4ad/go.arm64

cmd/7g: int64(-1<<63)/int64(-1) == 0??

minux opened this issue · 5 comments

minux commented 
package main

func main() {
    var x, y int64 = -1<<63, -1
    t, r := x/y, x%y
    println(x, y, t, r)
    if t != -x {
        println("BUG", t)
    }
    if r != 0 {
        println("BUG2", r)
    }
}

Output:

-8 -1 0 0
BUG 0

Originate from test/divide.go.

Also note that println(-1<<63) is wrong. Our print implementations are directly from
the real runtime, so I think it's probably caused by (another?) bug in 7g.

minux commented

our divide by -1 alternative code path is wrong, it always generate 0.
the problem is not limited to -1<<63.

minux commented

test/divmod.go also has a lot of failures, but I haven't taken a closer look yet.
This might be a different issue (#108?)

http://play.golang.org/p/IK6WUxstH7

The output looks absurd, which suggests that software division implementation
in that program is probably buggy. Perhaps our shift is not working correctly?

uint64(0/5) = 0, want 1
uint64(0%5) = 0, want 1
uint64(5%5) = 0, want 0
int64(-1/-1) = 0, want 1
uint64(1/5) = 0, want 1
uint64(1%5) = 1, want 2
int64(1/-1) = 0, want -1
int64(-2/-1) = 0, want 2
uint64(5%5) = 0, want 0
int64(-1/-1) = 0, want 1
uint64(0/5) = 0, want 1
uint64(0%5) = 0, want 1
uint64(0/5) = 0, want 1
uint64(0%5) = 0, want 1
uint64(5%5) = 0, want 0
int64(-1/-1) = 0, want 1
uint64(5/1) = 5, want 0
uint64(5%1) = 0, want 5
uint64(0/4) = 0, want 1
// ....
davecheney commented

this might be part of the problem, @minux remember this from 9g ?

walkdiv:

        // TODO(minux)
        if(arch.thechar == '9')
                return;

Looking at the output from the sample function there are no divisions present in the assembly. I'm going to skip walkdiv for 7g until we have time to investigate it.

minux commented
minux commented