[PATCH] D146973: [Clang] Implicitly include LLVM libc headers for the GPU

[Joseph Huber via Phabricator via cfe-commits]

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There is currently work to support basic `libc` functionality on the
GPU. Some basic information about the projects can be found at
https://libc.llvm.org/gpu_mode.html. Typically, including the system
headers on the GPU will result in an error. For this reason the LLVM
`libc` project will generate its own headers that can be used with the
GPU.

The problem is that these headers will use the same name as the system headers.
For that reason, D146970 <https://reviews.llvm.org/D146970> places it in the `llvm-libc` subfolder. In order to
still pick these files up, this patch adds changes in clang to default to
searching this directory when targeting the GPU. This lets offloading languages
such as OpenMP use the system `string.h` when compiling for the host and then
the LLVM libc `string.h` when targeting the GPU.

Depends on D146970 <https://reviews.llvm.org/D146970>


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D146973

Files:
  clang/lib/Driver/ToolChains/Clang.cpp
  clang/test/Driver/gpu-libc-headers.c


Index: clang/test/Driver/gpu-libc-headers.c
===================================================================
--- /dev/null
+++ clang/test/Driver/gpu-libc-headers.c
@@ -0,0 +1,22 @@
+// REQUIRES: nvptx-registered-target
+// REQUIRES: amdgpu-registered-target
+
+// RUN:   %clang -### --target=x86_64-unknown-linux-gnu -fopenmp=libomp \
+// RUN:     -fopenmp-targets=amdgcn-amd-amdhsa -Xopenmp-target=amdgcn-amd-amdhsa --offload-arch=gfx908  \
+// RUN:     -nogpulib %s 2>&1 | FileCheck %s --check-prefix=CHECK-HEADERS
+// RUN:   %clang -### --target=x86_64-unknown-linux-gnu -fopenmp=libomp \
+// RUN:     -fopenmp-targets=nvptx64-nvidia-cuda -Xopenmp-target=nvptx64-nvidia-cuda --offload-arch=sm_70  \
+// RUN:     -nogpulib %s 2>&1 | FileCheck %s --check-prefix=CHECK-HEADERS
+// RUN:   %clang -### --target=nvptx64-nvidia-cuda -march=sm_70 -nogpulib %s 2>&1 | \
+// RUN:     FileCheck %s --check-prefix=CHECK-HEADERS
+// RUN:   %clang -### --target=amdgcn-amd-amdhsa -mcpu=gfx1030 -nogpulib %s 2>&1 | \
+// RUN:     FileCheck %s --check-prefix=CHECK-HEADERS
+// CHECK-HEADERS: "-cc1"{{.*}}"-c-isystem" "{{.*}}include/llvm-libc"
+
+// RUN:   %clang -### --target=amdgcn-amd-amdhsa -mcpu=gfx1030 -nogpulib \
+// RUN:     -nogpuinc %s 2>&1 | FileCheck %s --check-prefix=CHECK-HEADERS-DISABLED
+// RUN:   %clang -### --target=amdgcn-amd-amdhsa -mcpu=gfx1030 -nogpulib \
+// RUN:     -nostdinc %s 2>&1 | FileCheck %s --check-prefix=CHECK-HEADERS-DISABLED
+// RUN:   %clang -### --target=amdgcn-amd-amdhsa -mcpu=gfx1030 -nogpulib \
+// RUN:     -nobuiltininc %s 2>&1 | FileCheck %s --check-prefix=CHECK-HEADERS-DISABLED
+// CHECK-HEADERS-DISABLED-NOT: "-cc1"{{.*}}"-c-isystem" "{{.*}}include/llvm-libc"
Index: clang/lib/Driver/ToolChains/Clang.cpp
===================================================================
--- clang/lib/Driver/ToolChains/Clang.cpp
+++ clang/lib/Driver/ToolChains/Clang.cpp
@@ -1214,6 +1214,23 @@
     CmdArgs.push_back("__clang_openmp_device_functions.h");
   }
 
+  // If we are compiling for a GPU target we want to override the system headers
+  // with ones created by the 'libc' project if present.
+  if (!Args.hasArg(options::OPT_nostdinc) &&
+      !Args.hasArg(options::OPT_nogpuinc) &&
+      !Args.hasArg(options::OPT_nobuiltininc) &&
+      (getToolChain().getTriple().isNVPTX() ||
+       getToolChain().getTriple().isAMDGCN())) {
+
+      // Add include/llvm-libc/* to our system include path. This lets us use
+      // GPU-specific system headers first.
+      SmallString<128> P(llvm::sys::path::parent_path(D.InstalledDir));
+      llvm::sys::path::append(P, "include");
+      llvm::sys::path::append(P, "llvm-libc");
+      CmdArgs.push_back("-c-isystem");
+      CmdArgs.push_back(Args.MakeArgString(P));
+  }
+
   // Add -i* options, and automatically translate to
   // -include-pch/-include-pth for transparent PCH support. It's
   // wonky, but we include looking for .gch so we can support seamless


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