[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 ↗

(On Diff #372358)

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 ↗

(On Diff #372369)

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

35

@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

29

openmp/libomptarget/DeviceRTL/include/Debug.h

 void __assert_fail(const char *assertion, const char *file, unsigned line,
                    const char *function);
 }
 
 #define ASSERT(e) __assert_assume(e, #e, __FILE__, __LINE__)
 
 ///}
 
-// TODO: We need to allow actual printf.
-#define PRINTF(fmt, ...) (void)fmt;
+/// Print
+/// TODO: For now we have to use macros to gaurd the code because Clang lower
+/// `printf` to different function calls on NVPTX and AMDGCN platforms, and it
+/// doesn't work for AMDGCN. After it can work on AMDGCN, we will remove the
+/// macro.
+/// {
+
+#ifndef __AMDGCN__
+extern "C" {
+int printf(const char *format, ...);
+}
+
+#define PRINTF(fmt, ...) (void)printf(fmt, __VA_ARGS__);
 #define PRINT(str) PRINTF("%s", str)
+#else
+#define PRINT(str)
+#endif
 
 ///}
 
 /// Enter a debugging scope for performing function traces. Enabled with
 /// FunctionTracting set in the debug kind.
 #define FunctionTracingRAII()                                                  \
   DebugEntryRAII Entry(__LINE__, __PRETTY_FUNCTION__);