Strange behaviour with loops [1.15 backport] #45187
Description
@randall77 requested issue #45175 to be considered for backport to the next 1.15 minor release.
Here's a simpler reproducer:
package main //go:noinline func f(c bool) int { b := true x := 0 y := 1 for b { b = false y = x x = 2 if c { return 3 } } return y } func main() { println(f(false)) }This reproducer incorrectly prints
2.
It should print0. And it does, with-gcflags=-d=ssa/short_circuit/off.
The problem is that during short circuit, we don't take into account that the arguments to a phi are from the previous iteration, not the current iteration. That confusion leads to assigningythe current iteration's value ofx, not the previous iteration's value ofx.The bug looks like it started in 1.15.
In
shortcircuit.go, we need to be a bit more choosy inreplaceUsesto not replace uses inside a phi, as we're replacing the current iteration's values, not those which are from the previous iteration. At least, something like that; it may not be quite so simple.@gopherbot please open backport issues.