmp-cuda-force-full-runtime">, Group<f_Group>,
   Flags<[CC1Option, NoArgumentUnused, HelpHidden]>;
 def fno_openmp_cuda_force_full_runtime : Flag<["-"], "fno-openmp-cuda-force-full-runtime">, Group<f_Group>,
   Flags<[NoArgumentUnused, HelpHidden]>;
 def fopenmp_cuda_number_of_sm_EQ : Joined<["-"], "fopenmp-cuda-number-of-sm=">, Group<f_Group>,
   Flags<[CC1Option, NoArgumentUnused, HelpHidden]>;
 def fopenmp_cuda_blocks_per_sm_EQ : Joined<["-"], "fopenmp-cuda-blocks-per-sm=">, Group<f_Group>,
   Flags<[CC1Option, NoArgumentUnused, HelpHidden]>;
+def fopenmp_optimistic_collapse : Flag<["-"], "fopenmp-optimistic-collapse">, Group<f_Group>,
+  Flags<[CC1Option, NoArgumentUnused, HelpHidden]>;
+def fno_openmp_optimistic_collapse : Flag<["-"], "fno-openmp-optimistic-collapse">, Group<f_Group>,
+  Flags<[NoArgumentUnused, HelpHidden]>;
 def fno_optimize_sibling_calls : Flag<["-"], "fno-optimize-sibling-calls">, Group<f_Group>;
 def foptimize_sibling_calls : Flag<["-"], "foptimize-sibling-calls">, Group<f_Group>;
 def fno_escaping_block_tail_calls : Flag<["-"], "fno-escaping-block-tail-calls">, Group<f_Group>, Flags<[CC1Option]>;
 def fescaping_block_tail_calls : Flag<["-"], "fescaping-block-tail-calls">, Group<f_Group>;
 def force__cpusubtype__ALL : Flag<["-"], "force_cpusubtype_ALL">;
 def force__flat__namespace : Flag<["-"], "force_flat_namespace">;
 def force__load : Separate<["-"], "force_load">;
 def force_addr : Joined<["-"], "fforce-addr">, Group<clang_ignored_f_Group>;

lib/Driver/ToolChains/Clang.cpp

case Driver::OMPRT_IOMP5:
       if (!Args.hasFlag(options::OPT_fopenmp_use_tls,
                         options::OPT_fnoopenmp_use_tls, /*Default=*/true))
         CmdArgs.push_back("-fnoopenmp-use-tls");
       Args.AddLastArg(CmdArgs, options::OPT_fopenmp_simd,
                       options::OPT_fno_openmp_simd);
       Args.AddAllArgs(CmdArgs, options::OPT_fopenmp_version_EQ);
       Args.AddAllArgs(CmdArgs, options::OPT_fopenmp_cuda_number_of_sm_EQ);
       Args.AddAllArgs(CmdArgs, options::OPT_fopenmp_cuda_blocks_per_sm_EQ);
+      if (Args.hasFlag(options::OPT_fopenmp_optimistic_collapse,
+                       options::OPT_fno_openmp_optimistic_collapse,
+                       /*Default=*/false))
+        CmdArgs.push_back("-fopenmp-optimistic-collapse");
 
       // When in OpenMP offloading mode with NVPTX target, forward
       // cuda-mode flag
       if (Args.hasFlag(options::OPT_fopenmp_cuda_mode,
                        options::OPT_fno_openmp_cuda_mode, /*Default=*/false))
         CmdArgs.push_back("-fopenmp-cuda-mode");
 
       // When in OpenMP offloading mode with NVPTX target, check if full runtime

lib/Frontend/CompilerInvocation.cpp

if (Opts.OpenMPIsDevice && T.isNVPTX()) {
     Opts.OpenMPCUDANumSMs =
         getLastArgIntValue(Args, options::OPT_fopenmp_cuda_number_of_sm_EQ,
                            Opts.OpenMPCUDANumSMs, Diags);
     Opts.OpenMPCUDABlocksPerSM =
         getLastArgIntValue(Args, options::OPT_fopenmp_cuda_blocks_per_sm_EQ,
                            Opts.OpenMPCUDABlocksPerSM, Diags);
   }
 
+  // Prevent auto-widening the representation of loop counters during an
+  // OpenMP collapse clause.
+  Opts.OpenMPOptimisticCollapse =
+      Args.hasArg(options::OPT_fopenmp_optimistic_collapse) ? 1 : 0;
+
   // Get the OpenMP target triples if any.
   if (Arg *A = Args.getLastArg(options::OPT_fopenmp_targets_EQ)) {
 
     for (unsigned i = 0; i < A->getNumValues(); ++i) {
       llvm::Triple TT(A->getValue(i));
 
       if (TT.getArch() == llvm::Triple::UnknownArch ||
           !(TT.getArch() == llvm::Triple::ppc ||

lib/Sema/SemaOpenMP.cpp

if (!NewStep->isValueDependent()) {
     bool IsConstPos =
         IsConstant && Result.isSigned() && (Subtract == Result.isNegative());
     bool IsConstZero = IsConstant && !Result.getBoolValue();
 
     // != with increment is treated as <; != with decrement is treated as >
     if (!TestIsLessOp.hasValue())
       TestIsLessOp = IsConstPos || (IsUnsigned && !Subtract);
     if (UB && (IsConstZero ||
-               (TestIsLessOp.getValue() ? 
+               (TestIsLessOp.getValue() ?
                   (IsConstNeg || (IsUnsigned && Subtract)) :
                   (IsConstPos || (IsUnsigned && !Subtract))))) {
       SemaRef.Diag(NewStep->getExprLoc(),
                    diag::err_omp_loop_incr_not_compatible)
           << LCDecl << TestIsLessOp.getValue() << NewStep->getSourceRange();
       SemaRef.Diag(ConditionLoc,
                    diag::note_omp_loop_cond_requres_compatible_incr)
           << TestIsLessOp.getValue() << ConditionSrcRange;
if (CE->getNumArgs() == 2) {
           return setUB(CE->getArg(1), Op == OO_Less || Op == OO_LessEqual,
                        Op == OO_Less || Op == OO_Greater, CE->getSourceRange(),
                        CE->getOperatorLoc());
         if (getInitLCDecl(CE->getArg(1)) == LCDecl)
           return setUB(CE->getArg(0), Op == OO_Greater || Op == OO_GreaterEqual,
                        Op == OO_Less || Op == OO_Greater, CE->getSourceRange(),
                        CE->getOperatorLoc());
         break;
-      case OO_ExclaimEqual: 
+      case OO_ExclaimEqual:
         return setUB(getInitLCDecl(CE->getArg(0)) == LCDecl ?
                      CE->getArg(1) : CE->getArg(0),
                      /*LessOp=*/llvm::None,
                      /*StrictOp=*/true,
                      CE->getSourceRange(),
                      CE->getOperatorLoc());
         break;
       default:
Expr *OpenMPIterationSpaceChecker::buildPreCond(
 
   ExprResult NewLB = tryBuildCapture(SemaRef, LB, Captures);
   ExprResult NewUB = tryBuildCapture(SemaRef, UB, Captures);
   if (!NewLB.isUsable() || !NewUB.isUsable())
     return nullptr;
 
   ExprResult CondExpr =
       SemaRef.BuildBinOp(S, DefaultLoc,
-                         TestIsLessOp.getValue() ? 
+                         TestIsLessOp.getValue() ?
                            (TestIsStrictOp ? BO_LT : BO_LE) :
                            (TestIsStrictOp ? BO_GT : BO_GE),
                          NewLB.get(), NewUB.get());
   if (CondExpr.isUsable()) {
     if (!SemaRef.Context.hasSameUnqualifiedType(CondExpr.get()->getType(),
                                                 SemaRef.Context.BoolTy))
       CondExpr = SemaRef.PerformImplicitConversion(
           CondExpr.get(), SemaRef.Context.BoolTy, /*Action=*/Sema::AA_Casting,
if (LastIteration64.isUsable())
               .PerformImplicitConversion(N->IgnoreImpCasts(), N->getType(),
                                          Sema::AA_Converting,
                                          /*AllowExplicit=*/true)
               .get());
   }
 
   // Choose either the 32-bit or 64-bit version.
   ExprResult LastIteration = LastIteration64;
-  if (LastIteration32.isUsable() &&
+  if (SemaRef.getLangOpts().OpenMPOptimisticCollapse ||
+      (LastIteration32.isUsable() &&
        C.getTypeSize(LastIteration32.get()->getType()) == 32 &&
        (AllCountsNeedLessThan32Bits || NestedLoopCount == 1 ||
         fitsInto(
             /*Bits=*/32,
             LastIteration32.get()->getType()->hasSignedIntegerRepresentation(),
-           LastIteration64.get(), SemaRef)))
+            LastIteration64.get(), SemaRef))))
     LastIteration = LastIteration32;
   QualType VType = LastIteration.get()->getType();
   QualType RealVType = VType;
   QualType StrideVType = VType;
   if (isOpenMPTaskLoopDirective(DKind)) {
     VType =
         SemaRef.Context.getIntTypeForBitwidth(/*DestWidth=*/64, /*Signed=*/0);
     StrideVType =
Sema::ActOnOpenMPMapClause(ArrayRef<OpenMPMapModifierKind> MapTypeModifiers,
   unsigned Count = 0;
   for (unsigned I = 0, E = MapTypeModifiers.size(); I < E; ++I) {
     if (MapTypeModifiers[I] != OMPC_MAP_MODIFIER_unknown &&
         llvm::find(Modifiers, MapTypeModifiers[I]) != std::end(Modifiers)) {
       Diag(MapTypeModifiersLoc[I], diag::err_omp_duplicate_map_type_modifier);
       continue;
     }
     assert(Count < OMPMapClause::NumberOfModifiers &&
-           "Modifiers exceed the allowed number of map type modifiers"); 
+           "Modifiers exceed the allowed number of map type modifiers");
     Modifiers[Count] = MapTypeModifiers[I];
     ModifiersLoc[Count] = MapTypeModifiersLoc[I];
     ++Count;
   }
 
   // We need to produce a map clause even if we don't have variables so that
   // other diagnostics related with non-existing map clauses are accurate.
   return OMPMapClause::Create(Context, StartLoc, LParenLoc, EndLoc,

test/OpenMP/nvptx_target_teams_distribute_parallel_for_codegen.cpp

 // Test target codegen - host bc file has to be created first.
 // RUN: %clang_cc1 -verify -fopenmp -fopenmp-version=45 -fopenmp-cuda-mode -x c++ -triple powerpc64le-unknown-unknown -fopenmp-targets=nvptx64-nvidia-cuda -emit-llvm-bc %s -o %t-ppc-host.bc
-// RUN: %clang_cc1 -verify -fopenmp -fopenmp-version=45 -fopenmp-cuda-mode -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.bc -o - | FileCheck %s --check-prefix CHECK --check-prefix CHECK-64
+// RUN: %clang_cc1 -verify -fopenmp -fopenmp-version=45 -fopenmp-cuda-mode -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.bc -o - | FileCheck %s --check-prefix CHECK --check-prefix CHECK-64 --check-prefix CHECK-DIV64
+// RUN: %clang_cc1 -verify -fopenmp -fopenmp-version=45 -fopenmp-cuda-mode -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.bc -fopenmp-optimistic-collapse -o - | FileCheck %s --check-prefix CHECK --check-prefix CHECK-DIV32
 // RUN: %clang_cc1 -verify -fopenmp -fopenmp-version=45 -fopenmp-cuda-mode -x c++ -triple i386-unknown-unknown -fopenmp-targets=nvptx-nvidia-cuda -emit-llvm-bc %s -o %t-x86-host.bc
 // RUN: %clang_cc1 -verify -fopenmp -fopenmp-version=45 -fopenmp-cuda-mode -x c++ -triple nvptx-unknown-unknown -fopenmp-targets=nvptx-nvidia-cuda -emit-llvm %s -fopenmp-is-device -fopenmp-host-ir-file-path %t-x86-host.bc -o - | FileCheck %s --check-prefix CHECK --check-prefix CHECK-32
 // RUN: %clang_cc1 -verify -fopenmp -fopenmp-version=45 -fopenmp-cuda-mode -fexceptions -fcxx-exceptions -x c++ -triple nvptx-unknown-unknown -fopenmp-targets=nvptx-nvidia-cuda -emit-llvm %s -fopenmp-is-device -fopenmp-host-ir-file-path %t-x86-host.bc -o - | FileCheck %s --check-prefix CHECK --check-prefix CHECK-32
 // expected-no-diagnostics
 #ifndef HEADER
 #define HEADER
 
 // Check that the execution mode of all 5 target regions on the gpu is set to SPMD Mode.
-// CHECK-DAG: {{@__omp_offloading_.+l32}}_exec_mode = weak constant i8 0
-// CHECK-DAG: {{@__omp_offloading_.+l38}}_exec_mode = weak constant i8 0
-// CHECK-DAG: {{@__omp_offloading_.+l43}}_exec_mode = weak constant i8 0
-// CHECK-DAG: {{@__omp_offloading_.+l48}}_exec_mode = weak constant i8 0
-// CHECK-DAG: {{@__omp_offloading_.+l56}}_exec_mode = weak constant i8 0
+// CHECK-DAG: {{@__omp_offloading_.+l34}}_exec_mode = weak constant i8 0
+// CHECK-DAG: {{@__omp_offloading_.+l40}}_exec_mode = weak constant i8 0
+// CHECK-DAG: {{@__omp_offloading_.+l45}}_exec_mode = weak constant i8 0
+// CHECK-DAG: {{@__omp_offloading_.+l50}}_exec_mode = weak constant i8 0
+// CHECK-DAG: {{@__omp_offloading_.+l58}}_exec_mode = weak constant i8 0
+// CHECK-DAG: {{@__omp_offloading_.+l65}}_exec_mode = weak constant i8 0
 
 #define N 1000
 #define M 10
 
 template<typename tx>
 tx ftemplate(int n) {
   tx a[N];
   short aa[N];

 }
 
 // CHECK-DAG: [[MEM_TY:%.+]] = type { [128 x i8] }
 // CHECK-DAG: [[SHARED_GLOBAL_RD:@.+]] = common addrspace(3) global [[MEM_TY]] zeroinitializer
 // CHECK-DAG: [[KERNEL_PTR:@.+]] = internal addrspace(3) global i8* null
 // CHECK-DAG: [[KERNEL_SIZE:@.+]] = internal unnamed_addr constant i{{64|32}} 4
 // CHECK-DAG: [[KERNEL_SHARED:@.+]] = internal unnamed_addr constant i16 1
 
-// CHECK-LABEL: define {{.*}}void {{@__omp_offloading_.+}}_l32(
+// CHECK-LABEL: define {{.*}}void {{@__omp_offloading_.+}}_l34(
 // CHECK-DAG: [[THREAD_LIMIT:%.+]] = call i32 @llvm.nvvm.read.ptx.sreg.ntid.x()
 // CHECK: call void @__kmpc_spmd_kernel_init(i32 [[THREAD_LIMIT]], i16 0, i16 0)
 // CHECK: call void @__kmpc_get_team_static_memory(i16 1, i8* addrspacecast (i8 addrspace(3)* getelementptr inbounds ([[MEM_TY]], [[MEM_TY]] addrspace(3)* [[SHARED_GLOBAL_RD]], i32 0, i32 0, i32 0) to i8*), i{{64|32}} 4, i16 1, i8** addrspacecast (i8* addrspace(3)* [[KERNEL_PTR]] to i8**))
 // CHECK: [[TEAM_ALLOC:%.+]] = load i8*, i8* addrspace(3)* [[KERNEL_PTR]],
 // CHECK: [[BC:%.+]] = bitcast i8* [[TEAM_ALLOC]] to [[REC:%.+]]*
 // CHECK: getelementptr inbounds [[REC]], [[REC]]* [[BC]], i{{[0-9]+}} 0, i{{[0-9]+}} 0
 // CHECK: call void @__kmpc_for_static_init_4({{.+}}, {{.+}}, {{.+}} 91,
 // CHECK: {{call|invoke}} void [[OUTL1:@.+]](

 // CHECK-DAG: [[OMP_LB_VAL_3:%.+]] = load{{.+}}, {{.+}} [[OMP_LB]],
 // CHECK: store{{.+}} [[OMP_LB_VAL_3]], {{.+}}* [[OMP_IV]],
 
 // CHECK: [[DIST_INNER_LOOP_END]]:
 // CHECK: call void @__kmpc_for_static_fini(
 // CHECK: call void @__kmpc_spmd_kernel_deinit_v2(i16 0)
 // CHECK: ret void
 
-// CHECK: define internal void [[OUTL4]](
+// CHECK-32: define internal void [[OUTL4]](
+// CHECK-64: define internal void [[OUTL4]](
 // CHECK: call void @__kmpc_for_static_init_4({{.+}}, {{.+}}, {{.+}} 33,
 // CHECK: call void @__kmpc_for_static_fini(
 // CHECK: ret void
 
-// CHECK: define weak void @__omp_offloading_{{.*}}_l56(i[[SZ:64|32]] %{{[^,]+}}, [10 x [10 x i32]]* dereferenceable{{.*}})
+// CHECK: define weak void @__omp_offloading_{{.*}}_l58(i[[SZ:64|32]] %{{[^,]+}}, [10 x [10 x i32]]* dereferenceable{{.*}})
 // CHECK: call void [[OUTLINED:@__omp_outlined.*]](i32* %{{.+}}, i32* %{{.+}}, i[[SZ]] %{{.*}}, i[[SZ]] %{{.*}}, i[[SZ]] %{{.*}}, [10 x [10 x i32]]* %{{.*}})
 // CHECK: define internal void [[OUTLINED]](i32* noalias %{{.*}}, i32* noalias %{{.*}} i[[SZ]] %{{.+}}, i[[SZ]] %{{.+}}, i[[SZ]] %{{.+}}, [10 x [10 x i32]]* dereferenceable{{.*}})
+// CHECK-DIV64: div i64
+// CHECK-DIV32-NO: div i64
 
-// CHECK: define weak void @__omp_offloading_{{.*}}_l63(i[[SZ:64|32]] %{{[^,]+}}, [1000 x i32]* dereferenceable{{.*}}, i32* %{{[^)]+}})
+// CHECK: define weak void @__omp_offloading_{{.*}}_l65(i[[SZ:64|32]] %{{[^,]+}}, [1000 x i32]* dereferenceable{{.*}}, i32* %{{[^)]+}})
 // CHECK: call void [[OUTLINED:@__omp_outlined.*]](i32* %{{.+}}, i32* %{{.+}}, i[[SZ]] %{{.*}}, i[[SZ]] %{{.*}}, i[[SZ]] %{{.*}}, [1000 x i32]* %{{.*}}, i32* %{{.*}})
 // CHECK: define internal void [[OUTLINED]](i32* noalias %{{.*}}, i32* noalias %{{.*}} i[[SZ]] %{{.+}}, i[[SZ]] %{{.+}}, i[[SZ]] %{{.+}}, [1000 x i32]* dereferenceable{{.*}}, i32* %{{.*}})
 
 #endif