Merge #1372

1372: rp: add division intrinsics r=Dirbaio a=pennae

rp2040-hal adds division intrinsics using the hardware divider unit in the SIO, as does the pico-sdk itself. using the hardware is faster than the compiler_rt implementations, and more compact too.

since embassy does not expose the hardware divider in any way (yet?) we could go even further an remove the state-saving code rp2040-hal needs, but that doesn't seem to be worth it.

Co-authored-by: pennae <github@quasiparticle.net>

embassy-rp/src/intrinsics.rs

@@ -274,3 +274,201 @@ macro_rules! intrinsics {
         intrinsics!($($rest)*);
     };
 }
+
+// Credit: taken from `rp-hal` (also licensed Apache+MIT)
+// https://github.com/rp-rs/rp-hal/blob/main/rp2040-hal/src/sio.rs
+
+// This takes advantage of how AAPCS defines a 64-bit return on 32-bit registers
+// by packing it into r0[0:31] and r1[32:63].  So all we need to do is put
+// the remainder in the high order 32 bits of a 64 bit result.   We can also
+// alias the division operators to these for a similar reason r0 is the
+// result either way and r1 a scratch register, so the caller can't assume it
+// retains the argument value.
+#[cfg(target_arch = "arm")]
+core::arch::global_asm!(
+    ".macro hwdivider_head",
+    "ldr    r2, =(0xd0000000)", // SIO_BASE
+    // Check the DIRTY state of the divider by shifting it into the C
+    // status bit.
+    "ldr    r3, [r2, #0x078]", // DIV_CSR
+    "lsrs   r3, #2",           // DIRTY = 1, so shift 2 down
+    // We only need to save the state when DIRTY, otherwise we can just do the
+    // division directly.
+    "bcs    2f",
+    "1:",
+    // Do the actual division now, we're either not DIRTY, or we've saved the
+    // state and branched back here so it's safe now.
+    ".endm",
+    ".macro hwdivider_tail",
+    // 8 cycle delay to wait for the result.  Each branch takes two cycles
+    // and fits into a 2-byte Thumb instruction, so this is smaller than
+    // 8 NOPs.
+    "b      3f",
+    "3: b   3f",
+    "3: b   3f",
+    "3: b   3f",
+    "3:",
+    // Read the quotient last, since that's what clears the dirty flag.
+    "ldr    r1, [r2, #0x074]", // DIV_REMAINDER
+    "ldr    r0, [r2, #0x070]", // DIV_QUOTIENT
+    // Either return to the caller or back to the state restore.
+    "bx     lr",
+    "2:",
+    // Since we can't save the signed-ness of the calculation, we have to make
+    // sure that there's at least an 8 cycle delay before we read the result.
+    // The push takes 5 cycles, and we've already spent at least 7 checking
+    // the DIRTY state to get here.
+    "push   {{r4-r6, lr}}",
+    // Read the quotient last, since that's what clears the dirty flag.
+    "ldr    r3, [r2, #0x060]", // DIV_UDIVIDEND
+    "ldr    r4, [r2, #0x064]", // DIV_UDIVISOR
+    "ldr    r5, [r2, #0x074]", // DIV_REMAINDER
+    "ldr    r6, [r2, #0x070]", // DIV_QUOTIENT
+    // If we get interrupted here (before a write sets the DIRTY flag) it's
+    // fine, since we have the full state, so the interruptor doesn't have to
+    // restore it.  Once the write happens and the DIRTY flag is set, the
+    // interruptor becomes responsible for restoring our state.
+    "bl     1b",
+    // If we are interrupted here, then the interruptor will start an incorrect
+    // calculation using a wrong divisor, but we'll restore the divisor and
+    // result ourselves correctly. This sets DIRTY, so any interruptor will
+    // save the state.
+    "str    r3, [r2, #0x060]", // DIV_UDIVIDEND
+    // If we are interrupted here, the the interruptor may start the
+    // calculation using incorrectly signed inputs, but we'll restore the
+    // result ourselves. This sets DIRTY, so any interruptor will save the
+    // state.
+    "str    r4, [r2, #0x064]", // DIV_UDIVISOR
+    // If we are interrupted here, the interruptor will have restored
+    // everything but the quotient may be wrongly signed.  If the calculation
+    // started by the above writes is still ongoing it is stopped, so it won't
+    // replace the result we're restoring.  DIRTY and READY set, but only
+    // DIRTY matters to make the interruptor save the state.
+    "str    r5, [r2, #0x074]", // DIV_REMAINDER
+    // State fully restored after the quotient write.  This sets both DIRTY
+    // and READY, so whatever we may have interrupted can read the result.
+    "str    r6, [r2, #0x070]", // DIV_QUOTIENT
+    "pop    {{r4-r6, pc}}",
+    ".endm",
+);
+
+macro_rules! division_function {
+    (
+        $name:ident $($intrinsic:ident)* ( $argty:ty ) {
+            $($begin:literal),+
+        }
+    ) => {
+        #[cfg(all(target_arch = "arm", feature = "intrinsics"))]
+        core::arch::global_asm!(
+            // Mangle the name slightly, since this is a global symbol.
+            concat!(".global _rphal_", stringify!($name)),
+            concat!(".type _rphal_", stringify!($name), ", %function"),
+            ".align 2",
+            concat!("_rphal_", stringify!($name), ":"),
+            $(
+                concat!(".global ", stringify!($intrinsic)),
+                concat!(".type ", stringify!($intrinsic), ", %function"),
+                concat!(stringify!($intrinsic), ":"),
+            )*
+
+            "hwdivider_head",
+            $($begin),+ ,
+            "hwdivider_tail",
+        );
+
+        #[cfg(all(target_arch = "arm", not(feature = "intrinsics")))]
+        core::arch::global_asm!(
+            // Mangle the name slightly, since this is a global symbol.
+            concat!(".global _rphal_", stringify!($name)),
+            concat!(".type _rphal_", stringify!($name), ", %function"),
+            ".align 2",
+            concat!("_rphal_", stringify!($name), ":"),
+
+            "hwdivider_head",
+            $($begin),+ ,
+            "hwdivider_tail",
+        );
+
+        #[cfg(target_arch = "arm")]
+        extern "aapcs" {
+            // Connect a local name to global symbol above through FFI.
+            #[link_name = concat!("_rphal_", stringify!($name)) ]
+            fn $name(n: $argty, d: $argty) -> u64;
+        }
+
+        #[cfg(not(target_arch = "arm"))]
+        #[allow(unused_variables)]
+        unsafe fn $name(n: $argty, d: $argty) -> u64 { 0 }
+    };
+}
+
+division_function! {
+    unsigned_divmod __aeabi_uidivmod __aeabi_uidiv ( u32 ) {
+        "str    r0, [r2, #0x060]", // DIV_UDIVIDEND
+        "str    r1, [r2, #0x064]"  // DIV_UDIVISOR
+    }
+}
+
+division_function! {
+    signed_divmod __aeabi_idivmod __aeabi_idiv ( i32 ) {
+        "str    r0, [r2, #0x068]", // DIV_SDIVIDEND
+        "str    r1, [r2, #0x06c]"  // DIV_SDIVISOR
+    }
+}
+
+fn divider_unsigned(n: u32, d: u32) -> DivResult<u32> {
+    let packed = unsafe { unsigned_divmod(n, d) };
+    DivResult {
+        quotient: packed as u32,
+        remainder: (packed >> 32) as u32,
+    }
+}
+
+fn divider_signed(n: i32, d: i32) -> DivResult<i32> {
+    let packed = unsafe { signed_divmod(n, d) };
+    // Double casts to avoid sign extension
+    DivResult {
+        quotient: packed as u32 as i32,
+        remainder: (packed >> 32) as u32 as i32,
+    }
+}
+
+/// Result of divide/modulo operation
+struct DivResult<T> {
+    /// The quotient of divide/modulo operation
+    pub quotient: T,
+    /// The remainder of divide/modulo operation
+    pub remainder: T,
+}
+
+intrinsics! {
+    extern "C" fn __udivsi3(n: u32, d: u32) -> u32 {
+        divider_unsigned(n, d).quotient
+    }
+
+    extern "C" fn __umodsi3(n: u32, d: u32) -> u32 {
+        divider_unsigned(n, d).remainder
+    }
+
+    extern "C" fn __udivmodsi4(n: u32, d: u32, rem: Option<&mut u32>) -> u32 {
+        let quo_rem = divider_unsigned(n, d);
+        if let Some(rem) = rem {
+            *rem = quo_rem.remainder;
+        }
+        quo_rem.quotient
+    }
+
+    extern "C" fn __divsi3(n: i32, d: i32) -> i32 {
+        divider_signed(n, d).quotient
+    }
+
+    extern "C" fn __modsi3(n: i32, d: i32) -> i32 {
+        divider_signed(n, d).remainder
+    }
+
+    extern "C" fn __divmodsi4(n: i32, d: i32, rem: &mut i32) -> i32 {
+        let quo_rem = divider_signed(n, d);
+        *rem = quo_rem.remainder;
+        quo_rem.quotient
+    }
+}