avr: __udivmodqi4

Author: tones111

compiler-builtins/src/int/udiv.rs

@@ -43,6 +43,41 @@ intrinsics! {
 
         ((rem as u64) << 32) | (div as u64)
     }
+    
+    #[naked]
+    pub unsafe extern "C" fn __udivmodqi4() {
+        // compute unsigned 8-bit `n / d` and `n % d`.
+        //
+        // Note: GCC implements a [non-standard calling convention](https://gcc.gnu.org/wiki/avr-gcc#Exceptions_to_the_Calling_Convention) for this function.
+        // Inputs:
+        //     R24: dividend
+        //     R22: divisor
+        // Outputs:
+        //     R24: quotient  (dividend / divisor)
+        //     R25: remainder (dividend % divisor)
+        // Clobbers:
+        //     R23: loop counter
+        core::arch::naked_asm!(
+            // This assembly routine implements the [long division](https://en.wikipedia.org/wiki/Division_algorithm#Long_division) algorithm.
+            // Bits shift out of the dividend and into the quotient, so R24 is used for both.
+            "clr R25",      // remainder = 0
+
+            "ldi R23, 8",   // for each bit
+            "1:",
+            "lsl R24",      //     shift the dividend MSb
+            "rol R25",      //     into the remainder LSb
+
+            "cp  R25, R22", //     if remainder >= divisor
+            "brlo 2f",    
+            "sub R25, R22", //         remainder -= divisor
+            "sbr R24, 1",   //         quotient |= 1
+            "2:",
+
+            "dec R23",      // end loop
+            "brne 1b",
+            "ret",
+        );
+    }
 }
 
 intrinsics! {