math/big: fix aliasing bug in Float.Quo
TBR r, adonovan
Change-Id: I1a38e2d724bf1147c7307a7e5ae855c42c60428c
Reviewed-on: https://go-review.googlesource.com/4875
Reviewed-by: Robert Griesemer <gri@golang.org>
diff --git a/src/math/big/float.go b/src/math/big/float.go
index dd67001..314fd68 100644
--- a/src/math/big/float.go
+++ b/src/math/big/float.go
@@ -967,12 +967,16 @@
// to shorten x for faster division. But we must be extra careful
// with rounding in that case.
+ // Compute d before division since there may be aliasing of x.mant
+ // (via xadj) or y.mant with z.mant.
+ d := len(xadj) - len(y.mant)
+
// divide
var r nat
z.mant, r = z.mant.div(nil, xadj, y.mant)
// determine exponent
- e := int64(x.exp) - int64(y.exp) - int64(len(xadj)-len(y.mant)-len(z.mant))*_W
+ e := int64(x.exp) - int64(y.exp) - int64(d-len(z.mant))*_W
// normalize mantissa
z.setExp(e - int64(fnorm(z.mant)))
