internal/runtime/maps: add GOEXPERIMENT=mapsplitgroup for KKKKVVVV slot order

Map groups are currently:

type group struct {
    ctrl  uint64
    slots [8]slot
}

type slot struct {
    key  K
    elem E
}

If the element type is struct{}, the slot will be padded so that the
address of the elem is unique rather than pointing outside the alloc.
This has the effect of map[K]struct{} wasting space due to the extra
byte and padding, making it no better than map[K]bool.

This CL changes the group layout to instead place keys and elems
together, as they used to be before swiss maps:

type group struct {
    ctrl  uint64
    keys  [8]K
    elems [8]V
}

This is an alternative to CL 701976, which I suspect will have better
performance. Keys placed together should lead to better cache behavior,
at the cost of more expensive elem lookups, since the elems are not a
fixed offset from their keys.

This change is locked behind GOEXPERIMENT=mapsplitgroup.

Updates #70835
Updates #71368

Change-Id: Ide8d1406ae4ab636f86edc40e0640cc80653197c
Reviewed-on: https://go-review.googlesource.com/c/go/+/711560
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

1 files changed, 13 insertions, 2 deletions

diff --git a/src/runtime/runtime-gdb.py b/src/runtime/runtime-gdb.py
index 9d2446a1eb..a56711e0bc 100644
--- a/src/runtime/runtime-gdb.py
+++ b/src/runtime/runtime-gdb.py
@@ -169,6 +169,10 @@ class MapTypePrinter:
 		cnt = 0
 		# Yield keys and elements in group.
 		# group is a value of type *group[K,V]
+		# The group layout depends on GOEXPERIMENT=mapsplitgroup:
+		#   split:       group.keys[i] / group.elems[i]
+		#   interleaved: group.slots[i].key / group.slots[i].elem
+		# Detect which layout by checking for the 'keys' field.
 		def group_slots(group):
 			ctrl = group['ctrl']
 
@@ -179,8 +183,15 @@ class MapTypePrinter:
 					continue
 
 				# Full
-				yield str(cnt), group['slots'][i]['key']
-				yield str(cnt+1), group['slots'][i]['elem']
+				# The group layout depends on GOEXPERIMENT=mapsplitgroup:
+				#   split:       group.keys[i] / group.elems[i]
+				#   interleaved: group.slots[i].key / group.slots[i].elem
+				try:
+					yield str(cnt), group['slots'][i]['key']
+					yield str(cnt+1), group['slots'][i]['elem']
+				except gdb.error:
+					yield str(cnt), group['keys'][i]
+					yield str(cnt+1), group['elems'][i]
 
 		# The linker DWARF generation
 		# (cmd/link/internal/ld.(*dwctxt).synthesizemaptypes) records