cfg_target_feature doesn't work for all supported rustc features

[raphaelcohn]

Viz,

target_feature does not work for rdrnd but does work for avx2 (rustc rustc 1.27.0-nightly (ac3c2288f 2018-04-18), Mac OS X El Capitan).

This is created with cargo rustc -- -C target-feature=+avx2

#[cfg(all(target_arch = "x86_64", target_feature = "avx2"))]
#[inline(always)]
pub fn generate_hyper_thread_safe_random_u64() -> u64
{
	println!("Hello World");
}

However, this is NOT created with cargo rustc -- -C target-feature=+rdrnd

#[cfg(all(target_arch = "x86_64", target_feature = "rdrnd"))]
#[inline(always)]
pub fn generate_hyper_thread_safe_random_u64() -> u64
{
	println!("Hello World");
}

ie, changing avx2 for rdrnd causes the function to not be compiled. Both avx2 and rdnrd are present inrustc --print target-features

[raphaelcohn]

Ah-ha! The code in https://github.com/rust-lang/rust/blob/186db76159c57f4af442d8aa5e7c1a330ee0004b/src/librustc_trans/llvm_util.rs, line 133 ("rdrand", None), mis-labels rdrnd as rdrand.

So the list from rustc --print target-features diverges from that in that file. So it seems there's more than one source of truth.

Since target_feature is not yet stable, perhaps we could at least make the two lists match up?

Changing #[cfg(all(target_arch = "x86_64", target_feature = "rdrnd"))] to #[cfg(all(target_arch = "x86_64", target_feature = "rdrand"))] (rdrnd to rdrand) fixes things.

[newpavlov]

It was discussed here. In short rdrnd is name used by LLVM for compatibility with GCC, while rdrand is a vendor name. It was decided to use vendor names for target_feature, but looks like it didn't get translated to rustc options, which I think it should.

[newpavlov]

I think it should be closed in favor of #49653?

[raphaelcohn]

@newpavlov It seems a sensible place to consolidate it to.

[raphaelcohn]

Closing as it's no longer relevant.