internal/core/adt: do not reuse large vertexMap maps

https://cuelang.org/cl/1226958 was a decent improvement thanks to
the reuse of vertexMap maps. However, Go is currently fairly slow
at clearing maps with large capacities, even if they have a small length
due to few entries.

For that reason, and since most maps we need are on the small side,
add a cutoff to keep most of the ~2.5% reduction in allocs
while avoiding the performance degradation due to the use of clear.

│ old │ new │
│ sec/op │ sec/op vs base │
ExportJudson 3.932 ± 1% 3.673 ± 2% -6.57% (p=0.000 n=8)

│ old │ new │
│ B/op │ B/op vs base │
ExportJudson 1.918Gi ± 0% 1.926Gi ± 0% +0.42% (p=0.000 n=8)

│ old │ new │
│ allocs/op │ allocs/op vs base │
ExportJudson 15.71M ± 0% 15.72M ± 0% +0.02% (p=0.000 n=8)

Signed-off-by: Daniel Martí <[e-mail]>
Change-Id: I79b3c3f39faf234e72a04c59daad7187f8772b0b
Reviewed-on: https://cue.gerrithub.io/c/cue-lang/cue/+/1226971
TryBot-Result: CUEcueckoo <[e-mail]>
Unity-Result: CUE porcuepine <[e-mail]>
Reviewed-by: Marcel van Lohuizen <[e-mail]>

internal/core/adt/overlay.go

···
 
 func (c *OpContext) popOverlay() {
 	i := len(c.overlays) - 1
-	clear(c.overlays[i].vertexMap)
+	// TODO(mvdan): unfortunately, clearing maps with large capacities
+	// is fairly slow as of Go 1.25, and most uses only need few entries.
+	// Do not reuse large maps to avoid this pitfall for now.
+	// See: https://github.com/golang/go/issues/70617
+	if l := len(c.overlays[i].vertexMap); l > 512 {
+		c.overlays[i].vertexMap = nil
+	} else {
+		clear(c.overlays[i].vertexMap)
+	}
 	c.overlays = c.overlays[:i]
 }