Add an option so perf jit dump can optionally use TSC for timestamps.

[Kuinox]

I'm trying to make perf with Intel PT works with RyuJIT.
It currently doesn't work because Intel PT use TSC as a timestamp, and RyuJIT uses the monotonic clock for the dump timestamp.

The change would read the environment variable flag JITDUMP_USE_ARCH_TIMESTAMP, and when this flag is true, change the behavior of GetTimeStampNS to use __rdtsc, and also set the file header flag JITDUMP_FLAGS_ARCH_TIMESTAMP

I'm open to do the contribution, but I have next to no experience with C++.

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[jkotas]

This issue about Linux perf tool support. Historically, this fell into area-Diagnostics (for example, check dotnet/coreclr#26897, #229, #99986, ...). @tommcdon Is it intentional that we want Linux perf tool support to be tracked under CodeGen now?

[tommcdon]

This issue about Linux perf tool support. Historically, this fell into area-Diagnostics (for example, check dotnet/coreclr#26897, #229, #99986, ...). @tommcdon Is it intentional that we want Linux perf tool support to be tracked under CodeGen now?

Thanks Jan! Moving to dotnet-diagnostics-coreclr - I had originally read this issue as being an RyuJIT request

[Kuinox]

Since I will try to implement this, can I get some guidance if there is a specific method to call to get __rdtsc there, what to do when not running on AMD64 (compile flags for AMD64 ?)

[jkotas]

there is a specific method to call to get __rdtsc there

You can #include <x86intrin.h> that defines __rdtsc() method. Alternatively, you can implement it using inline assembly. There are plenty of examples in other projects that implement suport for JITDUMP_FLAGS_ARCH_TIMESTAMP: https://github.com/search?q=JITDUMP_FLAGS_ARCH_TIMESTAMP&type=code

what to do when not running on AMD64 (compile flags for AMD64 ?)

We use HOST_AMD64 define in situations like this, e.g. #if defined(HOST_AMD64).

[Kuinox]

You can #include <x86intrin.h> that defines

This question was also if I needed to use some helper method like what GetTimestampNs uses.