cmd/compile: 64 bits shifts on arm get wrong results #48476
Description
package main
//go:noinline
func f(x, y uint64) uint64 {
s := "\x04"
c := s[0]
return x<<c<<4
}
func main() {
println(f(1, 1))
}
Should print 256 (0x100), instead prints 1099511628032 (0x10000000100) on arm.
The problem is we build a SLLconst with a shift amount > 32. That in itself is ok, but we then copy the shift amount into a ORshiftLL, where the top bits are magically discarded.
The SLL with a large constant shift amount comes from the 64->32 bit lowering of shifts. We compute things like s-32 and 32-s (unsigned) for shift amounts in [0,63).