[riscv] Simplify TCP/IP checksum calculation

Use the tighter provable constraint

  carry.2^n + x <= (2^n - 1) + (2^n - 1)
                <= 2^n + (2^n - 2)

and so

  x + carry <= (2^n - 2) + 1
            <= (2^n - 1)

to eliminate some unnecessary folding steps, and hold the folded value
in the most significant bits of the register rather than the least
significant bits so that the final one's complement negation can be
accomplished naturally without requiring an explicit 0xffff constant.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/riscv/core/riscv_tcpip.S

@@ -56,7 +56,7 @@ tcpip_continue_chksum:
 	 * a1: data pointer
 	 * a2: end of data pointer
 	 * a3: end of data pointer minus a constant offset of interest
-	 * a4: checksum high bits (guaranteed to never carry) / constant 0xffff
+	 * a4: checksum high bits (guaranteed to never carry)
 	 * a5: temporary register
 	 */
 	not	a0, a0
@@ -100,40 +100,30 @@ post_aligned:
 	lbu	a5, (a1)
 	add	a4, a4, a5
 1:
-	/* Fold down to xlen+1 bits */
+	/* Fold down to xlen bits */
 	add	a0, a0, a4
 	sltu	a4, a0, a4
-
-	/* Fold down to (xlen/2)+2 bits */
-	slli	a5, a0, ( __riscv_xlen / 2 )
-	srli	a0, a0, ( __riscv_xlen / 2 )
-	srli	a5, a5, ( __riscv_xlen / 2 )
 	add	a0, a0, a4
-	add	a0, a0, a5
 
-	/* Load constant 0xffff for use in subsequent folding */
-	li	a4, 0xffff
+	/* Fold down to (high) xlen/2 bits */
+	slli	a4, a0, ( __riscv_xlen / 2 )
+	add	a0, a0, a4
+	sltu	a4, a0, a4
+	slli	a4, a4, ( __riscv_xlen / 2 )
+	add	a0, a0, a4
 
 #if __riscv_xlen >= 64
-	/* Fold down to (xlen/4)+3 bits (if xlen >= 64) */
-	and	a5, a0, a4
-	srli	a0, a0, ( __riscv_xlen / 4 )
-	add	a0, a0, a5
+	/* Fold down to (high) xlen/4 bits (if xlen >= 64) */
+	srli	a4, a0, ( __riscv_xlen / 2 )
+	slli	a4, a4, ( __riscv_xlen * 3 / 4 )
+	add	a0, a0, a4
+	sltu	a4, a0, a4
+	slli	a4, a4, ( __riscv_xlen * 3 / 4 )
+	add	a0, a0, a4
 #endif
 
-	/* Fold down to 16+1 bits */
-	and	a5, a0, a4
-	srli	a0, a0, 16
-	add	a0, a0, a5
-
-	/* Fold down to 16 bits */
-	srli	a5, a0, 16
-	add	a0, a0, a5
-	srli	a5, a0, 17
-	add	a0, a0, a5
-	and	a0, a0, a4
-
-	/* Negate and return */
-	xor	a0, a0, a4
+	/* Negate, move to low bits, and return */
+	not	a0, a0
+	srli	a0, a0, ( __riscv_xlen - 16 )
 	ret
 	.size	tcpip_continue_chksum, . - tcpip_continue_chksum