cmd/compile: be more careful about pointer incrementing in range loops
For range loops, we use a pointer to the backing store that gets
incremented on each iteration of the loop.
The problem with this scheme is that at the end of the last iteration,
we may briefly have a pointer that points past the end of the backing store
of the slice that is being iterated over. We cannot let the garbage collector
see that pointer.
To fix this problem, have the incremented pointer live briefly as
a uintptr instead of a normal pointer, so it doesn't keep anything
alive. Convert back to a normal pointer just after the loop condition
is checked, but before anything that requires a real pointer representation
(in practice, any call, which is what could cause a GC scan or stack copy).
Fixes #56699
Change-Id: Ia928d23f85a211565357603668bea4e5c534f989
Reviewed-on: https://go-review.googlesource.com/c/go/+/449995 Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Keith Randall <khr@golang.org> Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>