apd: inline Context.round

This commit reworks the call to `Context.round` to allow for mid-stack
function inlining. To do this, we remove the conditional call to one
of two functions, which is considered too complex to inline.

Author: nvb

context.go

@@ -1220,8 +1220,8 @@ func (c *Context) quantize(d, v *Decimal, exp int32) Condition {
 			// target eliminates this problem.
 
 			d.Exponent = -diff
-			// Avoid the c.Precision == 0 check.
-			res = nc.Rounding.Round(nc, d, d)
+			// Round even if nc.Precision == 0.
+			res = nc.Rounding.Round(nc, d, d, false /* disableIfPrecisionZero */)
 			// Adjust for 0.9 -> 1.0 rollover.
 			if d.Exponent > 0 {
 				d.Coeff.Mul(&d.Coeff, bigTen)

round.go

@@ -21,13 +21,9 @@ func (c *Context) Round(d, x *Decimal) (Condition, error) {
 	return c.goError(c.round(d, x))
 }
 
+//gcassert:inline
 func (c *Context) round(d, x *Decimal) Condition {
-	if c.Precision == 0 {
-		d.Set(x)
-		return d.setExponent(c, unknownNumDigits, 0, int64(d.Exponent))
-	}
-	res := c.Rounding.Round(c, d, x)
-	return res
+	return c.Rounding.Round(c, d, x, true /* disableIfPrecisionZero */)
 }
 
 // Rounder specifies the behavior of rounding.
@@ -63,12 +59,17 @@ func (r Rounder) ShouldAddOne(result *BigInt, neg bool, half int) bool {
 }
 
 // Round sets d to rounded x.
-func (r Rounder) Round(c *Context, d, x *Decimal) Condition {
+func (r Rounder) Round(c *Context, d, x *Decimal, disableIfPrecisionZero bool) Condition {
 	d.Set(x)
 	nd := x.NumDigits()
 	xs := x.Sign()
 	var res Condition
 
+	if disableIfPrecisionZero && c.Precision == 0 {
+		// Rounding has been disabled.
+		return d.setExponent(c, nd, res, int64(d.Exponent))
+	}
+
 	// adj is the adjusted exponent: exponent + clength - 1
 	if adj := int64(x.Exponent) + nd - 1; xs != 0 && adj < int64(c.MinExponent) {
 		// Subnormal is defined before rounding.