Stowage/go
2
0
Fork 0
mirror of https://github.com/golang/go.git synced 2025-12-08 06:10:04 +00:00
Code Issues Projects Releases Packages Wiki Activity

runtime: use conservative scanning for debug calls

Browse source 
A debugger can inject a call at almost any PC, which causes
significant complications with stack scanning and growth. Currently,
the runtime solves this using precise stack maps and register maps at
nearly all PCs, but these extra maps require roughly 5% of the binary.
These extra maps were originally considered worth this space because
they were intended to be used for non-cooperative preemption, but are
now used only for debug call injection.

This CL switches from using precise maps to instead using conservative
frame scanning, much like how non-cooperative preemption works. When a
call is injected, the runtime flushes all potential pointer registers
to the stack, and then treats that frame as well as the interrupted
frame conservatively.

The limitation of conservative frame scanning is that we cannot grow
the goroutine stack. That's doable because the previous CL switched to
performing debug calls on a new goroutine, where they are free to grow
the stack.

With this CL, there are no remaining uses of precise register maps
(though we still use the unsafe-point information that's encoded in
the register map PCDATA stream), and stack maps are only used at call
sites.

For #36365.

Change-Id: Ie217b6711f3741ccc437552d8ff88f961a73cee0
Reviewed-on: https://go-review.googlesource.com/c/go/+/229300
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
This commit is contained in:
Austin Clements 2020-04-15 15:44:57 -04:00
parent 3633d2c545
commit 57d751370c
4 changed files with 19 additions and 28 deletions
Download patch file Download diff file Expand all files Collapse all files

23
src/runtime/stack.go
View file

@ -1216,29 +1216,8 @@ func getStackMap(frame *stkframe, cache *pcvalueCache, debug bool) (locals, args
minsize = sys.MinFrameSize
}
if size > minsize {
var stkmap *stackmap
stackid := pcdata
if f.funcID != funcID_debugCallV1 {
stkmap = (*stackmap)(funcdata(f, _FUNCDATA_LocalsPointerMaps))
} else {
// debugCallV1's stack map is the register map
// at its call site.
callerPC := frame.lr
caller := findfunc(callerPC)
if !caller.valid() {
println("runtime: debugCallV1 called by unknown caller", hex(callerPC))
throw("bad debugCallV1")
}
stackid = int32(-1)
if callerPC != caller.entry {
callerPC--
stackid = pcdatavalue(caller, _PCDATA_RegMapIndex, callerPC, cache)
}
if stackid == -1 {
stackid = 0 // in prologue
}
stkmap = (*stackmap)(funcdata(caller, _FUNCDATA_RegPointerMaps))
}
stkmap := (*stackmap)(funcdata(f, _FUNCDATA_LocalsPointerMaps))
if stkmap == nil || stkmap.n <= 0 {
print("runtime: frame ", funcname(f), " untyped locals ", hex(frame.varp-size), "+", hex(size), "\n")
throw("missing stackmap")