Introduce a select_implementation macro

Currently there is a macro called `llvm_intrinsically_optimized` that
uses an intrinsic rather than the function implementation if the
configuration is correct. Add a new macro `select_implementation` that
is somewhat cleaner to use.

In the future, we can update this macro with more fields to specify
other implementations that may be selected, such as something
architecture-specific or e.g. using a generic implementation for `f32`
routines, rather than those that convert to `f64`.

This introduces a `macros` module within `math/support`. We will be able
to move more things here later.

library/compiler-builtins/libm/src/math/mod.rs

@@ -74,6 +74,7 @@ macro_rules! div {
     };
 }
 
+// FIXME: phase this out, to be replaced by the more flexible `select_implementation`
 macro_rules! llvm_intrinsically_optimized {
     (#[cfg($($clause:tt)*)] $e:expr) => {
         #[cfg(all(intrinsics_enabled, not(feature = "force-soft-floats"), $($clause)*))]
@@ -85,6 +86,38 @@ macro_rules! llvm_intrinsically_optimized {
     };
 }
 
+// Private modules
+#[macro_use]
+mod support;
+mod arch;
+mod expo2;
+mod fenv;
+mod k_cos;
+mod k_cosf;
+mod k_expo2;
+mod k_expo2f;
+mod k_sin;
+mod k_sinf;
+mod k_tan;
+mod k_tanf;
+mod rem_pio2;
+mod rem_pio2_large;
+mod rem_pio2f;
+
+// Private re-imports
+use self::expo2::expo2;
+use self::k_cos::k_cos;
+use self::k_cosf::k_cosf;
+use self::k_expo2::k_expo2;
+use self::k_expo2f::k_expo2f;
+use self::k_sin::k_sin;
+use self::k_sinf::k_sinf;
+use self::k_tan::k_tan;
+use self::k_tanf::k_tanf;
+use self::rem_pio2::rem_pio2;
+use self::rem_pio2_large::rem_pio2_large;
+use self::rem_pio2f::rem_pio2f;
+
 // Public modules
 mod acos;
 mod acosf;
@@ -301,36 +334,6 @@ pub use self::tgammaf::tgammaf;
 pub use self::trunc::trunc;
 pub use self::truncf::truncf;
 
-// Private modules
-mod arch;
-mod expo2;
-mod fenv;
-mod k_cos;
-mod k_cosf;
-mod k_expo2;
-mod k_expo2f;
-mod k_sin;
-mod k_sinf;
-mod k_tan;
-mod k_tanf;
-mod rem_pio2;
-mod rem_pio2_large;
-mod rem_pio2f;
-
-// Private re-imports
-use self::expo2::expo2;
-use self::k_cos::k_cos;
-use self::k_cosf::k_cosf;
-use self::k_expo2::k_expo2;
-use self::k_expo2f::k_expo2f;
-use self::k_sin::k_sin;
-use self::k_sinf::k_sinf;
-use self::k_tan::k_tan;
-use self::k_tanf::k_tanf;
-use self::rem_pio2::rem_pio2;
-use self::rem_pio2_large::rem_pio2_large;
-use self::rem_pio2f::rem_pio2f;
-
 #[inline]
 fn get_high_word(x: f64) -> u32 {
     (x.to_bits() >> 32) as u32

library/compiler-builtins/libm/src/math/support/macros.rs

@@ -0,0 +1,34 @@
+/// Choose among using an intrinsic (if available) and falling back to the default function body.
+/// Returns directly if the intrinsic version is used, otherwise continues to the rest of the
+/// function.
+///
+/// Use this if the intrinsic is likely to be more performant on the platform(s) specified
+/// in `intrinsic_available`.
+///
+/// The `cfg` used here is controlled by `build.rs` so the passed meta does not need to account
+/// for e.g. the `unstable-intrinsics` or `force-soft-float` features.
+macro_rules! select_implementation {
+    (
+        name: $fname:ident,
+        // Configuration meta for when to call intrinsics and let LLVM figure it out
+        $( use_intrinsic: $use_intrinsic:meta, )?
+        args: $($arg:ident),+ ,
+    ) => {
+        // FIXME: these use paths that are a pretty fragile (`super`). We should figure out
+        // something better w.r.t. how this is vendored into compiler-builtins.
+
+        // Never use intrinsics if we are forcing soft floats, and only enable with the
+        // `unstable-intrinsics` feature.
+        #[cfg(intrinsics_enabled)]
+        select_implementation! {
+            @cfg $( $use_intrinsic )?;
+            if true {
+                return  super::arch::intrinsics::$fname( $($arg),+ );
+            }
+        }
+    };
+
+    // Coalesce helper to construct an expression only if a config is provided
+    (@cfg ; $ex:expr) => { };
+    (@cfg $provided:meta; $ex:expr) => { #[cfg($provided)] $ex };
+}

library/compiler-builtins/libm/src/math/support/mod.rs

@@ -0,0 +1,2 @@
+#[macro_use]
+pub mod macros;