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Improve hash function

This is the hash function found in k&r second edition
mixed with djb2 because it uses xor instead of adding.
This new version gets a better distribution of the
symbols in the hash table.

--- a/cc1/symbol.c

+++ b/cc1/symbol.c

@@ -7,7 +7,7 @@

 #include "../inc/sizes.h"

 #include "cc1.h"

-#define NR_SYM_HASH 32

+#define NR_SYM_HASH 64

 static unsigned curctx;

 static short localcnt;

@@ -19,11 +19,11 @@

 static inline unsigned

 hash(const char *s)

 {

-	unsigned h, ch;

+	unsigned c, h;

-	for (h = 0; ch = *s++; h += ch)

-		/* nothing */;

-	return h & NR_SYM_HASH - 1;

+	for (h = 0; c = *s; ++s)

+		h ^= 33 * c;

+	return h & NR_SYM_HASH-1;

 }

 void