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cmd/compile: captureless closures are constants

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In particular, we can initialize globals with them at link time instead
of generating code for them in an init() function.  Less code, less
startup cost.

But the real reason for this change is binary size.  This change reduces
the binary size of hello world by ~4%.

The culprit is fmt.ssFree, a global variable which is a sync.Pool of
scratch scan states.  It is initalized with a captureless closure as the
pool's New action.  That action in turn references all the scanf code.

If you never call any of the fmt.Scanf* routines, ssFree is never used.
But before this change, ssFree is still referenced by fmt's init
function.  That keeps ssFree and all the code it references in the
binary.  With this change, ssFree is initialized at link time.  As a
result, fmt.init never mentions ssFree.  If you don't call fmt.Scanf*,
ssFree is unreferenced and it and the scanf code are not included.

This change is an easy fix for what is generally a much harder problem,
the unnecessary initializing of unused globals (and retention of code
that they reference).  Ideally we should have separate init code for
each global and only include that code if the corresponding global is
live.  (We'd need to make sure that the initializing code has no side
effects, except on the global being initialized.)  That is a much harder
change.

Update #6853

Change-Id: I19d1e33992287882c83efea6ce113b7cfc504b67
Reviewed-on: https://go-review.googlesource.com/17398
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
This commit is contained in:
Keith Randall 2015-12-03 13:20:58 -08:00 • committed by Russ Cox
parent 5ef899111e
commit 4f97ec0866
2 changed files with 77 additions and 11 deletions
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24
src/cmd/compile/internal/gc/sinit.go
View file

@ -378,10 +378,6 @@ func staticassign(l *Node, r *Node, out **NodeList) bool {
}
switch r.Op {
//dump("not static", r);
default:
break
case ONAME:
return staticcopy(l, r, out)
@ -404,12 +400,8 @@ func staticassign(l *Node, r *Node, out **NodeList) bool {
case OPTRLIT:
switch r.Left.Op {
//dump("not static ptrlit", r);
default:
break
// Init pointer.
case OARRAYLIT, OMAPLIT, OSTRUCTLIT:
// Init pointer.
a := staticname(r.Left.Type, 1)
inittemps[r] = a
@ -421,6 +413,7 @@ func staticassign(l *Node, r *Node, out **NodeList) bool {
}
return true
}
//dump("not static ptrlit", r);
case OSTRARRAYBYTE:
if l.Class == PEXTERN && r.Left.Op == OLITERAL {
@ -452,7 +445,6 @@ func staticassign(l *Node, r *Node, out **NodeList) bool {
}
fallthrough
// fall through
case OSTRUCTLIT:
initplan(r)
@ -477,11 +469,21 @@ func staticassign(l *Node, r *Node, out **NodeList) bool {
return true
// TODO: Table-driven map insert.
case OMAPLIT:
// TODO: Table-driven map insert.
break
case OCLOSURE:
if r.Func.Cvars == nil {
// Closures with no captured variables are globals,
// so the assignment can be done at link time.
n := *l
gdata(&n, r.Func.Closure.Func.Nname, Widthptr)
return true
}
}
//dump("not static", r);
return false
}