math/big: make nat.setUint64 vet-friendly

nat.setUint64 is nicely generic.
By assuming 32- or 64-bit words, however,
we can write simpler code,
and eliminate some shifts
in dead code that vet complains about.

Generated code for 64 bit systems is unaltered.
Generated code for 32 bit systems is much better.
For 386, the routine length drops from 325
bytes of code to 271 bytes of code, with fewer loops.

Change-Id: I1bc14c06272dee37a7fcb48d33dd1e621eba945d
Reviewed-on: https://go-review.googlesource.com/38070
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/vet/all/whitelist/64bit.txt

@@ -22,7 +22,5 @@ math/big/arith.go: (xi&yi | (xi|yi)&^zi) might be too small for shift of 63
 math/big/arith.go: (yi&^xi | (yi|^xi)&zi) might be too small for shift of 63
 math/big/arith.go: xi &^ zi might be too small for shift of 63
 math/big/arith.go: (zi &^ xi) might be too small for shift of 63
-math/big/nat.go: t too small for shift of 64
-math/big/nat.go: x too small for shift of 64
 math/big/nat.go: yi might be too small for shift of 60
 math/big/nat.go: yi might be too small for shift of 60

src/math/big/nat.go

@@ -68,24 +68,14 @@ func (z nat) setWord(x Word) nat {
 }
 
 func (z nat) setUint64(x uint64) nat {
-	// single-digit values
+	// single-word value
 	if w := Word(x); uint64(w) == x {
 		return z.setWord(w)
 	}
-
-	// compute number of words n required to represent x
-	n := 0
-	for t := x; t > 0; t >>= _W {
-		n++
-	}
-
-	// split x into n words
-	z = z.make(n)
-	for i := range z {
-		z[i] = Word(x & _M)
-		x >>= _W
-	}
-
+	// 2-word value
+	z = z.make(2)
+	z[1] = Word(x >> 32)
+	z[0] = Word(x)
 	return z
 }