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std: Remove internal definitions of cfg_if! macro

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This is duplicated in a few locations throughout the sysroot to work
around issues with not exporting a macro in libstd but still wanting it
available to sysroot crates to define blocks. Nowadays though we can
simply depend on the `cfg-if` crate on crates.io, allowing us to use it
from there!
This commit is contained in:
Alex Crichton 2019-06-10 08:12:14 -07:00
parent a73ecb3d9c
commit 8eb7f36a3b
18 changed files with 49 additions and 233 deletions
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50
src/libcore/hint.rs
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@ -111,31 +111,31 @@ pub fn spin_loop() {
/// This function is a no-op, and does not even read from `dummy`.
#[inline]
#[unstable(feature = "test", issue = "27812")]
#[allow(unreachable_code)] // this makes #[cfg] a bit easier below.
pub fn black_box<T>(dummy: T) -> T {
cfg_if! {
if #[cfg(any(
target_arch = "asmjs",
all(
target_arch = "wasm32",
target_os = "emscripten"
)
))] {
#[inline]
unsafe fn black_box_impl<T>(d: T) -> T {
// these targets do not support inline assembly
let ret = crate::ptr::read_volatile(&d);
crate::mem::forget(d);
ret
}
} else {
#[inline]
unsafe fn black_box_impl<T>(d: T) -> T {
// we need to "use" the argument in some way LLVM can't
// introspect.
asm!("" : : "r"(&d));
d
}
}
// We need to "use" the argument in some way LLVM can't introspect, and on
// targets that support it we can typically leverage inline assembly to do
// this. LLVM's intepretation of inline assembly is that it's, well, a black
// box. This isn't the greatest implementation since it probably deoptimizes
// more than we want, but it's so far good enough.
#[cfg(not(any(
target_arch = "asmjs",
all(
target_arch = "wasm32",
target_os = "emscripten"
)
)))]
unsafe {
asm!("" : : "r"(&dummy));
return dummy;
}
// Not all platforms support inline assembly so try to do something without
// inline assembly which in theory still hinders at least some optimizations
// on those targets. This is the "best effort" scenario.
unsafe {
let ret = crate::ptr::read_volatile(&dummy);
crate::mem::forget(dummy);
ret
}
unsafe { black_box_impl(dummy) }
}