cmd/compile: mis-calculates a constant

Question

dvyukov opened this issue 9 years ago · 2 comments

For the following program:

package main
var f = 7 / "0"[0]

go tool compile says:

tmp.go:2: constant 1197 overflows byte

Now sure where 1197 came from, but it must be 0 and the program must compile successfully.

go version devel +514014c Thu Jun 18 15:54:35 2015 +0200 linux/amd64

Answer 1 · 2015-06-23T22:31:07.000Z

I've looked into this a bit, and that particular value is coming from gc.Umagic:

ord("0") = 48
Umagic(48) = 171 with shift of 5
7 * 171 = 1197

In gc.walkdiv there is different logic used depending on if the divisor is a power of two or not, so it sometimes works:

Same root cause as #11358

Answer 2 · 2015-06-24T15:06:27.000Z