[clang] 2cbdb5b - [clang][OpenMP] Add OpenMP GPU optimization flag framework (#178914)
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cfe-commits at lists.llvm.org
Fri Feb 6 05:38:24 PST 2026
Author: Nicole Aschenbrenner
Date: 2026-02-06T07:38:18-06:00
New Revision: 2cbdb5bf3e1b0f5c9d135a8b53815c34ba0ce3ce
URL: https://github.com/llvm/llvm-project/commit/2cbdb5bf3e1b0f5c9d135a8b53815c34ba0ce3ce
DIFF: https://github.com/llvm/llvm-project/commit/2cbdb5bf3e1b0f5c9d135a8b53815c34ba0ce3ce.diff
LOG: [clang][OpenMP] Add OpenMP GPU optimization flag framework (#178914)
This patch series adds a framework of OpenMP GPU optimization flags to
enable more efficient code generation for GPU offloading. The series
consists of three patches:
1. Add negative flag variants (-fno-*) for existing assume options to
allow explicit disabling of optimizations.
2. Add -fopenmp-target-ignore-env-vars flag to indicate that OpenMP
runtime can ignore environment variables during code generation,
enabling optimizations like skipping runtime checks and eliminating
conditional branches.
3. Add -fopenmp-target-fast meta-flag that implies the above
optimization flags. This convenience flag is automatically enabled by
-Ofast and provides a simple interface for aggressive GPU optimizations.
The flags benefit all GPU targets (NVPTX, AMDGPU, Intel GPU) by
providing a standard way to enable common GPU optimization patterns.
Individual flags can be selectively overridden while keeping others
enabled.
Added:
clang/test/Driver/openmp-target-fast-flag.c
Modified:
clang/include/clang/Options/Options.td
clang/lib/Driver/ToolChains/Clang.cpp
Removed:
################################################################################
diff --git a/clang/include/clang/Options/Options.td b/clang/include/clang/Options/Options.td
index 5ecb516c2cf68..3e3239bef677e 100644
--- a/clang/include/clang/Options/Options.td
+++ b/clang/include/clang/Options/Options.td
@@ -3986,9 +3986,13 @@ def fno_openmp_assume_threads_oversubscription : Flag<["-"], "fno-openmp-assume-
def fopenmp_assume_no_thread_state : Flag<["-"], "fopenmp-assume-no-thread-state">,
HelpText<"Assert no thread in a parallel region modifies an ICV">,
MarshallingInfoFlag<LangOpts<"OpenMPNoThreadState">>;
+def fno_openmp_assume_no_thread_state : Flag<["-"], "fno-openmp-assume-no-thread-state">,
+ HelpText<"Assert that a thread in a parallel region may modify an ICV">;
def fopenmp_assume_no_nested_parallelism : Flag<["-"], "fopenmp-assume-no-nested-parallelism">,
HelpText<"Assert no nested parallel regions in the GPU">,
MarshallingInfoFlag<LangOpts<"OpenMPNoNestedParallelism">>;
+def fno_openmp_assume_no_nested_parallelism : Flag<["-"], "fno-openmp-assume-no-nested-parallelism">,
+ HelpText<"Assert that a nested parallel region may be used in the GPU">;
} // let Group = f_Group
} // let Visibility = [ClangOption, CC1Option, FC1Option]
@@ -4012,6 +4016,13 @@ def fopenmp_target_new_runtime : Flag<["-"], "fopenmp-target-new-runtime">,
Group<f_Group>, Flags<[HelpHidden]>, Visibility<[ClangOption, CC1Option]>;
def fno_openmp_target_new_runtime : Flag<["-"], "fno-openmp-target-new-runtime">,
Group<f_Group>, Flags<[HelpHidden]>, Visibility<[ClangOption, CC1Option]>;
+def fopenmp_target_fast : Flag<["-"], "fopenmp-target-fast">,
+ Group<f_Group>, Flags<[NoArgumentUnused, HelpHidden]>,
+ Visibility<[ClangOption]>,
+ HelpText<"Assert common GPU usage patterns to enable OpenMP runtime optimizations">;
+def fno_openmp_target_fast : Flag<["-"], "fno-openmp-target-fast">,
+ Group<f_Group>, Flags<[NoArgumentUnused, HelpHidden]>,
+ Visibility<[ClangOption]>;
defm openmp_optimistic_collapse : BoolFOption<"openmp-optimistic-collapse",
LangOpts<"OpenMPOptimisticCollapse">, DefaultFalse,
PosFlag<SetTrue, [], [ClangOption, CC1Option]>,
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index 5575b644c6814..616cc371ec4a0 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -6671,6 +6671,11 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
options::OPT_fno_offload_via_llvm, false) &&
(JA.isDeviceOffloading(Action::OFK_None) ||
JA.isDeviceOffloading(Action::OFK_OpenMP))) {
+
+ // Determine if target-fast optimizations should be enabled
+ bool TargetFastUsed =
+ Args.hasFlag(options::OPT_fopenmp_target_fast,
+ options::OPT_fno_openmp_target_fast, OFastEnabled);
switch (D.getOpenMPRuntime(Args)) {
case Driver::OMPRT_OMP:
case Driver::OMPRT_IOMP5:
@@ -6721,10 +6726,19 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
options::OPT_fno_openmp_assume_threads_oversubscription,
/*Default=*/false))
CmdArgs.push_back("-fopenmp-assume-threads-oversubscription");
- if (Args.hasArg(options::OPT_fopenmp_assume_no_thread_state))
+
+ // Handle -fopenmp-assume-no-thread-state (implied by target-fast)
+ if (Args.hasFlag(options::OPT_fopenmp_assume_no_thread_state,
+ options::OPT_fno_openmp_assume_no_thread_state,
+ /*Default=*/TargetFastUsed))
CmdArgs.push_back("-fopenmp-assume-no-thread-state");
- if (Args.hasArg(options::OPT_fopenmp_assume_no_nested_parallelism))
+
+ // Handle -fopenmp-assume-no-nested-parallelism (implied by target-fast)
+ if (Args.hasFlag(options::OPT_fopenmp_assume_no_nested_parallelism,
+ options::OPT_fno_openmp_assume_no_nested_parallelism,
+ /*Default=*/TargetFastUsed))
CmdArgs.push_back("-fopenmp-assume-no-nested-parallelism");
+
if (Args.hasArg(options::OPT_fopenmp_offload_mandatory))
CmdArgs.push_back("-fopenmp-offload-mandatory");
if (Args.hasArg(options::OPT_fopenmp_force_usm))
diff --git a/clang/test/Driver/openmp-target-fast-flag.c b/clang/test/Driver/openmp-target-fast-flag.c
new file mode 100644
index 0000000000000..4ded4be341801
--- /dev/null
+++ b/clang/test/Driver/openmp-target-fast-flag.c
@@ -0,0 +1,35 @@
+// REQUIRES: x86-registered-target, amdgpu-registered-target
+
+// RUN: %clang -### -fopenmp -fopenmp-targets=amdgcn-amd-amdhsa -Xopenmp-target=amdgcn-amd-amdhsa -march=gfx90a -nogpulib %s -O0 2>&1 \
+// RUN: | FileCheck -check-prefixes=DefaultTFast,DefaultTState,DefaultNoNestParallel %s
+
+// RUN: %clang -### -fopenmp -fopenmp-targets=amdgcn-amd-amdhsa -Xopenmp-target=amdgcn-amd-amdhsa -march=gfx90a -nogpulib -O0 -fopenmp-target-fast %s 2>&1 \
+// RUN: | FileCheck -check-prefixes=TState,NestParallel %s
+
+// RUN: %clang -### -fopenmp -fopenmp-targets=amdgcn-amd-amdhsa -Xopenmp-target=amdgcn-amd-amdhsa -march=gfx90a -nogpulib -O3 %s 2>&1 \
+// RUN: | FileCheck -check-prefixes=O3,DefaultTFast,DefaultTState,DefaultNoNestParallel %s
+
+// RUN: %clang -### -fopenmp -fopenmp-targets=amdgcn-amd-amdhsa -Xopenmp-target=amdgcn-amd-amdhsa -march=gfx90a -nogpulib -O3 -fno-openmp-target-fast %s 2>&1 \
+// RUN: | FileCheck -check-prefixes=O3,DefaultTState,DefaultNoNestParallel %s
+
+// RUN: %clang -### -fopenmp -fopenmp-targets=amdgcn-amd-amdhsa -Xopenmp-target=amdgcn-amd-amdhsa -march=gfx90a -nogpulib -Ofast %s 2>&1 \
+// RUN: | FileCheck -check-prefixes=OFast,TState,NestParallel %s
+
+// RUN: %clang -### -fopenmp -fopenmp-targets=amdgcn-amd-amdhsa -Xopenmp-target=amdgcn-amd-amdhsa -march=gfx90a -nogpulib -Ofast -fno-openmp-target-fast %s 2>&1 \
+// RUN: | FileCheck -check-prefixes=OFast,DefaultTState,DefaultNoNestParallel %s
+
+// RUN: %clang -### -fopenmp -fopenmp-targets=amdgcn-amd-amdhsa -Xopenmp-target=amdgcn-amd-amdhsa -march=gfx90a -nogpulib -O0 -fno-openmp-target-fast -fopenmp-target-fast %s 2>&1 \
+// RUN: | FileCheck -check-prefixes=TState,NestParallel %s
+
+// O3: -O3
+// OFast: -Ofast
+
+// DefaultTFast-NOT: {{"-f(no-)?openmp-target-fast"}}
+
+// TState: "-fopenmp-assume-no-thread-state"
+// TState-NOT: "-fno-openmp-assume-no-thread-state"
+// DefaultTState-NOT: {{"-f(no-)?openmp-assume-no-thread-state"}}
+
+// NestParallel: "-fopenmp-assume-no-nested-parallelism"
+// NestParallel-NOT: "-fno-openmp-assume-no-nested-parallelism"
+// DefaultNoNestParallel-NOT: {{"-f(-no-)?openmp-assume-no-nested-parallelism"}}
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