[PATCH] Do CRC 4 bits at a time

[Nathan Sidwell <nathan at acm dot org>]

Hi,
our current CRC routine processes 1 bit at a time, and permits arbitrary 
numbers of bits from 1 to 32.  However we only ever feed it multiples of 
8 bits to process.

So part of this patch changes the interface to use a crc32_unsigned_n 
worker function, which crcs a N-byte integer.

The other change is to do 4 bits at a time.  This is possible because 
the feedback syndrome is '0x04c11db7', which has the top 5 bits clear. 
But 5's an awkward number to work with, so just go with nibble at a time.

bootstrapped on x86_64-linux, ok for trunk?

nathan

--
Nathan Sidwell

2017-04-25  Nathan Sidwell  <nathan@acm.org>

	* tree.h (crc32_unsigned_n): Declare.
	(crc32_unsigned, crc32_unsigned): Make inline.
	* tree.c (crc32_unsigned_bits): Replace with ...
	(crc32_unsigned_n): ... this.
	(crc32_unsigned, crc32_byte): Remove.
	(crc32_string): Remove unnecessary braces.

Index: tree.c
===================================================================
--- tree.c	(revision 247207)
+++ tree.c	(working copy)
@@ -9611,38 +9611,34 @@ dump_tree_statistics (void)
 
 #define FILE_FUNCTION_FORMAT "_GLOBAL__%s_%s"
 
-/* Generate a crc32 of a byte.  */
+/* Generate a crc32 of the low BYTES bytes of VALUE.  */
 
-static unsigned
-crc32_unsigned_bits (unsigned chksum, unsigned value, unsigned bits)
+unsigned
+crc32_unsigned_n (unsigned chksum, unsigned value, unsigned bytes)
 {
-  unsigned ix;
-
-  for (ix = bits; ix--; value <<= 1)
+  /* This relies on the raw feedback's top 4 bits being zero.  */
+#define FEEDBACK(X) ((X) * 0x04c11db7)
+#define SYNDROME(X) (FEEDBACK ((X) & 1) ^ FEEDBACK ((X) & 2) \
+		     ^ FEEDBACK ((X) & 4) ^ FEEDBACK ((X) & 8))
+  static const unsigned syndromes[16] =
     {
-      unsigned feedback;
-      
-      feedback = (value ^ chksum) & 0x80000000 ? 0x04c11db7 : 0;
-      chksum <<= 1;
-      chksum ^= feedback;
-    }
-  return chksum;
-}
-
-/* Generate a crc32 of a 32-bit unsigned.  */
+      SYNDROME(0x0), SYNDROME(0x1), SYNDROME(0x2), SYNDROME(0x3),
+      SYNDROME(0x4), SYNDROME(0x5), SYNDROME(0x6), SYNDROME(0x7),
+      SYNDROME(0x8), SYNDROME(0x9), SYNDROME(0xa), SYNDROME(0xb),
+      SYNDROME(0xc), SYNDROME(0xd), SYNDROME(0xe), SYNDROME(0xf),
+    };
+#undef FEEDBACK
+#undef SYNDROME
 
-unsigned
-crc32_unsigned (unsigned chksum, unsigned value)
-{
-  return crc32_unsigned_bits (chksum, value, 32);
-}
+  value <<= (32 - bytes * 8);
+  for (unsigned ix = bytes * 2; ix--; value <<= 4)
+    {
+      unsigned feedback = syndromes[((value ^ chksum) >> 28) & 0xf];
 
-/* Generate a crc32 of a byte.  */
+      chksum = (chksum << 4) ^ feedback;
+    }
 
-unsigned
-crc32_byte (unsigned chksum, char byte)
-{
-  return crc32_unsigned_bits (chksum, (unsigned) byte << 24, 8);
+  return chksum;
 }
 
 /* Generate a crc32 of a string.  */
@@ -9651,9 +9647,7 @@ unsigned
 crc32_string (unsigned chksum, const char *string)
 {
   do
-    {
-      chksum = crc32_byte (chksum, *string);
-    }
+    chksum = crc32_byte (chksum, *string);
   while (*string++);
   return chksum;
 }
Index: tree.h
===================================================================
--- tree.h	(revision 247207)
+++ tree.h	(working copy)
@@ -4688,9 +4688,18 @@ inlined_function_outer_scope_p (const_tr
        function_args_iter_next (&(ITER)))
 
 /* In tree.c */
+extern unsigned crc32_unsigned_n (unsigned, unsigned, unsigned);
 extern unsigned crc32_string (unsigned, const char *);
-extern unsigned crc32_byte (unsigned, char);
-extern unsigned crc32_unsigned (unsigned, unsigned);
+static inline unsigned
+crc32_unsigned (unsigned chksum, unsigned value)
+{
+  return crc32_unsigned_n (chksum, value, 4);
+}
+static inline unsigned
+crc32_byte (unsigned chksum, char byte)
+{
+  return crc32_unsigned_n (chksum, byte, 1);
+}
 extern void clean_symbol_name (char *);
 extern tree get_file_function_name (const char *);
 extern tree get_callee_fndecl (const_tree);