From 60fd32a47fdffb95d3646c9fc75acc9beff67183 Mon Sep 17 00:00:00 2001 From: Keith Randall Date: Tue, 19 Apr 2016 08:31:04 -0700 Subject: cmd/compile: change the way we handle large map values mapaccess{1,2} returns a pointer to the value. When the key is not in the map, it returns a pointer to zeroed memory. Currently, for large map values we have a complicated scheme which dynamically allocates zeroed memory for this purpose. It is ugly code and requires an atomic.Load in a bunch of places we'd rather not have it. Switch to a scheme where callsites of mapaccess{1,2} which expect large return values pass in a pointer to zeroed memory that mapaccess can return if the key is not found. This avoids the atomic.Load on all map accesses with a few extra instructions only for the large value acccesses, plus a bit of bss space. There was a time (1.4 & 1.5?) where we did something like this but all the tricks to make the right size zero value were done by the linker. That scheme broke in the presence of dyamic linking. The scheme in this CL works even when dynamic linking. Fixes #12337 Change-Id: Ic2d0319944af33bbb59785938d9ab80958d1b4b1 Reviewed-on: https://go-review.googlesource.com/22221 Run-TryBot: Keith Randall TryBot-Result: Gobot Gobot Reviewed-by: Michael Hudson-Doyle --- src/runtime/map_test.go | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) (limited to 'src/runtime/map_test.go') diff --git a/src/runtime/map_test.go b/src/runtime/map_test.go index 9d2894cb6f..496f8e8868 100644 --- a/src/runtime/map_test.go +++ b/src/runtime/map_test.go @@ -317,6 +317,22 @@ func TestBigItems(t *testing.T) { } } +func TestMapHugeZero(t *testing.T) { + type T [4000]byte + m := map[int]T{} + x := m[0] + if x != (T{}) { + t.Errorf("map value not zero") + } + y, ok := m[0] + if ok { + t.Errorf("map value should be missing") + } + if y != (T{}) { + t.Errorf("map value not zero") + } +} + type empty struct { } -- cgit v1.3