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cmd/compile: mark Lsyms as readonly earlier

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The SSA backend has rules to read the contents of readonly Lsyms.
However, this rule was failing to trigger for many readonly Lsyms.
This is because the readonly attribute that was set on the Node.Name
was not propagated to its Lsym until the dump globals phase, after SSA runs.

To work around this phase ordering problem, introduce Node.SetReadonly,
which sets Node.Name.Readonly and also configures the Lsym
enough that SSA can use it.

This change also fixes a latent problem in the rewrite rule function,
namely that reads past the end of lsym.P were treated as entirely zero,
instead of merely requiring padding with trailing zeros.

This change also adds an amd64 rule needed to fully optimize
the results of this change. It would be better not to need this,
but the zero extension that should handle this for us
gets optimized away too soon (see #36897 for a similar problem).
I have not investigated whether other platforms also need new
rules to take full advantage of the new optimizations.

Compiled code for (interface{})(true) on amd64 goes from:

LEAQ	type.bool(SB), AX
MOVBLZX	""..stmp_0(SB), BX
LEAQ	runtime.staticbytes(SB), CX
ADDQ	CX, BX

to

LEAQ	type.bool(SB), AX
LEAQ	runtime.staticbytes+1(SB), BX

Prior to this change, the readonly symbol rewrite rules
fired a total of 884 times during make.bash.
Afterwards they fire 1807 times.

file    before    after     Δ       %
cgo     4827832   4823736   -4096   -0.085%
compile 24907768  24895656  -12112  -0.049%
fix     3376952   3368760   -8192   -0.243%
pprof   14751700  14747604  -4096   -0.028%
total   120343528 120315032 -28496  -0.024%

Change-Id: I59ea52138276c37840f69e30fb109fd376d579ec
Reviewed-on: https://go-review.googlesource.com/c/go/+/220499
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
This commit is contained in:
Josh Bleecher Snyder 2020-02-16 17:00:52 -08:00
parent 390c096ee9
commit eb5cd0fb40
15 changed files with 103 additions and 70 deletions
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10
src/cmd/compile/internal/gc/sinit.go
View file

@ -372,7 +372,7 @@ func (c initContext) String() string {
var statuniqgen int // name generator for static temps
// staticname returns a name backed by a static data symbol.
// Callers should call n.Name.SetReadonly(true) on the
// Callers should call n.MarkReadonly on the
// returned node for readonly nodes.
func staticname(t *types.Type) *Node {
// Don't use lookupN; it interns the resulting string, but these are all unique.
@ -652,7 +652,7 @@ func slicelit(ctxt initContext, n *Node, var_ *Node, init *Nodes) {
if mode&initConst != 0 && !isSmallSliceLit(n) {
vstat = staticname(t)
if ctxt == inInitFunction {
vstat.Name.SetReadonly(true)
vstat.MarkReadonly()
}
fixedlit(ctxt, initKindStatic, n, vstat, init)
}
@ -795,9 +795,9 @@ func maplit(n *Node, m *Node, init *Nodes) {
// make and initialize static arrays
vstatk := staticname(tk)
vstatk.Name.SetReadonly(true)
vstatk.MarkReadonly()
vstate := staticname(te)
vstate.Name.SetReadonly(true)
vstate.MarkReadonly()
datak := nod(OARRAYLIT, nil, nil)
datae := nod(OARRAYLIT, nil, nil)
@ -919,7 +919,7 @@ func anylit(n *Node, var_ *Node, init *Nodes) {
if var_.isSimpleName() && n.List.Len() > 4 {
// lay out static data
vstat := staticname(t)
vstat.Name.SetReadonly(true)
vstat.MarkReadonly()
ctxt := inInitFunction
if n.Op == OARRAYLIT {