[compiler-rt][XRay] xray_interface.h not C compliant

[Thyre]

Heya 👋

We're currently looking into integrating a prototype developed by @sebastiankreutzer and @pauladelmann, which enables the usage of XRay for compiler instrumentation in the performance measurement infrastructure Score-P. The prototype so far uses C++, but we are looking to port it to C, as this prevents some issues we might run into down the line. A lot of work in that direction is already done, but we noticed an issue with xray_interface.h.

xray_interface.h includes two headers:

llvm-project/compiler-rt/include/xray/xray_interface.h

Lines 17 to 18 in 7e7871d

	#include <cstddef>
	#include <cstdint>

Both headers are C++ heades, not compatible with C. A bit further down, we see std::size_t being used:

llvm-project/compiler-rt/include/xray/xray_interface.h

Line 71 in 7e7871d

extern int __xray_set_customevent_handler(void (*entry)(void *, std::size_t));

These issues seem to exist since XRay was introduced. Probably no tool tried to use this with C...
Fixing this should be straight-forward, replacing #include <cstddef> with #include <stddef.h>, #include <cstdint> with #include <stdint.h> and std::size_t with size_t.
But maybe we want to introduce a test to make sure that this header stays C compliant?

[frederick-vs-ja]

It seems intended that this header is C++-only, as shown by the comment.

llvm-project/compiler-rt/include/xray/xray_interface.h

Line 1 in 7e7871d

//===- xray_interface.h -----------------------------------------*- C++ -*-===//

But IIUC this header doesn't use any C++-specific core language feature.

It's a bit unfortunate that xray_records.h uses default member initializers which are absent in C.

[Thyre]

That's right. I haven't even looked at xray_records.h yet.

For us, having xray_interface.h a C compliant header should be sufficient for now, though it would probably be nice to have XRay completely as a C interface.

At the moment, I basically created a mockup of the header with the changes required to make it C compliant. For xray_interface.h, the changes required are small. I'll try to create a draft for this header today.