[Libomptarget] Add RPC-based printf implementation for OpenMP (#85638)

Summary:
This patch adds an implementation of `printf` that's provided by the GPU
C library runtime. This `pritnf` currently implemented using the same
wrapper handling that OpenMP sets up. This will be removed once we have
proper varargs support.

This `printf` differs from the one CUDA offers in that it is synchronous
and uses a finite size. Additionally we support pretty much every format
specifier except the `%n` option.

Depends on #85331

openmp/libomptarget/DeviceRTL/CMakeLists.txt

@@ -122,6 +122,11 @@ set(clang_opt_flags -O3 -mllvm -openmp-opt-disable -DSHARED_SCRATCHPAD_SIZE=512
 set(link_opt_flags  -O3        -openmp-opt-disable -attributor-enable=module -vectorize-slp=false )
 set(link_export_flag -passes=internalize -internalize-public-api-file=${source_directory}/exports)
 
+# If the user built with the GPU C library enabled we will use that instead.
+if(${LIBOMPTARGET_GPU_LIBC_SUPPORT})
+  list(APPEND clang_opt_flags -DOMPTARGET_HAS_LIBC)
+endif()
+
 # Prepend -I to each list element
 set (LIBOMPTARGET_LLVM_INCLUDE_DIRS_DEVICERTL "${LIBOMPTARGET_LLVM_INCLUDE_DIRS}")
 list(TRANSFORM LIBOMPTARGET_LLVM_INCLUDE_DIRS_DEVICERTL PREPEND "-I")

openmp/libomptarget/DeviceRTL/src/LibC.cpp

@@ -53,10 +53,23 @@ void memset(void *dst, int C, size_t count) {
     dstc[I] = C;
 }
 
+// If the user built with the GPU C library enabled we will assume that we can
+// call it.
+#ifdef OMPTARGET_HAS_LIBC
+
+// TODO: Remove this handling once we have varargs support.
+extern struct FILE *stdout;
+int32_t rpc_fprintf(FILE *, const char *, void *, uint64_t);
+
+int32_t __llvm_omp_vprintf(const char *Format, void *Arguments, uint32_t Size) {
+  return rpc_fprintf(stdout, Format, Arguments, Size);
+}
+#else
 /// printf() calls are rewritten by CGGPUBuiltin to __llvm_omp_vprintf
 int32_t __llvm_omp_vprintf(const char *Format, void *Arguments, uint32_t Size) {
   return impl::omp_vprintf(Format, Arguments, Size);
 }
+#endif
 }
 
 #pragma omp end declare target

openmp/libomptarget/test/libc/printf.c

@@ -0,0 +1,36 @@
+// RUN: %libomptarget-compile-run-and-check-generic
+
+// REQUIRES: libc
+
+#include <stdio.h>
+
+int main() {
+  // CHECK: PASS
+#pragma omp target
+  { printf("PASS\n"); }
+
+  // CHECK: PASS
+#pragma omp target
+  { printf("%s\n", "PASS"); }
+
+  // CHECK: PASS
+  // CHECK: PASS
+  // CHECK: PASS
+  // CHECK: PASS
+  // CHECK: PASS
+  // CHECK: PASS
+  // CHECK: PASS
+  // CHECK: PASS
+#pragma omp target teams num_teams(4)
+#pragma omp parallel num_threads(2)
+  { printf("PASS\n"); }
+
+  // CHECK: PASS
+  char str[] = {'P', 'A', 'S', 'S', '\0'};
+#pragma omp target map(to : str)
+  { printf("%s\n", str); }
+
+  // CHECK: 11111111111
+#pragma omp target
+  { printf("%s%-.0f%4b%c%ld\n", "1111", 1.0, 0xf, '1', 1lu); }
+}