[libc] Support timing information in libc tests

This patch adds the necessary support to provide timing information in
`libc` tests. This is useful for determining which tests look what
amount of time. We also can use this as a test basis for providing more
fine-grained timing when implementing things on the GPU.

The main difficulty with this is the fact that the AMDGPU fixed
frequency clock operates at an unknown frequency. We need to read this
on a per-card basis from the driver and then copy it in. NVPTX on the
other hand has a fixed clock at a resolution of 1ns. I have also
increased the resolution of the print-outs as the majority of these are
below a millisecond for me.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D154446

1 files changed, 6 insertions, 0 deletions

diff --git a/libc/startup/gpu/amdgpu/start.cpp b/libc/startup/gpu/amdgpu/start.cpp
index 9a99955..7081ae6 100644
--- a/libc/startup/gpu/amdgpu/start.cpp
+++ b/libc/startup/gpu/amdgpu/start.cpp
@@ -15,6 +15,12 @@ extern "C" int main(int argc, char **argv, char **envp);
 
 namespace __llvm_libc {
 
+// The AMDGPU architecture provides a fixed frequency clock used for obtaining
+// real time. However, the frequency of this clock varies between cards and can
+// only be obtained via the driver. The loader will set this so we can use it.
+extern "C" [[gnu::visibility("protected")]] uint64_t
+    [[clang::address_space(4)]] __llvm_libc_clock_freq = 0;
+
 extern "C" uintptr_t __init_array_start[];
 extern "C" uintptr_t __init_array_end[];
 extern "C" uintptr_t __fini_array_start[];