[Clang] Do not implicitly link C libraries for the GPU targets (#109052)

Summary:
I initially thought that it would be convenient to automatically link
these libraries like they are for standard C/C++ targets. However, this
created issues when trying to use C++ as a GPU target. This patch moves
the logic to now implicitly pass it as part of the offloading toolchain
instead, if found. This means that the user needs to set the target
toolchain for the link job for automatic detection, but can still be
done manually via `-Xoffload-linker -lc`.

Author: jhuber6

clang/lib/Driver/ToolChains/AMDGPU.cpp

@@ -648,8 +648,6 @@ void amdgpu::Linker::ConstructJob(Compilation &C, const JobAction &JA,
         Args.MakeArgString("-plugin-opt=-mattr=" + llvm::join(Features, ",")));
   }
 
-  addGPULibraries(getToolChain(), Args, CmdArgs);
-
   CmdArgs.push_back("-o");
   CmdArgs.push_back(Output.getFilename());
   C.addCommand(std::make_unique<Command>(
@@ -1089,4 +1087,4 @@ bool AMDGPUToolChain::shouldSkipSanitizeOption(
     return true;
   }
   return false;
-}
+}

clang/lib/Driver/ToolChains/Clang.cpp

@@ -9223,6 +9223,25 @@ void LinkerWrapper::ConstructJob(Compilation &C, const JobAction &JA,
     A->claim();
   }
 
+  // Pass in the C library for GPUs if present and not disabled.
+  if (!Args.hasArg(options::OPT_nostdlib, options::OPT_r, options::OPT_nogpulib,
+                   options::OPT_nodefaultlibs, options::OPT_nolibc,
+                   options::OPT_nogpulibc)) {
+    forAllAssociatedToolChains(C, JA, getToolChain(), [&](const ToolChain &TC) {
+      // The device C library is only available for NVPTX and AMDGPU targets
+      // currently.
+      if (!TC.getTriple().isNVPTX() && !TC.getTriple().isAMDGPU())
+        return;
+      bool HasLibC = TC.getStdlibIncludePath().has_value();
+      if (HasLibC) {
+        CmdArgs.push_back(Args.MakeArgString(
+            "--device-linker=" + TC.getTripleString() + "=" + "-lc"));
+        CmdArgs.push_back(Args.MakeArgString(
+            "--device-linker=" + TC.getTripleString() + "=" + "-lm"));
+      }
+    });
+  }
+
   // If we disable the GPU C library support it needs to be forwarded to the
   // link job.
   if (!Args.hasFlag(options::OPT_gpulibc, options::OPT_nogpulibc, true))

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -510,22 +510,6 @@ void tools::addLinkerCompressDebugSectionsOption(
   }
 }
 
-void tools::addGPULibraries(const ToolChain &TC, const llvm::opt::ArgList &Args,
-                            llvm::opt::ArgStringList &CmdArgs) {
-  if (Args.hasArg(options::OPT_nostdlib, options::OPT_r,
-                  options::OPT_nodefaultlibs, options::OPT_nolibc,
-                  options::OPT_nogpulibc))
-    return;
-
-  // If the user's toolchain has the 'include/<triple>/` path, we assume it
-  // supports the standard C libraries for the GPU and include them.
-  bool HasLibC = TC.getStdlibIncludePath().has_value();
-  if (HasLibC) {
-    CmdArgs.push_back("-lc");
-    CmdArgs.push_back("-lm");
-  }
-}
-
 void tools::AddTargetFeature(const ArgList &Args,
                              std::vector<StringRef> &Features,
                              OptSpecifier OnOpt, OptSpecifier OffOpt,

clang/lib/Driver/ToolChains/CommonArgs.h

@@ -35,9 +35,6 @@ void addLinkerCompressDebugSectionsOption(const ToolChain &TC,
                                           const llvm::opt::ArgList &Args,
                                           llvm::opt::ArgStringList &CmdArgs);
 
-void addGPULibraries(const ToolChain &TC, const llvm::opt::ArgList &Args,
-                     llvm::opt::ArgStringList &CmdArgs);
-
 void claimNoWarnArgs(const llvm::opt::ArgList &Args);
 
 bool addSanitizerRuntimes(const ToolChain &TC, const llvm::opt::ArgList &Args,

clang/lib/Driver/ToolChains/Cuda.cpp

@@ -635,8 +635,6 @@ void NVPTX::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   for (StringRef Feature : Features)
     CmdArgs.append({"--feature", Args.MakeArgString(Feature)});
 
-  addGPULibraries(getToolChain(), Args, CmdArgs);
-
   // Add paths for the default clang library path.
   SmallString<256> DefaultLibPath =
       llvm::sys::path::parent_path(TC.getDriver().Dir);

clang/test/Driver/openmp-offload-gpu.c

@@ -377,4 +377,4 @@
 // RUN:      --cuda-path=%S/Inputs/CUDA_102/usr/local/cuda \
 // RUN:      --offload-arch=sm_52 -nogpulibc -nogpuinc %s 2>&1 \
 // RUN:   | FileCheck --check-prefix=LIBC-GPU %s
-// LIBC-GPU: clang-linker-wrapper{{.*}}"--device-compiler=-nolibc"
+// LIBC-GPU-NOT: clang-linker-wrapper{{.*}}"--device-linker"

libc/cmake/modules/prepare_libc_gpu_build.cmake

@@ -21,10 +21,10 @@ if(LIBC_TARGET_TRIPLE)
   set(CMAKE_REQUIRED_FLAGS "--target=${LIBC_TARGET_TRIPLE}")
 endif()
 if(LIBC_TARGET_ARCHITECTURE_IS_AMDGPU)
-  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nogpulib -nostdlib")
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nogpulib")
 elseif(LIBC_TARGET_ARCHITECTURE_IS_NVPTX)
   set(CMAKE_REQUIRED_FLAGS
-      "${CMAKE_REQUIRED_FLAGS} -flto -c -Wno-unused-command-line-argument -nostdlib")
+      "${CMAKE_REQUIRED_FLAGS} -flto -c -Wno-unused-command-line-argument")
 endif()
 
 # Optionally set up a job pool to limit the number of GPU tests run in parallel.

libcxx/cmake/caches/AMDGPU.cmake

@@ -33,4 +33,4 @@ set(LIBCXX_ADDITIONAL_COMPILE_FLAGS
     "-nogpulib;-flto;-fconvergent-functions;-Xclang;-mcode-object-version=none" CACHE STRING "")
 set(LIBCXXABI_ADDITIONAL_COMPILE_FLAGS
     "-nogpulib;-flto;-fconvergent-functions;-Xclang;-mcode-object-version=none" CACHE STRING "")
-set(CMAKE_REQUIRED_FLAGS "-nogpulib -nodefaultlibs" CACHE STRING "")
+set(CMAKE_REQUIRED_FLAGS "-nogpulib" CACHE STRING "")

libcxx/cmake/caches/NVPTX.cmake

@@ -33,4 +33,4 @@ set(LIBCXX_ADDITIONAL_COMPILE_FLAGS
     "-nogpulib;-flto;-fconvergent-functions;--cuda-feature=+ptx63" CACHE STRING "")
 set(LIBCXXABI_ADDITIONAL_COMPILE_FLAGS
     "-nogpulib;-flto;-fconvergent-functions;--cuda-feature=+ptx63" CACHE STRING "")
-set(CMAKE_REQUIRED_FLAGS "-nogpulib -nodefaultlibs -flto -c" CACHE STRING "")
+set(CMAKE_REQUIRED_FLAGS "-nogpulib -flto -c" CACHE STRING "")