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runtime: document stkframe

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The meaning of some of the fields in stkframe is actually quite
subtle.

Change-Id: Iac765ff6fbf4c3b7c9f2453f5b4a2e5e640f5750
Reviewed-on: https://go-review.googlesource.com/c/go/+/424256
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Austin Clements <austin@google.com>
This commit is contained in:
Austin Clements 2022-07-13 13:36:23 -04:00 committed by Gopher Robot
parent 5063056bd1
commit f00fa0b98d
1 changed files with 48 additions and 11 deletions
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45
src/runtime/runtime2.go
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@ -985,11 +985,48 @@ type _panic struct {
goexit bool
}
// stack traces
// A stkframe holds information about a single physical stack frame.
type stkframe struct {
fn funcInfo // function being run
pc uintptr // program counter within fn
continpc uintptr // program counter where execution can continue, or 0 if not
// fn is the function being run in this frame. If there is
// inlining, this is the outermost function.
fn funcInfo
// pc is the program counter within fn.
//
// The meaning of this is subtle:
//
// - Typically, this frame performed a regular function call
// and this is the return PC (just after the CALL
// instruction). In this case, pc-1 reflects the CALL
// instruction itself and is the correct source of symbolic
// information.
//
// - If this frame "called" sigpanic, then pc is the
// instruction that panicked, and pc is the correct address
// to use for symbolic information.
//
// - If this is the innermost frame, then PC is where
// execution will continue, but it may not be the
// instruction following a CALL. This may be from
// cooperative preemption, in which case this is the
// instruction after the call to morestack. Or this may be
// from a signal or an un-started goroutine, in which case
// PC could be any instruction, including the first
// instruction in a function. Conventionally, we use pc-1
// for symbolic information, unless pc == fn.entry(), in
// which case we use pc.
pc uintptr
// continpc is the PC where execution will continue in fn, or
// 0 if execution will not continue in this frame.
//
// This is usually the same as pc, unless this frame "called"
// sigpanic, in which case it's either the address of
// deferreturn or 0 if this frame will never execute again.
//
// This is the PC to use to look up GC liveness for this frame.
continpc uintptr
lr uintptr // program counter at caller aka link register
sp uintptr // stack pointer at pc
fp uintptr // stack pointer at caller aka frame pointer