[OpenMP][DeviceRTL] Add the support for printf in a freestanding way

[tianshilei1992]

Summary

For NVPTX, printf can be used just with a function declaration. For AMDGCN, an
function definition is added, but it simply returns.

[jdoerfert]

I think clang will lower printf for nvptx targets to vprintf. Can you check that. Also check what it emits for amdgcn targets.

openmp/libomptarget/DeviceRTL/src/Debug.cpp

37

no need for these annotations.

[JonChesterfield]

I'm surprised that compiles for amdgpu, should be an error about variadic functions being unimplemented. Perhaps the cmake for the new runtime only builds on nvptx?

[tianshilei1992]

fixed wrong implementation

[jdoerfert]

So this compiles with nvptx and amdgcn properly?

[tianshilei1992]

In D109728#2999932, @jdoerfert wrote:

So this compiles with nvptx and amdgcn properly?

I checked nvptx, it works. Will Ask @JonChesterfield to help see if clang lowers it to vprintf as well.

[jdoerfert]

LG

[tianshilei1992]

openmp/libomptarget/DeviceRTL/src/Debug.cpp

37

Since AMD FE doesn't lower it to vprintf, what should we do here? If we enable the feature, there will be link error.

[JonChesterfield]

Stub it out with a macro until printf exists for amdgpu. Hopefully that won't be too much longer

[tianshilei1992]

gaurd the code with macro for now

openmp/libomptarget/DeviceRTL/include/Debug.h

33

@JonChesterfield This should be sufficient, right?

[JonChesterfield]

If we use PRINT everywhere, sure. Else we need PRINTF too.

I had #define printf(...) in mind instead of the MACROs, but strictly speaking one probably shouldn't override a libc function like that.

[tianshilei1992]

add PRINTF for AMDGCN as well

[jdoerfert]

LG, 2 nits.

openmp/libomptarget/DeviceRTL/include/Debug.h

27

openmp/libomptarget/DeviceRTL/include/Debug.h

 extern "C" {
 void __assert_assume(bool cond, const char *exp, const char *file, int line);
 }
 
 #define ASSERT(e) __assert_assume(e, #e, __FILE__, __LINE__)
 
 ///}
 
-// TODO: We need to allow actual printf.
-#define PRINTF(fmt, ...) (void)fmt;
+/// Print
+/// {
+extern "C" {
+int printf(const char *format, ...);
+int vprintf(const char *format, __builtin_va_list args);
+}
+
+#define PRINTF(fmt, ...) (void)printf(fmt, __VA_ARGS__);
 #define PRINT(str) PRINTF("%s", str)
 
+///}
+
 #endif

openmp/libomptarget/DeviceRTL/src/Debug.cpp

 //===--- Debug.cpp -------- Debug utilities ----------------------- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This file contains debug utilities
 //
 //===----------------------------------------------------------------------===//
 
 #include "Debug.h"
 #include "Configuration.h"
 
 using namespace _OMP;
 
 #pragma omp declare target
 
 extern "C" {
 void __assert_assume(bool cond, const char *exp, const char *file, int line) {
   if (!cond && config::isDebugMode(config::DebugLevel::Assertion)) {
     PRINTF("ASSERTION failed: %s at %s, line %d\n", exp, file, line);
     __builtin_trap();
   }
 
   __builtin_assume(cond);
 }
 }
 
+/// AMDGCN implementations of the printf.
+///
+///{
+#pragma omp begin declare variant match(device = {arch(amdgcn)})
+
+extern "C" {
+int vprintf(const char *format, __builtin_va_list args) { return 0; }
+}
+
+#pragma omp end declare variant
+///}
+
 #pragma omp end declare target