Remove the need for #[cfg] in #[use_c_shim_if]

This commit tweaks the implementation of the synthetic
`#[use_c_shim_if]` attribute, renaming it to
`#[maybe_use_optimized_c_shim]` in the process. This no longer requires
specifying a `#[cfg]` clause indicating when the optimized intrinsic
should be used, but rather this is inferred and printed from the build
script.

The build script will now print out appropriate `#[cfg]` directives for
rustc to indicate what intrinsics it's compiling. This should remove the
need for us to keep the build script and the source in sync, but rather
the build script can simply take care of everything.

Author: alexcrichton

build.rs

@@ -1,5 +1,6 @@
 set -ex
 
+export CARGO_INCREMENTAL=0
 cargo=cargo
 
 # Test our implementation

ci/run.sh

@@ -87,11 +87,7 @@ intrinsics! {
         int_to_float!(i, i32, f64)
     }
 
-    #[use_c_shim_if(any(
-        all(target_arch = "x86", not(target_env = "msvc")),
-        all(target_arch = "x86_64", not(windows)),
-        all(target_arch = "x86_64", target_env = "msvc"),
-    ))]
+    #[maybe_use_optimized_c_shim]
     #[arm_aeabi_alias = __aeabi_l2f]
     pub extern "C" fn __floatdisf(i: i64) -> f32 {
         // On x86_64 LLVM will use native instructions for this conversion, we
@@ -103,7 +99,7 @@ intrinsics! {
         }
     }
 
-    #[use_c_shim_if(all(target_arch = "x86", not(target_env = "msvc")))]
+    #[maybe_use_optimized_c_shim]
     #[arm_aeabi_alias = __aeabi_l2d]
     pub extern "C" fn __floatdidf(i: i64) -> f64 {
         // On x86_64 LLVM will use native instructions for this conversion, we
@@ -135,19 +131,13 @@ intrinsics! {
         int_to_float!(i, u32, f64)
     }
 
-    #[use_c_shim_if(any(
-        all(target_arch = "x86", not(target_env = "msvc")),
-        all(target_arch = "x86_64", not(windows)),
-    ))]
+    #[maybe_use_optimized_c_shim]
     #[arm_aeabi_alias = __aeabi_ul2f]
     pub extern "C" fn __floatundisf(i: u64) -> f32 {
         int_to_float!(i, u64, f32)
     }
 
-    #[use_c_shim_if(any(
-        all(target_arch = "x86", not(target_env = "msvc")),
-        all(target_arch = "x86_64", not(windows)),
-    ))]
+    #[maybe_use_optimized_c_shim]
     #[arm_aeabi_alias = __aeabi_ul2d]
     pub extern "C" fn __floatundidf(i: u64) -> f64 {
         int_to_float!(i, u64, f64)

src/float/conv.rs

@@ -302,7 +302,7 @@ impl_wide_int!(u32, u64, 32);
 impl_wide_int!(u64, u128, 64);
 
 intrinsics! {
-    #[use_c_shim_if(/* always if C compilation is enabled */)]
+    #[maybe_use_optimized_c_shim]
     #[cfg(any(
         target_pointer_width = "16",
         target_pointer_width = "32",

src/int/mod.rs

@@ -84,7 +84,7 @@ trait UMulo: Int {
 impl UMulo for u128 {}
 
 intrinsics! {
-    #[use_c_shim_if(all(target_arch = "x86", not(target_env = "msvc")))]
+    #[maybe_use_optimized_c_shim]
     #[arm_aeabi_alias = __aeabi_lmul]
     pub extern "C" fn __muldi3(a: u64, b: u64) -> u64 {
         a.mul(b)

src/int/mul.rs

@@ -58,13 +58,13 @@ impl Divmod for i32 {}
 impl Divmod for i64 {}
 
 intrinsics! {
-    #[use_c_shim_if(all(target_arch = "arm", not(target_os = "ios"), not(thumb_1)))]
+    #[maybe_use_optimized_c_shim]
     #[arm_aeabi_alias = __aeabi_idiv]
     pub extern "C" fn __divsi3(a: i32, b: i32) -> i32 {
         a.div(b)
     }
 
-    #[use_c_shim_if(all(target_arch = "x86", not(target_env = "msvc")))]
+    #[maybe_use_optimized_c_shim]
     pub extern "C" fn __divdi3(a: i64, b: i64) -> i64 {
         a.div(b)
     }
@@ -74,15 +74,12 @@ intrinsics! {
         a.div(b)
     }
 
-    #[use_c_shim_if(all(target_arch = "arm",
-                    not(target_os = "ios"),
-                    not(target_env = "msvc"),
-                    not(thumb_1)))]
+    #[maybe_use_optimized_c_shim]
     pub extern "C" fn __modsi3(a: i32, b: i32) -> i32 {
         a.mod_(b)
     }
 
-    #[use_c_shim_if(all(target_arch = "x86", not(target_env = "msvc")))]
+    #[maybe_use_optimized_c_shim]
     pub extern "C" fn __moddi3(a: i64, b: i64) -> i64 {
         a.mod_(b)
     }
@@ -92,8 +89,7 @@ intrinsics! {
         a.mod_(b)
     }
 
-    #[use_c_shim_if(all(target_arch = "arm", not(target_env = "msvc"),
-                    not(target_os = "ios"), not(thumb_1)))]
+    #[maybe_use_optimized_c_shim]
     pub extern "C" fn __divmodsi4(a: i32, b: i32, rem: &mut i32) -> i32 {
         a.divmod(b, rem, |a, b| __divsi3(a, b))
     }

src/int/sdiv.rs

@@ -74,7 +74,7 @@ impl Lshr for u64 {}
 impl Lshr for u128 {}
 
 intrinsics! {
-    #[use_c_shim_if(all(target_arch = "x86", not(target_env = "msvc")))]
+    #[maybe_use_optimized_c_shim]
     #[arm_aeabi_alias = __aeabi_llsl]
     pub extern "C" fn __ashldi3(a: u64, b: u32) -> u64 {
         a.ashl(b)
@@ -84,7 +84,7 @@ intrinsics! {
         a.ashl(b)
     }
 
-    #[use_c_shim_if(all(target_arch = "x86", not(target_env = "msvc")))]
+    #[maybe_use_optimized_c_shim]
     #[arm_aeabi_alias = __aeabi_lasr]
     pub extern "C" fn __ashrdi3(a: i64, b: u32) -> i64 {
         a.ashr(b)
@@ -94,7 +94,7 @@ intrinsics! {
         a.ashr(b)
     }
 
-    #[use_c_shim_if(all(target_arch = "x86", not(target_env = "msvc")))]
+    #[maybe_use_optimized_c_shim]
     #[arm_aeabi_alias = __aeabi_llsr]
     pub extern "C" fn __lshrdi3(a: u64, b: u32) -> u64 {
         a.lshr(b)

src/int/shift.rs

@@ -152,9 +152,7 @@ macro_rules! udivmod_inner {
 }
 
 intrinsics! {
-    #[use_c_shim_if(all(target_arch = "arm",
-                        not(target_os = "ios"),
-                        not(thumb_1)))]
+    #[maybe_use_optimized_c_shim]
     #[arm_aeabi_alias = __aeabi_uidiv]
     /// Returns `n / d`
     pub extern "C" fn __udivsi3(n: u32, d: u32) -> u32 {
@@ -212,20 +210,14 @@ intrinsics! {
         (q << 1) | carry
     }
 
-    #[use_c_shim_if(all(target_arch = "arm",
-                        not(target_os = "ios"),
-                        not(target_env = "msvc"),
-                        not(thumb_1)))]
+    #[maybe_use_optimized_c_shim]
     /// Returns `n % d`
     pub extern "C" fn __umodsi3(n: u32, d: u32) -> u32 {
         let q = __udivsi3(n, d);
         n - q * d
     }
 
-    #[use_c_shim_if(all(target_arch = "arm",
-                        not(target_os = "ios"),
-                        not(target_env = "msvc"),
-                        not(thumb_1)))]
+    #[maybe_use_optimized_c_shim]
     /// Returns `n / d` and sets `*rem = n % d`
     pub extern "C" fn __udivmodsi4(n: u32, d: u32, rem: Option<&mut u32>) -> u32 {
         let q = __udivsi3(n, d);
@@ -235,13 +227,13 @@ intrinsics! {
         q
     }
 
-    #[use_c_shim_if(all(target_arch = "x86", not(target_env = "msvc")))]
+    #[maybe_use_optimized_c_shim]
     /// Returns `n / d`
     pub extern "C" fn __udivdi3(n: u64, d: u64) -> u64 {
         __udivmoddi4(n, d, None)
     }
 
-    #[use_c_shim_if(all(target_arch = "x86", not(target_env = "msvc")))]
+    #[maybe_use_optimized_c_shim]
     /// Returns `n % d`
     pub extern "C" fn __umoddi3(n: u64, d: u64) -> u64 {
         let mut rem = 0;

src/int/udiv.rs

@@ -30,8 +30,8 @@
 ///
 /// A quick overview of attributes supported right now are:
 ///
-/// * `use_c_shim_if` - takes a #[cfg] directive and falls back to the
-///   C-compiled version if `use_c` is specified.
+/// * `maybe_use_optimized_c_shim` - indicates that the Rust implementation is
+///   ignored if an optimized C version was compiled.
 /// * `aapcs_on_arm` - forces the ABI of the function to be `"aapcs"` on ARM and
 ///   the specified ABI everywhere else.
 /// * `unadjusted_on_win64` - like `aapcs_on_arm` this switches to the
@@ -51,15 +51,14 @@ macro_rules! intrinsics {
     // to the architecture-specific versions which should be more optimized. The
     // purpose of this macro is to easily allow specifying this.
     //
-    // The argument to `use_c_shim_if` is a `#[cfg]` directive which, when true,
-    // will cause this crate's exported version of `$name` to just redirect to
-    // the C implementation. No symbol named `$name` will be in the object file
-    // for this crate itself.
-    //
-    // When the `#[cfg]` directive is false, or when the `c` feature is
-    // disabled, the provided implementation is used instead.
+    // The `#[maybe_use_optimized_c_shim]` attribute indicates that this
+    // intrinsic may have an optimized C version. In these situations the build
+    // script, if the C code is enabled and compiled, will emit a cfg directive
+    // to get passed to rustc for our compilation. If that cfg is set we skip
+    // the Rust implementation, but if the attribute is not enabled then we
+    // compile in the Rust implementation.
     (
-        #[use_c_shim_if($($cfg_clause:tt)*)]
+        #[maybe_use_optimized_c_shim]
         $(#[$($attr:tt)*])*
         pub extern $abi:tt fn $name:ident( $($argname:ident:  $ty:ty),* ) -> $ret:ty {
             $($body:tt)*
@@ -68,7 +67,7 @@ macro_rules! intrinsics {
         $($rest:tt)*
     ) => (
 
-        #[cfg(all(use_c, $($cfg_clause)*))]
+        #[cfg($name = "optimized-c")]
         pub extern $abi fn $name( $($argname: $ty),* ) -> $ret {
             extern $abi {
                 fn $name($($argname: $ty),*) -> $ret;
@@ -78,7 +77,7 @@ macro_rules! intrinsics {
             }
         }
 
-        #[cfg(not(all(use_c, $($cfg_clause)*)))]
+        #[cfg(not($name = "optimized-c"))]
         intrinsics! {
             $(#[$($attr)*])*
             pub extern $abi fn $name( $($argname: $ty),* ) -> $ret {