The gc library allocates a _panic struct on the stack. This does not
work for gccgo, because when a deferred function recovers the panic we
unwind the stack up to that point so that returning from the function
will work correctly.
Allocating on the stack fine if the panic is not recovered, and it
works fine if the panic is recovered by a function that
returns. However, it fails if the panic is recovered by a function
that itself panics, and if that second panic is then recovered by a
function higher up on the stack. When we unwind the stack to that
second panic, the g will wind up pointing at a panic farther down on
the stack. Even then everything will often work fine, except when the
deferred function catching the second panic makes a bunch of calls
that use stack space before returning. In that case the code can
overwrite the panic struct, which will then cause disaster when we
remove the struct from the linked list, as the link field will be
garbage. This case is rare enough that all the x86 tests were passing,
but there was a failure on ppc64le.
Before https://golang.org/cl/33414 we allocated the panic struct on
the heap, so go back to doing that again.
Fixesgolang/go#18228.
Reviewed-on: https://go-review.googlesource.com/34027
From-SVN: r243444
b2264b0964