x64: enable VTune support by default

[abrown]

After significant work in the ittapi-rs crate, this dependency should
build without issue on Wasmtime's supported operating systems: Windows,
Linux, and macOS. The difference in the release binary is <20KB, so this
change makes vtune a default build feature. This change upgrades
ittapi-rs to v0.2.0 and updates the documentation.

[abrown]

Some more details on this change:

• I wouldn't mind a second look at what was necessary to implement the multi-OS support in ittapi-rs; e.g., right now there are bindings built for each supported OS but unsupported OS-es will cause a build failure because they export nothing (see the lib.rs file)
• I ran cargo build --release before and after this change on a Fedora machine: without the vtune feature, the binary was 18,674,952 bytes; with vtune on by default, the binary was 18,691,768 bytes. That seemed like an appropriately-small increase.
• Thanks to @bnjbvr and @jlb6740 for working on this as well!

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[bnjbvr]

Thanks! Having written similar patches to test support on Windows before, this is not sufficient to enable vtune support more widely: there is also a cfg[target_os = "linux"] that need to be tweaked in profiling.rs iirc. There we should precisely enable this if the target os is linux or windows or mac, to not break compiling to other OS:es.

Then for the default feature: wonder if, in the Cargo.toml file, we can remove it from the list of dependencies (even optional), and then have per-os specific dependencies that both add the dependency and enable the feature?

[alexcrichton]

In addition to what @bnjbvr mentioned to make this default I think this block and this block need updating to include vtune

[abrown]

cfg[target_os = "linux"] that need to be tweaked in profiling.rs iirc

This is resolved by the latest commit.

we can remove it from the list of dependencies (even optional), and then have per-os specific dependencies that both add the dependency and enable the feature

We could, yes, but it seems like we should do one or the other: either enable the vtune feature exclusively in Cargo like you suggest OR have the cfg(...) conditions in the code itself. This could be resolved in a later PR, I think.

I think this block and this block need updating to include vtune

One of those was already "defaulted" by this PR but I added a default to wasmtime-jit to be consistent.

[abrown]

Oh, hold on: we also need to check for x86_64 in cfg(...) as well, right?

[alexcrichton]

Oh reading more closely ... personally I think it's ok to skip platform-specific checks in the #[cfg] because Wasmtime probably only really compiles on Linux/macOS/Windows anyway and other platforms likely hit compile errors elsewhere so this wouldn't be the only issue. If Wasmtime is ported to other platforms I think it'd be good to leave this in as "hey this crate probably needs an update" if possible

[abrown]

@bnjbvr, @alexcrichton: this removes the conditional compilation for the OS based on @alexcrichton's comment--this way, if someone ports Wasmtime to another OS but does not turn the vtune feature off, they will get a compile error in ittapi-rs which will motivate adding the OS support there.

I did add a target_arch = "x86_64" check since this feature only makes sense to be enabled on those machines anyways. ittapi-rs seems to compile fine on other architectures so there should be no problem there but this makes it more clear what environment the vtune feature is intended for.

[alexcrichton]

Looks good to me! I'll defer final approval to @bnjbvr though.

[bnjbvr]

Works fine with the wasmtime CLI binary on Linux! Thanks.

[bnjbvr]

hmm, it seems the end of this comment after -- would need to be removed, as it contains : but with no extra indication thereafter?

[abrown]

It's setting up the list of ways to enable VTune at runtime (if I understand what you mean).

[abrown]

I'll leave it and we can fix it up later if we think of a more clear way to word this.

[bnjbvr]

Oh, i misread it as a list entry as well, my bad. Maybe can reformulate slightly then:

Suggested change

	the `ittapi` library of JIT events. But it must still be enabled at
	runtime--enable runtime support based on how you use Wasmtime:
	the `ittapi` library of JIT events. But it must still be enabled at
	runtime with one of the following: