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cmd/compile: initialize sparse slice literals dynamically

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When a slice composite literal is sparse, initialize it dynamically
instead of statically.

s := []int{5:5, 20:20}

To initialize the backing store for s, use 2 constant writes instead
of copying from a static array with 21 entries.

This CL also fixes pathologies in the compiler when the slice is
*very* sparse.

Fixes #23780

Change-Id: Iae95c6e6f6a0e2994675cbc750d7a4dd6436b13b
Reviewed-on: https://go-review.googlesource.com/c/151319
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
This commit is contained in:
Keith Randall 2018-11-26 14:33:32 -08:00 • committed by Keith Randall
parent 1602e49701
commit 6fff980cf1
3 changed files with 28 additions and 2 deletions
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2
src/cmd/compile/internal/gc/syntax.go
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@ -622,7 +622,7 @@ const (
OMAPLIT // Type{List} (composite literal, Type is map)
OSTRUCTLIT // Type{List} (composite literal, Type is struct)
OARRAYLIT // Type{List} (composite literal, Type is array)
OSLICELIT // Type{List} (composite literal, Type is slice)
OSLICELIT // Type{List} (composite literal, Type is slice) Right.Int64() = slice length.
OPTRLIT // &Left (left is composite literal)
OCONV // Type(Left) (type conversion)
OCONVIFACE // Type(Left) (type conversion, to interface)