gccrs: Add port of FNV hash used during legacy symbol mangling

This hash was ported from the Go runtime, as we needed a hash for the legacy
symbol mangling system. All symbols in Rust contain a hash of some
metadata for uniqueness on generic functions.

	gcc/rust/
	* util/fnv-hash.h: New.

gcc/rust/util/fnv-hash.h

@@ -0,0 +1,95 @@
+// Copyright (C) 2020-2022 Free Software Foundation, Inc.
+
+// This file is part of GCC.
+
+// GCC is free software; you can redistribute it and/or modify it under
+// the terms of the GNU General Public License as published by the Free
+// Software Foundation; either version 3, or (at your option) any later
+// version.
+
+// GCC is distributed in the hope that it will be useful, but WITHOUT ANY
+// WARRANTY; without even the implied warranty of MERCHANTABILITY or
+// FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+// for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with GCC; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+#ifndef RUST_FNV_HASH_H
+#define RUST_FNV_HASH_H
+
+namespace Rust {
+namespace Hash {
+
+const uint64_t offset128Lower = 0x62b821756295c58d;
+const uint64_t offset128Higher = 0x6c62272e07bb0142;
+const uint64_t prime128Lower = 0x13b;
+const uint64_t prime128Shift = 24;
+
+// ported from https://github.com/golang/go/blob/master/src/hash/fnv/fnv.go
+class FNV128
+{
+public:
+  FNV128 () { reset (); }
+
+  void reset ()
+  {
+    buf[0] = offset128Higher;
+    buf[1] = offset128Lower;
+  }
+
+  void write (const unsigned char *in, size_t len)
+  {
+    for (size_t i = 0; i < len; i++)
+      {
+	unsigned char c = in[i];
+
+	// https://stackoverflow.com/questions/28868367/getting-the-high-part-of-64-bit-integer-multiplication
+	uint64_t a = prime128Lower;
+	uint64_t b = buf[1];
+
+	uint64_t a_lo = (uint32_t) a;
+	uint64_t a_hi = a >> 32;
+	uint64_t b_lo = (uint32_t) b;
+	uint64_t b_hi = b >> 32;
+
+	uint64_t a_x_b_hi = a_hi * b_hi;
+	uint64_t a_x_b_mid = a_hi * b_lo;
+	uint64_t b_x_a_mid = b_hi * a_lo;
+	uint64_t a_x_b_lo = a_lo * b_lo;
+
+	uint64_t carry_bit
+	  = ((uint64_t) (uint32_t) a_x_b_mid + (uint64_t) (uint32_t) b_x_a_mid
+	     + (a_x_b_lo >> 32))
+	    >> 32;
+
+	uint64_t multhi
+	  = a_x_b_hi + (a_x_b_mid >> 32) + (b_x_a_mid >> 32) + carry_bit;
+
+	uint64_t s0 = multhi;		      // high
+	uint64_t s1 = prime128Lower * buf[1]; // low
+
+	s0 += buf[1] << (prime128Shift + prime128Lower * buf[0]);
+
+	// Update the values
+	buf[1] = s1;
+	buf[0] = s0;
+	buf[1] ^= (uint64_t) c;
+      }
+  }
+
+  void sum (uint64_t *hi, uint64_t *lo) const
+  {
+    *hi = buf[0];
+    *lo = buf[1];
+  }
+
+private:
+  uint64_t buf[2];
+};
+
+} // namespace Hash
+} // namespace Rust
+
+#endif // RUST_FNV_HASH_H