Small comment:

switch {} // prevent inlining

this no longer works. In fact, at tip f1_ssa is inlined.

Also, disabling inlining fixes the issue: the code prints nothing.

Happy accident. The fact that the code behaves differently with inlining disabled and enabled it's a clear bug in the code generated. Most likely it's a rule that folds the comparisons, but I'd have to minimize the code first to figure out which. Alternatively one can bisect and find which patch is the culprit.

A few more examples:

http://play.golang.org/p/-HaD6RhlJj
http://play.golang.org/p/wtaffQRc2a
http://play.golang.org/p/6Mb6gmLQyE

This is one is minimized:

package main
import "fmt"
import "os"
func main() {
a6 := uint8(253)
got := a6 >> 0
if got != 253 {
fmt.Printf("f1_ssa(0, false, false, 1, 2, 2) = %v, wanted 253\n", got)
os.Exit(1)
}
}

Git bisect says it was introduced in https://go-review.googlesource.com/#/c/21256/

There are a few things wrong:
a6 := uint8(253) -> (Const8 [-3]) . I think it should be (Const8 253) instead.

This also happens:
(Neq8 (Const8 [-3]) (Const8 [-3])) -> (ConstBool [true])

which is an instantiation of
(Neq8 (Const8 [c]) (Const8 [d])) -> (ConstBool [b2i(c != d)])

where c = 253 (coming from right shift) and d = -3 (coming from the constant above).

@randall77

In 21256 I changed the semantics of the AuxInt field to always be sign-extended. So -3 is right, 253 is wrong. It is ending up with 253 anyway because of a bad rewrite rule. CL pending.

CL https://golang.org/cl/21586 mentions this issue.