cmd/compile, runtime: emit only GC data for stack objects

Currently, for stack objects, the compiler emits metadata that
includes the offset and type descriptor for each object. The type
descriptor symbol has many fields, and it references many other
symbols, e.g. field/element types, equality functions, names.

Observe that what we actually need at runtime is only the GC
metadata that are needed to scan the object, and the GC metadata
are "leaf" symbols (which doesn't reference other symbols). Emit
only the GC data instead. This avoids bringing live the type
descriptor as well as things referenced by it (if it is not
otherwise live).

This reduces binary sizes:

                     old          new
hello (println)    1187776      1133856 (-4.5%)
hello (fmt)        1902448      1844416 (-3.1%)
cmd/compile       22670432     22438576 (-1.0%)
cmd/link           6346272      6225408 (-1.9%)

No significant change in compiler speed.

name        old time/op       new time/op       delta
Template          184ms ± 2%        186ms ± 5%    ~     (p=0.905 n=9+10)
Unicode          78.4ms ± 5%       76.3ms ± 3%  -2.60%  (p=0.009 n=10+10)
GoTypes           1.09s ± 1%        1.08s ± 1%  -0.73%  (p=0.027 n=10+8)
Compiler         85.6ms ± 3%       84.6ms ± 4%    ~     (p=0.143 n=10+10)
SSA               7.23s ± 1%        7.25s ± 1%    ~     (p=0.780 n=10+9)
Flate             116ms ± 5%        115ms ± 6%    ~     (p=0.912 n=10+10)
GoParser          201ms ± 4%        195ms ± 1%    ~     (p=0.089 n=10+10)
Reflect           455ms ± 1%        458ms ± 2%    ~     (p=0.050 n=9+9)
Tar               155ms ± 2%        155ms ± 3%    ~     (p=0.436 n=10+10)
XML               202ms ± 2%        200ms ± 2%    ~     (p=0.053 n=10+9)

Change-Id: I33a7f383d79afba1a482cac6da0cf5b7de9c0ec4
Reviewed-on: https://go-review.googlesource.com/c/go/+/313514
Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

1 files changed, 20 insertions, 2 deletions

diff --git a/src/cmd/compile/internal/liveness/plive.go b/src/cmd/compile/internal/liveness/plive.go
index 424d248590..f5c2ef7709 100644
--- a/src/cmd/compile/internal/liveness/plive.go
+++ b/src/cmd/compile/internal/liveness/plive.go
@@ -1431,8 +1431,26 @@ func (lv *liveness) emitStackObjects() *obj.LSym {
 		// Note: arguments and return values have non-negative Xoffset,
 		// in which case the offset is relative to argp.
 		// Locals have a negative Xoffset, in which case the offset is relative to varp.
-		off = objw.Uintptr(x, off, uint64(v.FrameOffset()))
-		off = objw.SymPtr(x, off, reflectdata.TypeLinksym(v.Type()), 0)
+		// We already limit the frame size, so the offset and the object size
+		// should not be too big.
+		frameOffset := v.FrameOffset()
+		if frameOffset != int64(int32(frameOffset)) {
+			base.Fatalf("frame offset too big: %v %d", v, frameOffset)
+		}
+		off = objw.Uint32(x, off, uint32(frameOffset))
+
+		t := v.Type()
+		sz := t.Width
+		if sz != int64(int32(sz)) {
+			base.Fatalf("stack object too big: %v of type %v, size %d", v, t, sz)
+		}
+		lsym, useGCProg, ptrdata := reflectdata.GCSym(t)
+		if useGCProg {
+			ptrdata = -ptrdata
+		}
+		off = objw.Uint32(x, off, uint32(sz))
+		off = objw.Uint32(x, off, uint32(ptrdata))
+		off = objw.SymPtr(x, off, lsym, 0)
 	}
 
 	if base.Flag.Live != 0 {