[go] runtime: conservatively scan the extended register state on ARM64
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Alexander Musman (Gerrit)
Jan 31, 2026, 4:49:20 PM (6 days ago) Jan 31
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Alexander Musman has uploaded the change for review![]( "Open in Gerrit")
Commit message
runtime: conservatively scan the extended register state on ARM64
Conservatively scan the extended register state on ARM64
when GC scans asynchronously preempted goroutines.
This ensures that any pointers that appear only
in vector registers at preemption time are kept alive.
The extended register state saved during async preemption is currently
assumed to contain only scalar data. However, as suggested in CL 738261,
using ARM64 vector registers (NEON) for small memory moves may cause
pointers to be loaded into these registers.
This can be extended to other architectures to allow using vector
registers for operations that may involve pointers.
Change-Id: I5f5ce98d6ed6f7cde34b33da0aea1f880c2fcf41
Change diff
diff --git a/src/runtime/mgcmark.go b/src/runtime/mgcmark.go
index 714b9a5..3f7579d 100644
--- a/src/runtime/mgcmark.go
+++ b/src/runtime/mgcmark.go
@@ -967,6 +967,11 @@
scanblock(uintptr(unsafe.Pointer(&gp.sched.ctxt)), goarch.PtrSize, &oneptrmask[0], gcw, &state)
}
+ // Scan conservatively the extended register state for the archs where it may contain pointers.
+ if GOARCH == "arm64" && gp.asyncSafePoint {
+ xRegScan(gp, gcw, &state)
+ }
+
// Scan the stack. Accumulate a list of stack objects.
var u unwinder
for u.init(gp, 0); u.valid(); u.next() {
diff --git a/src/runtime/preempt_noxreg.go b/src/runtime/preempt_noxreg.go
index 977bf0b..ee6f879 100644
--- a/src/runtime/preempt_noxreg.go
+++ b/src/runtime/preempt_noxreg.go
@@ -25,3 +25,5 @@
func xRegRestore(gp *g) {}
func (*xRegPerP) free() {}
+
+func xRegScan(gp *g, gcw *gcWork, state *stackScanState) {}
diff --git a/src/runtime/preempt_xreg.go b/src/runtime/preempt_xreg.go
index cc52c5f..c9eda53 100644
--- a/src/runtime/preempt_xreg.go
+++ b/src/runtime/preempt_xreg.go
@@ -10,8 +10,9 @@
// While asynchronous preemption stores general-purpose (GP) registers on the
// preempted goroutine's own stack, extended register state can be used to save
// non-GP state off the stack. In particular, this is meant for large vector
-// register files. Currently, we assume this contains only scalar data, though
-// we could change this constraint by conservatively scanning this memory.
+// register files. Currently, we assume this contains only scalar data,
+// except for arm64 which conservatively scans this memory to enable small size
+// memoves using non-GP registers.
//
// For an architecture to support extended register state, it must provide a Go
// definition of an xRegState type for storing the state, and its asyncPreempt
@@ -20,6 +21,7 @@
package runtime
import (
+ "internal/abi"
"internal/runtime/sys"
"unsafe"
)
@@ -135,3 +137,33 @@
unlock(&xRegAlloc.lock)
}
}
+
+// xRegScan conservatively scans the extended register state.
+//
+// This is supposed to be called only by scanstack when it handles async preemption.
+func xRegScan(gp *g, gcw *gcWork, state *stackScanState) {
+ // The scan is only needed for the archs where pointers may appear in the extended register state.
+ if GOARCH != "arm64" {
+ return
+ }
+ // Regular async preemption always provides the extended register state.
+ if gp.xRegs.state == nil {
+ var u unwinder
+ for u.init(gp, 0); u.valid(); u.next() {
+ if u.frame.fn.valid() && u.frame.fn.funcID == abi.FuncID_debugCallV2 {
+ return
+ }
+ }
+ println("runtime: gp=", gp, ", goid=", gp.goid)
+ throw("gp.xRegs.state == nil on a scanstack attempt during async preemption")
+ }
+ b := uintptr(unsafe.Pointer(&gp.xRegs.state.regs))
+ n := uintptr(unsafe.Sizeof(gp.xRegs.state.regs))
+ if debugScanConservative {
+ print("begin scan xRegs of goroutine ", gp.goid, " at [", hex(b), ",", hex(b+n), ")\n")
+ }
+ scanConservative(b, n, nil, gcw, state)
+ if debugScanConservative {
+ print("end scan xRegs of goroutine ", gp.goid, "\n")
+ }
+}
Change information
Files:
- M src/runtime/mgcmark.go
- M src/runtime/preempt_noxreg.go
- M src/runtime/preempt_xreg.go
Change size: S
Delta: 3 files changed, 41 insertions(+), 2 deletions(-)
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Alexander Musman (Gerrit)
Feb 5, 2026, 1:49:00 PM (2 days ago) Feb 5
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Attention needed from Austin Clements, Dmitry Vyukov and Michael Knyszek
Alexander Musman uploaded new patchset![]( "Open in Gerrit")
Alexander Musman uploaded patch set #2 to this change.
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Alexander Musman (Gerrit)
Feb 5, 2026, 2:09:25 PM (2 days ago) Feb 5
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Alexander Musman added 1 comment![]( "Open in Gerrit")
Patchset-level comments
File-level comment, Patchset 2 (Latest):
Alexander Musman . resolved
This CL is part of a chain with CL 738261, which adds SIMD-based
small memory moves on ARM64. Keith Randall requested conservative
scanning of saved SIMD state to ensure pointers in vector registers
aren't missed during GC of async-preempted goroutines. See discussion
in CL 738261.
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Keith Randall (Gerrit)
Feb 6, 2026, 8:10:44 PM (8 hours ago) Feb 6
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Keith Randall (Gerrit)
Feb 6, 2026, 8:11:10 PM (8 hours ago) Feb 6
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