Put the asymmetric division behind a feature flag

Author: AaronKutch

Cargo.toml

@@ -60,6 +60,9 @@ no-lang-items = []
 # Only used in the compiler's build system
 rustc-dep-of-std = ['compiler-builtins', 'core']
 
+# Used for faster u128 division on x86_64
+asymmetric-asm = []
+
 [[example]]
 name = "intrinsics"
 required-features = ["compiler-builtins"]

src/int/sdiv.rs

@@ -53,7 +53,7 @@ intrinsics! {
 // `_trifecta` is efficient for large divisions, even when division
 // hardware is not availiable at all.
 
-#[cfg(not(target = "x86_64"))]
+#[cfg(any(not(feature = "asymmetric-asm"), not(target = "x86_64")))]
 intrinsics! {
     #[win64_128bit_abi_hack]
     /// Returns `n / d`
@@ -68,7 +68,7 @@ intrinsics! {
     }
 }
 
-#[cfg(target = "x86_64")]
+#[cfg(all(feature = "asymmetric-asm", target = "x86_64"))]
 intrinsics! {
     #[win64_128bit_abi_hack]
     /// Returns `a / b`

src/int/specialized_div_rem/mod.rs

@@ -18,7 +18,7 @@ mod asymmetric;
 
 /// This function is unsafe, because if the quotient of `duo` and `div` does not
 /// fit in a `u64`, a floating point exception is thrown.
-#[cfg(target = "x86_64")]
+#[cfg(all(feature = "asymmetric-asm", target = "x86_64"))]
 #[inline]
 unsafe fn u128_by_u64_div_rem(duo: u128, div: u64) -> (u64, u64) {
     let quo: u64;

src/int/udiv.rs

@@ -56,7 +56,7 @@ intrinsics! {
 // `_trifecta` is efficient for large divisions, even when division
 // hardware is not availiable at all.
 
-#[cfg(not(target = "x86_64"))]
+#[cfg(any(not(feature = "asymmetric-asm"), not(target = "x86_64")))]
 intrinsics! {
     #[win64_128bit_abi_hack]
     /// Returns `n / d`
@@ -81,7 +81,7 @@ intrinsics! {
     }
 }
 
-#[cfg(target = "x86_64")]
+#[cfg(all(feature = "asymmetric-asm", target = "x86_64"))]
 intrinsics! {
     #[win64_128bit_abi_hack]
     /// Returns `n / d`