Implement __bswap[sdt]i2 intrinsics

These can be emitted by gcc, at least if requested specifically via __builtin_bswap{32,64,128}.

library/compiler-builtins/README.md

@@ -154,6 +154,9 @@ rely on CI.
 - [ ] arm/unordsf2vfp.S
 - [x] ashldi3.c
 - [x] ashrdi3.c
+- [x] bswapdi2.c
+- [x] bswapsi2.c
+- [x] bswapti2.c
 - [x] comparedf2.c
 - [x] comparesf2.c
 - [x] divdf3.c

library/compiler-builtins/build.rs

@@ -161,6 +161,9 @@ fn configure_check_cfg() {
         "__ashlsi3",
         "__ashrdi3",
         "__ashrsi3",
+        "__bswapsi2",
+        "__bswapdi2",
+        "__bswapti2",
         "__clzsi2",
         "__divdi3",
         "__divsi3",

library/compiler-builtins/src/int/bswap.rs

@@ -0,0 +1,22 @@
+intrinsics! {
+    #[maybe_use_optimized_c_shim]
+    #[avr_skip]
+    /// Swaps bytes in 32-bit number
+    pub extern "C" fn __bswapsi2(x: u32) -> u32 {
+        x.swap_bytes()
+    }
+
+    #[maybe_use_optimized_c_shim]
+    #[avr_skip]
+    /// Swaps bytes in 64-bit number
+    pub extern "C" fn __bswapdi2(x: u64) -> u64 {
+        x.swap_bytes()
+    }
+
+    #[maybe_use_optimized_c_shim]
+    #[avr_skip]
+    /// Swaps bytes in 128-bit number
+    pub extern "C" fn __bswapti2(x: u128) -> u128 {
+        x.swap_bytes()
+    }
+}

library/compiler-builtins/src/int/mod.rs

@@ -4,6 +4,7 @@ mod specialized_div_rem;
 
 pub mod addsub;
 mod big;
+pub mod bswap;
 pub mod leading_zeros;
 pub mod mul;
 pub mod sdiv;

library/compiler-builtins/testcrate/tests/misc.rs

@@ -92,6 +92,40 @@ fn leading_zeros() {
     })
 }
 
+#[test]
+#[cfg(not(target_arch = "avr"))]
+fn bswap() {
+    use compiler_builtins::int::bswap::{__bswapdi2, __bswapsi2};
+    fuzz(N, |x: u32| {
+        assert_eq!(x.swap_bytes(), __bswapsi2(x));
+    });
+    fuzz(N, |x: u64| {
+        assert_eq!(x.swap_bytes(), __bswapdi2(x));
+    });
+
+    assert_eq!(__bswapsi2(0x12345678u32), 0x78563412u32);
+    assert_eq!(__bswapsi2(0x00000001u32), 0x01000000u32);
+    assert_eq!(__bswapdi2(0x123456789ABCDEF0u64), 0xF0DEBC9A78563412u64);
+    assert_eq!(__bswapdi2(0x0200000001000000u64), 0x0000000100000002u64);
+
+    #[cfg(any(target_pointer_width = "32", target_pointer_width = "64"))]
+    {
+        use compiler_builtins::int::bswap::__bswapti2;
+        fuzz(N, |x: u128| {
+            assert_eq!(x.swap_bytes(), __bswapti2(x));
+        });
+
+        assert_eq!(
+            __bswapti2(0x123456789ABCDEF013579BDF02468ACEu128),
+            0xCE8A4602DF9B5713F0DEBC9A78563412u128
+        );
+        assert_eq!(
+            __bswapti2(0x04000000030000000200000001000000u128),
+            0x00000001000000020000000300000004u128
+        );
+    }
+}
+
 // This is approximate because of issues related to
 // https://github.com/rust-lang/rust/issues/73920.
 // TODO how do we resolve this indeterminacy?