crc32: add notw macro

instead of materializing a constant 0xffffffff, do a NOT and then
zero-extend the result. my qemu doesn't support the zext.w instruction,
so we have to do two shifts instead. these can potentially be fused
into a single instruction by the processor.

crc32.s

@@ -127,6 +127,14 @@ iloop2:
 
 #............................................
 
+.macro notw rd, rs
+    not    \rd, \rs
+    #zext.w \rd, \rd
+    #add.uw  \rd, \rs, 0
+    slli   \rd, \rd, 32
+    srli   \rd, \rd, 32
+.endm
+
 # inputs:
 # a0: crc
 # a1: buf
@@ -142,9 +150,7 @@ crc_update:
     mv t2, s1
     mv t3, s2
     # invert crc
-    li   t0, -1
-    srli t0, t0, 32
-    xor  a0, a0, t0
+    notw a0, a0
 
     mv s0, a0 # crc
     mv s1, a1 # buffer
@@ -174,6 +180,5 @@ udone:
     # invert crc
     #li   t0, -1
     #srli t0, t0, 32
-    li t0, 0xFFFFFFFF
-    xor a0, a0, t0
+    notw a0, a0
     ret