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runtime: explain and enforce that _panic values live on the stack

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It's a bit mysterious that _defer.sp is a uintptr that gets
stack-adjusted explicitly while _panic.argp is an unsafe.Pointer that
doesn't, but turns out to be critically important when a deferred
function grows the stack before doing a recover.

Add a comment explaining that this works because _panic values live on
the stack. Enforce this by marking _panic go:notinheap.

Change-Id: I9ca49e84ee1f86d881552c55dccd0662b530836b
Reviewed-on: https://go-review.googlesource.com/99735
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
This commit is contained in:
Austin Clements 2018-03-08 17:48:22 -05:00
parent 60a9e5d613
commit 5d22cebb12
1 changed files with 11 additions and 1 deletions
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12
src/runtime/runtime2.go
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@ -710,7 +710,17 @@ type _defer struct {
link *_defer
}
// panics
// A _panic holds information about an active panic.
//
// This is marked go:notinheap because _panic values must only ever
// live on the stack.
//
// The argp and link fields are stack pointers, but don't need special
// handling during stack growth: because they are pointer-typed and
// _panic values only live on the stack, regular stack pointer
// adjustment takes care of them.
//
//go:notinheap
type _panic struct {
argp unsafe.Pointer // pointer to arguments of deferred call run during panic; cannot move - known to liblink
arg interface{} // argument to panic