runtime: replace GC programs with simpler encoding, faster decoder
Small types record the location of pointers in their memory layout
by using a simple bitmap. In Go 1.4 the bitmap held 4-bit entries,
and in Go 1.5 the bitmap holds 1-bit entries, but in both cases using
a bitmap for a large type containing arrays does not make sense:
if someone refers to the type [1<<28]*byte in a program in such
a way that the type information makes it into the binary, it would be
a waste of space to write a 128 MB (for 4-bit entries) or even 32 MB
(for 1-bit entries) bitmap full of 1s into the binary or even to keep
one in memory during the execution of the program.
For large types containing arrays, it is much more compact to describe
the locations of pointers using a notation that can express repetition
than to lay out a bitmap of pointers. Go 1.4 included such a notation,
called ``GC programs'' but it was complex, required recursion during
decoding, and was generally slow. Dmitriy measured the execution of
these programs writing directly to the heap bitmap as being 7x slower
than copying from a preunrolled 4-bit mask (and frankly that code was
not terribly fast either). For some tests, unrollgcprog1 was seen costing
as much as 3x more than the rest of malloc combined.
This CL introduces a different form for the GC programs. They use a
simple Lempel-Ziv-style encoding of the 1-bit pointer information,
in which the only operations are (1) emit the following n bits
and (2) repeat the last n bits c more times. This encoding can be
generated directly from the Go type information (using repetition
only for arrays or large runs of non-pointer data) and it can be decoded
very efficiently. In particular the decoding requires little state and
no recursion, so that the entire decoding can run without any memory
accesses other than the reads of the encoding and the writes of the
decoded form to the heap bitmap. For recursive types like arrays of
arrays of arrays, the inner instructions are only executed once, not
n times, so that large repetitions run at full speed. (In contrast, large
repetitions in the old programs repeated the individual bit-level layout
of the inner data over and over.) The result is as much as 25x faster
decoding compared to the old form.
Because the old decoder was so slow, Go 1.4 had three (or so) cases
for how to set the heap bitmap bits for an allocation of a given type:
(1) If the type had an even number of words up to 32 words, then
the 4-bit pointer mask for the type fit in no more than 16 bytes;
store the 4-bit pointer mask directly in the binary and copy from it.
(1b) If the type had an odd number of words up to 15 words, then
the 4-bit pointer mask for the type, doubled to end on a byte boundary,
fit in no more than 16 bytes; store that doubled mask directly in the
binary and copy from it.
(2) If the type had an even number of words up to 128 words,
or an odd number of words up to 63 words (again due to doubling),
then the 4-bit pointer mask would fit in a 64-byte unrolled mask.
Store a GC program in the binary, but leave space in the BSS for
the unrolled mask. Execute the GC program to construct the mask the
first time it is needed, and thereafter copy from the mask.
(3) Otherwise, store a GC program and execute it to write directly to
the heap bitmap each time an object of that type is allocated.
(This is the case that was 7x slower than the other two.)
Because the new pointer masks store 1-bit entries instead of 4-bit
entries and because using the decoder no longer carries a significant
overhead, after this CL (that is, for Go 1.5) there are only two cases:
(1) If the type is 128 words or less (no condition about odd or even),
store the 1-bit pointer mask directly in the binary and use it to
initialize the heap bitmap during malloc. (Implemented in CL 9702.)
(2) There is no case 2 anymore.
(3) Otherwise, store a GC program and execute it to write directly to
the heap bitmap each time an object of that type is allocated.
Executing the GC program directly into the heap bitmap (case (3) above)
was disabled for the Go 1.5 dev cycle, both to avoid needing to use
GC programs for typedmemmove and to avoid updating that code as
the heap bitmap format changed. Typedmemmove no longer uses this
type information; as of CL 9886 it uses the heap bitmap directly.
Now that the heap bitmap format is stable, we reintroduce GC programs
and their space savings.
Benchmarks for heapBitsSetType, before this CL vs this CL:
name old mean new mean delta
SetTypePtr 7.59ns × (0.99,1.02) 5.16ns × (1.00,1.00) -32.05% (p=0.000)
SetTypePtr8 21.0ns × (0.98,1.05) 21.4ns × (1.00,1.00) ~ (p=0.179)
SetTypePtr16 24.1ns × (0.99,1.01) 24.6ns × (1.00,1.00) +2.41% (p=0.001)
SetTypePtr32 31.2ns × (0.99,1.01) 32.4ns × (0.99,1.02) +3.72% (p=0.001)
SetTypePtr64 45.2ns × (1.00,1.00) 47.2ns × (1.00,1.00) +4.42% (p=0.000)
SetTypePtr126 75.8ns × (0.99,1.01) 79.1ns × (1.00,1.00) +4.25% (p=0.000)
SetTypePtr128 74.3ns × (0.99,1.01) 77.6ns × (1.00,1.01) +4.55% (p=0.000)
SetTypePtrSlice 726ns × (1.00,1.01) 712ns × (1.00,1.00) -1.95% (p=0.001)
SetTypeNode1 20.0ns × (0.99,1.01) 20.7ns × (1.00,1.00) +3.71% (p=0.000)
SetTypeNode1Slice 112ns × (1.00,1.00) 113ns × (0.99,1.00) ~ (p=0.070)
SetTypeNode8 23.9ns × (1.00,1.00) 24.7ns × (1.00,1.01) +3.18% (p=0.000)
SetTypeNode8Slice 294ns × (0.99,1.02) 287ns × (0.99,1.01) -2.38% (p=0.015)
SetTypeNode64 52.8ns × (0.99,1.03) 51.8ns × (0.99,1.01) ~ (p=0.069)
SetTypeNode64Slice 1.13µs × (0.99,1.05) 1.14µs × (0.99,1.00) ~ (p=0.767)
SetTypeNode64Dead 36.0ns × (1.00,1.01) 32.5ns × (0.99,1.00) -9.67% (p=0.000)
SetTypeNode64DeadSlice 1.43µs × (0.99,1.01) 1.40µs × (1.00,1.00) -2.39% (p=0.001)
SetTypeNode124 75.7ns × (1.00,1.01) 79.0ns × (1.00,1.00) +4.44% (p=0.000)
SetTypeNode124Slice 1.94µs × (1.00,1.01) 2.04µs × (0.99,1.01) +4.98% (p=0.000)
SetTypeNode126 75.4ns × (1.00,1.01) 77.7ns × (0.99,1.01) +3.11% (p=0.000)
SetTypeNode126Slice 1.95µs × (0.99,1.01) 2.03µs × (1.00,1.00) +3.74% (p=0.000)
SetTypeNode128 85.4ns × (0.99,1.01) 122.0ns × (1.00,1.00) +42.89% (p=0.000)
SetTypeNode128Slice 2.20µs × (1.00,1.01) 2.36µs × (0.98,1.02) +7.48% (p=0.001)
SetTypeNode130 83.3ns × (1.00,1.00) 123.0ns × (1.00,1.00) +47.61% (p=0.000)
SetTypeNode130Slice 2.30µs × (0.99,1.01) 2.40µs × (0.98,1.01) +4.37% (p=0.000)
SetTypeNode1024 498ns × (1.00,1.00) 537ns × (1.00,1.00) +7.96% (p=0.000)
SetTypeNode1024Slice 15.5µs × (0.99,1.01) 17.8µs × (1.00,1.00) +15.27% (p=0.000)
The above compares always using a cached pointer mask (and the
corresponding waste of memory) against using the programs directly.
Some slowdown is expected, in exchange for having a better general algorithm.
The GC programs kick in for SetTypeNode128, SetTypeNode130, SetTypeNode1024,
along with the slice variants of those.
It is possible that the cutoff of 128 words (bits) should be raised
in a followup CL, but even with this low cutoff the GC programs are
faster than Go 1.4's "fast path" non-GC program case.
Benchmarks for heapBitsSetType, Go 1.4 vs this CL:
name old mean new mean delta
SetTypePtr 6.89ns × (1.00,1.00) 5.17ns × (1.00,1.00) -25.02% (p=0.000)
SetTypePtr8 25.8ns × (0.97,1.05) 21.5ns × (1.00,1.00) -16.70% (p=0.000)
SetTypePtr16 39.8ns × (0.97,1.02) 24.7ns × (0.99,1.01) -37.81% (p=0.000)
SetTypePtr32 68.8ns × (0.98,1.01) 32.2ns × (1.00,1.01) -53.18% (p=0.000)
SetTypePtr64 130ns × (1.00,1.00) 47ns × (1.00,1.00) -63.67% (p=0.000)
SetTypePtr126 241ns × (0.99,1.01) 79ns × (1.00,1.01) -67.25% (p=0.000)
SetTypePtr128 2.07µs × (1.00,1.00) 0.08µs × (1.00,1.00) -96.27% (p=0.000)
SetTypePtrSlice 1.05µs × (0.99,1.01) 0.72µs × (0.99,1.02) -31.70% (p=0.000)
SetTypeNode1 16.0ns × (0.99,1.01) 20.8ns × (0.99,1.03) +29.91% (p=0.000)
SetTypeNode1Slice 184ns × (0.99,1.01) 112ns × (0.99,1.01) -39.26% (p=0.000)
SetTypeNode8 29.5ns × (0.97,1.02) 24.6ns × (1.00,1.00) -16.50% (p=0.000)
SetTypeNode8Slice 624ns × (0.98,1.02) 285ns × (1.00,1.00) -54.31% (p=0.000)
SetTypeNode64 135ns × (0.96,1.08) 52ns × (0.99,1.02) -61.32% (p=0.000)
SetTypeNode64Slice 3.83µs × (1.00,1.00) 1.14µs × (0.99,1.01) -70.16% (p=0.000)
SetTypeNode64Dead 134ns × (0.99,1.01) 32ns × (1.00,1.01) -75.74% (p=0.000)
SetTypeNode64DeadSlice 3.83µs × (0.99,1.00) 1.40µs × (1.00,1.01) -63.42% (p=0.000)
SetTypeNode124 240ns × (0.99,1.01) 79ns × (1.00,1.01) -67.05% (p=0.000)
SetTypeNode124Slice 7.27µs × (1.00,1.00) 2.04µs × (1.00,1.00) -71.95% (p=0.000)
SetTypeNode126 2.06µs × (0.99,1.01) 0.08µs × (0.99,1.01) -96.23% (p=0.000)
SetTypeNode126Slice 64.4µs × (1.00,1.00) 2.0µs × (1.00,1.00) -96.85% (p=0.000)
SetTypeNode128 2.09µs × (1.00,1.01) 0.12µs × (1.00,1.00) -94.15% (p=0.000)
SetTypeNode128Slice 65.4µs × (1.00,1.00) 2.4µs × (0.99,1.03) -96.39% (p=0.000)
SetTypeNode130 2.11µs × (1.00,1.00) 0.12µs × (1.00,1.00) -94.18% (p=0.000)
SetTypeNode130Slice 66.3µs × (1.00,1.00) 2.4µs × (0.97,1.08) -96.34% (p=0.000)
SetTypeNode1024 16.0µs × (1.00,1.01) 0.5µs × (1.00,1.00) -96.65% (p=0.000)
SetTypeNode1024Slice 512µs × (1.00,1.00) 18µs × (0.98,1.04) -96.45% (p=0.000)
SetTypeNode124 uses a 124 data + 2 ptr = 126-word allocation.
Both Go 1.4 and this CL are using pointer bitmaps for this case,
so that's an overall 3x speedup for using pointer bitmaps.
SetTypeNode128 uses a 128 data + 2 ptr = 130-word allocation.
Both Go 1.4 and this CL are running the GC program for this case,
so that's an overall 17x speedup when using GC programs (and
I've seen >20x on other systems).
Comparing Go 1.4's SetTypeNode124 (pointer bitmap) against
this CL's SetTypeNode128 (GC program), the slow path in the
code in this CL is 2x faster than the fast path in Go 1.4.
The Go 1 benchmarks are basically unaffected compared to just before this CL.
Go 1 benchmarks, before this CL vs this CL:
name old mean new mean delta
BinaryTree17 5.87s × (0.97,1.04) 5.91s × (0.96,1.04) ~ (p=0.306)
Fannkuch11 4.38s × (1.00,1.00) 4.37s × (1.00,1.01) -0.22% (p=0.006)
FmtFprintfEmpty 90.7ns × (0.97,1.10) 89.3ns × (0.96,1.09) ~ (p=0.280)
FmtFprintfString 282ns × (0.98,1.04) 287ns × (0.98,1.07) +1.72% (p=0.039)
FmtFprintfInt 269ns × (0.99,1.03) 282ns × (0.97,1.04) +4.87% (p=0.000)
FmtFprintfIntInt 478ns × (0.99,1.02) 481ns × (0.99,1.02) +0.61% (p=0.048)
FmtFprintfPrefixedInt 399ns × (0.98,1.03) 400ns × (0.98,1.05) ~ (p=0.533)
FmtFprintfFloat 563ns × (0.99,1.01) 570ns × (1.00,1.01) +1.37% (p=0.000)
FmtManyArgs 1.89µs × (0.99,1.01) 1.92µs × (0.99,1.02) +1.88% (p=0.000)
GobDecode 15.2ms × (0.99,1.01) 15.2ms × (0.98,1.05) ~ (p=0.609)
GobEncode 11.6ms × (0.98,1.03) 11.9ms × (0.98,1.04) +2.17% (p=0.000)
Gzip 648ms × (0.99,1.01) 648ms × (1.00,1.01) ~ (p=0.835)
Gunzip 142ms × (1.00,1.00) 143ms × (1.00,1.01) ~ (p=0.169)
HTTPClientServer 90.5µs × (0.98,1.03) 91.5µs × (0.98,1.04) +1.04% (p=0.045)
JSONEncode 31.5ms × (0.98,1.03) 31.4ms × (0.98,1.03) ~ (p=0.549)
JSONDecode 111ms × (0.99,1.01) 107ms × (0.99,1.01) -3.21% (p=0.000)
Mandelbrot200 6.01ms × (1.00,1.00) 6.01ms × (1.00,1.00) ~ (p=0.878)
GoParse 6.54ms × (0.99,1.02) 6.61ms × (0.99,1.03) +1.08% (p=0.004)
RegexpMatchEasy0_32 160ns × (1.00,1.01) 161ns × (1.00,1.00) +0.40% (p=0.000)
RegexpMatchEasy0_1K 560ns × (0.99,1.01) 559ns × (0.99,1.01) ~ (p=0.088)
RegexpMatchEasy1_32 138ns × (0.99,1.01) 138ns × (1.00,1.00) ~ (p=0.380)
RegexpMatchEasy1_1K 877ns × (1.00,1.00) 878ns × (1.00,1.00) ~ (p=0.157)
RegexpMatchMedium_32 251ns × (0.99,1.00) 251ns × (1.00,1.01) +0.28% (p=0.021)
RegexpMatchMedium_1K 72.6µs × (1.00,1.00) 72.6µs × (1.00,1.00) ~ (p=0.539)
RegexpMatchHard_32 3.84µs × (1.00,1.00) 3.84µs × (1.00,1.00) ~ (p=0.378)
RegexpMatchHard_1K 117µs × (1.00,1.00) 117µs × (1.00,1.00) ~ (p=0.067)
Revcomp 904ms × (0.99,1.02) 904ms × (0.99,1.01) ~ (p=0.943)
Template 125ms × (0.99,1.02) 127ms × (0.99,1.01) +1.79% (p=0.000)
TimeParse 627ns × (0.99,1.01) 622ns × (0.99,1.01) -0.88% (p=0.000)
TimeFormat 655ns × (0.99,1.02) 655ns × (0.99,1.02) ~ (p=0.976)
For the record, Go 1 benchmarks, Go 1.4 vs this CL:
name old mean new mean delta
BinaryTree17 4.61s × (0.97,1.05) 5.91s × (0.98,1.03) +28.35% (p=0.000)
Fannkuch11 4.40s × (0.99,1.03) 4.41s × (0.99,1.01) ~ (p=0.212)
FmtFprintfEmpty 102ns × (0.99,1.01) 84ns × (0.99,1.02) -18.38% (p=0.000)
FmtFprintfString 302ns × (0.98,1.01) 303ns × (0.99,1.02) ~ (p=0.203)
FmtFprintfInt 313ns × (0.97,1.05) 270ns × (0.99,1.01) -13.69% (p=0.000)
FmtFprintfIntInt 524ns × (0.98,1.02) 477ns × (0.99,1.00) -8.87% (p=0.000)
FmtFprintfPrefixedInt 424ns × (0.98,1.02) 386ns × (0.99,1.01) -8.96% (p=0.000)
FmtFprintfFloat 652ns × (0.98,1.02) 594ns × (0.97,1.05) -8.97% (p=0.000)
FmtManyArgs 2.13µs × (0.99,1.02) 1.94µs × (0.99,1.01) -8.92% (p=0.000)
GobDecode 17.1ms × (0.99,1.02) 14.9ms × (0.98,1.03) -13.07% (p=0.000)
GobEncode 13.5ms × (0.98,1.03) 11.5ms × (0.98,1.03) -15.25% (p=0.000)
Gzip 656ms × (0.99,1.02) 647ms × (0.99,1.01) -1.29% (p=0.000)
Gunzip 143ms × (0.99,1.02) 144ms × (0.99,1.01) ~ (p=0.204)
HTTPClientServer 88.2µs × (0.98,1.02) 90.8µs × (0.98,1.01) +2.93% (p=0.000)
JSONEncode 32.2ms × (0.98,1.02) 30.9ms × (0.97,1.04) -4.06% (p=0.001)
JSONDecode 121ms × (0.98,1.02) 110ms × (0.98,1.05) -8.95% (p=0.000)
Mandelbrot200 6.06ms × (0.99,1.01) 6.11ms × (0.98,1.04) ~ (p=0.184)
GoParse 6.76ms × (0.97,1.04) 6.58ms × (0.98,1.05) -2.63% (p=0.003)
RegexpMatchEasy0_32 195ns × (1.00,1.01) 155ns × (0.99,1.01) -20.43% (p=0.000)
RegexpMatchEasy0_1K 479ns × (0.98,1.03) 535ns × (0.99,1.02) +11.59% (p=0.000)
RegexpMatchEasy1_32 169ns × (0.99,1.02) 131ns × (0.99,1.03) -22.44% (p=0.000)
RegexpMatchEasy1_1K 1.53µs × (0.99,1.01) 0.87µs × (0.99,1.02) -43.07% (p=0.000)
RegexpMatchMedium_32 334ns × (0.99,1.01) 242ns × (0.99,1.01) -27.53% (p=0.000)
RegexpMatchMedium_1K 125µs × (1.00,1.01) 72µs × (0.99,1.03) -42.53% (p=0.000)
RegexpMatchHard_32 6.03µs × (0.99,1.01) 3.79µs × (0.99,1.01) -37.12% (p=0.000)
RegexpMatchHard_1K 189µs × (0.99,1.02) 115µs × (0.99,1.01) -39.20% (p=0.000)
Revcomp 935ms × (0.96,1.03) 926ms × (0.98,1.02) ~ (p=0.083)
Template 146ms × (0.97,1.05) 119ms × (0.99,1.01) -18.37% (p=0.000)
TimeParse 660ns × (0.99,1.01) 624ns × (0.99,1.02) -5.43% (p=0.000)
TimeFormat 670ns × (0.98,1.02) 710ns × (1.00,1.01) +5.97% (p=0.000)
This CL is a bit larger than I would like, but the compiler, linker, runtime,
and package reflect all need to be in sync about the format of these programs,
so there is no easy way to split this into independent changes (at least
while keeping the build working at each change).
Fixes #9625.
Fixes #10524.
Change-Id: I9e3e20d6097099d0f8532d1cb5b1af528804989a
Reviewed-on: https://go-review.googlesource.com/9888
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
diff --git a/src/cmd/internal/gc/reflect.go b/src/cmd/internal/gc/reflect.go
index 061b17b..6c0962f 100644
--- a/src/cmd/internal/gc/reflect.go
+++ b/src/cmd/internal/gc/reflect.go
@@ -5,8 +5,10 @@
package gc
import (
+ "cmd/internal/gcprog"
"cmd/internal/obj"
"fmt"
+ "os"
)
/*
@@ -771,6 +773,8 @@
// The linker magically takes the max of all the sizes.
zero := Pkglookup("zerovalue", Runtimepkg)
+ gcsym, useGCProg, ptrdata := dgcsym(t)
+
// We use size 0 here so we get the pointer to the zero value,
// but don't allocate space for the zero value unless we need it.
// TODO: how do we get this symbol into bss? We really want
@@ -787,14 +791,14 @@
// fieldAlign uint8
// kind uint8
// alg unsafe.Pointer
- // gc unsafe.Pointer
+ // gcdata unsafe.Pointer
// string *string
// *extraType
// ptrToThis *Type
// zero unsafe.Pointer
// }
ot = duintptr(s, ot, uint64(t.Width))
- ot = duintptr(s, ot, uint64(typeptrdata(t)))
+ ot = duintptr(s, ot, uint64(ptrdata))
ot = duint32(s, ot, typehash(t))
ot = duint8(s, ot, 0) // unused
@@ -811,8 +815,6 @@
ot = duint8(s, ot, t.Align) // align
ot = duint8(s, ot, t.Align) // fieldAlign
- gcprog := usegcprog(t)
-
i = kinds[t.Etype]
if t.Etype == TARRAY && t.Bound < 0 {
i = obj.KindSlice
@@ -823,7 +825,7 @@
if isdirectiface(t) {
i |= obj.KindDirectIface
}
- if gcprog {
+ if useGCProg {
i |= obj.KindGCProg
}
ot = duint8(s, ot, uint8(i)) // kind
@@ -832,48 +834,7 @@
} else {
ot = dsymptr(s, ot, algsym, 0)
}
-
- // gc
- if gcprog {
- var gcprog1 *Sym
- var gcprog0 *Sym
- gengcprog(t, &gcprog0, &gcprog1)
- if gcprog0 != nil {
- ot = dsymptr(s, ot, gcprog0, 0)
- } else {
- ot = duintptr(s, ot, 0)
- }
- ot = dsymptr(s, ot, gcprog1, 0)
- } else {
- var gcmask [16]uint8
- gengcmask(t, gcmask[:])
- x1 := uint64(0)
- for i := 0; i < 8; i++ {
- x1 = x1<<8 | uint64(gcmask[i])
- }
- var p string
- if Widthptr == 4 {
- p = fmt.Sprintf("gcbits.0x%016x", x1)
- } else {
- x2 := uint64(0)
- for i := 0; i < 8; i++ {
- x2 = x2<<8 | uint64(gcmask[i+8])
- }
- p = fmt.Sprintf("gcbits.0x%016x%016x", x1, x2)
- }
-
- sbits := Pkglookup(p, Runtimepkg)
- if sbits.Flags&SymUniq == 0 {
- sbits.Flags |= SymUniq
- for i := 0; i < 2*Widthptr; i++ {
- duint8(sbits, i, gcmask[i])
- }
- ggloblsym(sbits, 2*int32(Widthptr), obj.DUPOK|obj.RODATA|obj.LOCAL)
- }
-
- ot = dsymptr(s, ot, sbits, 0)
- ot = duintptr(s, ot, 0)
- }
+ ot = dsymptr(s, ot, gcsym, 0)
p := Tconv(t, obj.FmtLeft|obj.FmtUnsigned)
@@ -1419,228 +1380,193 @@
return s
}
-func usegcprog(t *Type) bool {
- if !haspointers(t) {
- return false
- }
- if t.Width == BADWIDTH {
- dowidth(t)
+// maxPtrmaskBytes is the maximum length of a GC ptrmask bitmap,
+// which holds 1-bit entries describing where pointers are in a given type.
+// 16 bytes is enough to describe 128 pointer-sized words, 512 or 1024 bytes
+// depending on the system. Above this length, the GC information is
+// recorded as a GC program, which can express repetition compactly.
+// In either form, the information is used by the runtime to initialize the
+// heap bitmap, and for large types (like 128 or more words), they are
+// roughly the same speed. GC programs are never much larger and often
+// more compact. (If large arrays are involved, they can be arbitrarily more
+// compact.)
+//
+// The cutoff must be large enough that any allocation large enough to
+// use a GC program is large enough that it does not share heap bitmap
+// bytes with any other objects, allowing the GC program execution to
+// assume an aligned start and not use atomic operations. In the current
+// runtime, this means all malloc size classes larger than the cutoff must
+// be multiples of four words. On 32-bit systems that's 16 bytes, and
+// all size classes >= 16 bytes are 16-byte aligned, so no real constraint.
+// On 64-bit systems, that's 32 bytes, and 32-byte alignment is guaranteed
+// for size classes >= 256 bytes. On a 64-bit sytem, 256 bytes allocated
+// is 32 pointers, the bits for which fit in 4 bytes. So maxPtrmaskBytes
+// must be >= 4.
+//
+// We use 16 because the GC programs do have some constant overhead
+// to get started, and processing 128 pointers seems to be enough to
+// amortize that overhead well.
+const maxPtrmaskBytes = 16
+
+// dgcsym emits and returns a data symbol containing GC information for type t,
+// along with a boolean reporting whether the UseGCProg bit should be set in
+// the type kind, and the ptrdata field to record in the reflect type information.
+func dgcsym(t *Type) (sym *Sym, useGCProg bool, ptrdata int64) {
+ ptrdata = typeptrdata(t)
+ if ptrdata/int64(Widthptr) <= maxPtrmaskBytes*8 {
+ sym = dgcptrmask(t)
+ return
}
- // Calculate size of the unrolled GC mask.
- nptr := typeptrdata(t) / int64(Widthptr)
-
- // Decide whether to use unrolled GC mask or GC program.
- // We could use a more elaborate condition, but this seems to work well in practice.
- // For small objects, the GC program can't give significant reduction.
- return nptr > int64(2*Widthptr*8)
+ useGCProg = true
+ sym, ptrdata = dgcprog(t)
+ return
}
-// Generates GC bitmask (1 bit per word).
-func gengcmask(t *Type, gcmask []byte) {
- for i := int64(0); i < 16; i++ {
- gcmask[i] = 0
+// dgcptrmask emits and returns the symbol containing a pointer mask for type t.
+func dgcptrmask(t *Type) *Sym {
+ ptrmask := make([]byte, (typeptrdata(t)/int64(Widthptr)+7)/8)
+ fillptrmask(t, ptrmask)
+ p := fmt.Sprintf("gcbits.%x", ptrmask)
+
+ sym := Pkglookup(p, Runtimepkg)
+ if sym.Flags&SymUniq == 0 {
+ sym.Flags |= SymUniq
+ for i, x := range ptrmask {
+ duint8(sym, i, x)
+ }
+ ggloblsym(sym, int32(len(ptrmask)), obj.DUPOK|obj.RODATA|obj.LOCAL)
+ }
+ return sym
+}
+
+// fillptrmask fills in ptrmask with 1s corresponding to the
+// word offsets in t that hold pointers.
+// ptrmask is assumed to fit at least typeptrdata(t)/Widthptr bits.
+func fillptrmask(t *Type, ptrmask []byte) {
+ for i := range ptrmask {
+ ptrmask[i] = 0
}
if !haspointers(t) {
return
}
- vec := bvalloc(int32(2 * Widthptr * 8))
+ vec := bvalloc(8 * int32(len(ptrmask)))
xoffset := int64(0)
onebitwalktype1(t, &xoffset, vec)
nptr := typeptrdata(t) / int64(Widthptr)
for i := int64(0); i < nptr; i++ {
if bvget(vec, int32(i)) == 1 {
- gcmask[i/8] |= 1 << (uint(i) % 8)
+ ptrmask[i/8] |= 1 << (uint(i) % 8)
}
}
}
-// Helper object for generation of GC programs.
-type ProgGen struct {
- s *Sym
- datasize int32
- data [256 / 8]uint8
- ot int64
+// dgcprog emits and returns the symbol containing a GC program for type t
+// along with the size of the data described by the program (in the range [typeptrdata(t), t.Width]).
+// In practice, the size is typeptrdata(t) except for non-trivial arrays.
+// For non-trivial arrays, the program describes the full t.Width size.
+func dgcprog(t *Type) (*Sym, int64) {
+ dowidth(t)
+ if t.Width == BADWIDTH {
+ Fatal("dgcprog: %v badwidth", t)
+ }
+ sym := typesymprefix(".gcprog", t)
+ var p GCProg
+ p.init(sym)
+ p.emit(t, 0)
+ offset := p.w.BitIndex() * int64(Widthptr)
+ p.end()
+ if ptrdata := typeptrdata(t); offset < ptrdata || offset > t.Width {
+ Fatal("dgcprog: %v: offset=%d but ptrdata=%d size=%d", t, offset, ptrdata, t.Width)
+ }
+ return sym, offset
}
-func proggeninit(g *ProgGen, s *Sym) {
- g.s = s
- g.datasize = 0
- g.ot = 0
- g.data = [256 / 8]uint8{}
+type GCProg struct {
+ sym *Sym
+ symoff int
+ w gcprog.Writer
}
-func proggenemit(g *ProgGen, v uint8) {
- g.ot = int64(duint8(g.s, int(g.ot), v))
+var Debug_gcprog int // set by -d gcprog
+
+func (p *GCProg) init(sym *Sym) {
+ p.sym = sym
+ p.symoff = 4 // first 4 bytes hold program length
+ p.w.Init(p.writeByte)
+ if Debug_gcprog > 0 {
+ fmt.Fprintf(os.Stderr, "compile: start GCProg for %v\n", sym)
+ p.w.Debug(os.Stderr)
+ }
}
-// Emits insData block from g->data.
-func proggendataflush(g *ProgGen) {
- if g.datasize == 0 {
+func (p *GCProg) writeByte(x byte) {
+ p.symoff = duint8(p.sym, p.symoff, x)
+}
+
+func (p *GCProg) end() {
+ p.w.End()
+ duint32(p.sym, 0, uint32(p.symoff-4))
+ ggloblsym(p.sym, int32(p.symoff), obj.DUPOK|obj.RODATA|obj.LOCAL)
+ if Debug_gcprog > 0 {
+ fmt.Fprintf(os.Stderr, "compile: end GCProg for %v\n", p.sym)
+ }
+}
+
+func (p *GCProg) emit(t *Type, offset int64) {
+ dowidth(t)
+ if !haspointers(t) {
return
}
- proggenemit(g, obj.InsData)
- proggenemit(g, uint8(g.datasize))
- s := (g.datasize + 7) / 8
- for i := int32(0); i < s; i++ {
- proggenemit(g, g.data[i])
+ if t.Width == int64(Widthptr) {
+ p.w.Ptr(offset / int64(Widthptr))
+ return
}
- g.datasize = 0
- g.data = [256 / 8]uint8{}
-}
-
-func proggendata(g *ProgGen, d uint8) {
- g.data[g.datasize/8] |= d << uint(g.datasize%8)
- g.datasize++
- if g.datasize == 255 {
- proggendataflush(g)
- }
-}
-
-// Skip v bytes due to alignment, etc.
-func proggenskip(g *ProgGen, off int64, v int64) {
- for i := off; i < off+v; i++ {
- if (i % int64(Widthptr)) == 0 {
- proggendata(g, 0)
- }
- }
-}
-
-// Emit insArray instruction.
-func proggenarray(g *ProgGen, len int64) {
- proggendataflush(g)
- proggenemit(g, obj.InsArray)
- for i := int32(0); i < int32(Widthptr); i, len = i+1, len>>8 {
- proggenemit(g, uint8(len))
- }
-}
-
-func proggenarrayend(g *ProgGen) {
- proggendataflush(g)
- proggenemit(g, obj.InsArrayEnd)
-}
-
-func proggenfini(g *ProgGen) int64 {
- proggendataflush(g)
- proggenemit(g, obj.InsEnd)
- return g.ot
-}
-
-// Generates GC program for large types.
-func gengcprog(t *Type, pgc0 **Sym, pgc1 **Sym) {
- nptr := (t.Width + int64(Widthptr) - 1) / int64(Widthptr)
- size := nptr + 1 // unroll flag in the beginning, used by runtime (see runtime.markallocated)
-
- // emity space in BSS for unrolled program
- *pgc0 = nil
-
- // Don't generate it if it's too large, runtime will unroll directly into GC bitmap.
- if size <= obj.MaxGCMask {
- gc0 := typesymprefix(".gc", t)
- ggloblsym(gc0, int32(size), obj.DUPOK|obj.NOPTR)
- *pgc0 = gc0
- }
-
- // program in RODATA
- gc1 := typesymprefix(".gcprog", t)
-
- var g ProgGen
- proggeninit(&g, gc1)
- xoffset := int64(0)
- gengcprog1(&g, t, &xoffset)
- ot := proggenfini(&g)
- ggloblsym(gc1, int32(ot), obj.DUPOK|obj.RODATA)
- *pgc1 = gc1
-}
-
-// Recursively walks type t and writes GC program into g.
-func gengcprog1(g *ProgGen, t *Type, xoffset *int64) {
switch t.Etype {
- case TINT8,
- TUINT8,
- TINT16,
- TUINT16,
- TINT32,
- TUINT32,
- TINT64,
- TUINT64,
- TINT,
- TUINT,
- TUINTPTR,
- TBOOL,
- TFLOAT32,
- TFLOAT64,
- TCOMPLEX64,
- TCOMPLEX128:
- proggenskip(g, *xoffset, t.Width)
- *xoffset += t.Width
-
- case TPTR32,
- TPTR64,
- TUNSAFEPTR,
- TFUNC,
- TCHAN,
- TMAP:
- proggendata(g, 1)
- *xoffset += t.Width
+ default:
+ Fatal("GCProg.emit: unexpected type %v", t)
case TSTRING:
- proggendata(g, 1)
- proggendata(g, 0)
- *xoffset += t.Width
+ p.w.Ptr(offset / int64(Widthptr))
- // Assuming IfacePointerOnly=1.
case TINTER:
- proggendata(g, 1)
- proggendata(g, 1)
- *xoffset += t.Width
+ p.w.Ptr(offset / int64(Widthptr))
+ p.w.Ptr(offset/int64(Widthptr) + 1)
case TARRAY:
if Isslice(t) {
- proggendata(g, 1)
- proggendata(g, 0)
- proggendata(g, 0)
- } else {
- t1 := t.Type
- if t1.Width == 0 {
- }
- // ignore
- if t.Bound <= 1 || t.Bound*t1.Width < int64(32*Widthptr) {
- for i := int64(0); i < t.Bound; i++ {
- gengcprog1(g, t1, xoffset)
- }
- } else if !haspointers(t1) {
- n := t.Width
- n -= -*xoffset & (int64(Widthptr) - 1) // skip to next ptr boundary
- proggenarray(g, (n+int64(Widthptr)-1)/int64(Widthptr))
- proggendata(g, 0)
- proggenarrayend(g)
- *xoffset -= (n+int64(Widthptr)-1)/int64(Widthptr)*int64(Widthptr) - t.Width
- } else {
- proggenarray(g, t.Bound)
- gengcprog1(g, t1, xoffset)
- *xoffset += (t.Bound - 1) * t1.Width
- proggenarrayend(g)
- }
+ p.w.Ptr(offset / int64(Widthptr))
+ return
}
+ if t.Bound == 0 {
+ // should have been handled by haspointers check above
+ Fatal("GCProg.emit: empty array")
+ }
+
+ // Flatten array-of-array-of-array to just a big array by multiplying counts.
+ count := t.Bound
+ elem := t.Type
+ for Isfixedarray(elem) {
+ count *= elem.Bound
+ elem = elem.Type
+ }
+
+ if !p.w.ShouldRepeat(elem.Width/int64(Widthptr), count) {
+ // Cheaper to just emit the bits.
+ for i := int64(0); i < count; i++ {
+ p.emit(elem, offset+i*elem.Width)
+ }
+ return
+ }
+ p.emit(elem, offset)
+ p.w.ZeroUntil((offset + elem.Width) / int64(Widthptr))
+ p.w.Repeat(elem.Width/int64(Widthptr), count-1)
case TSTRUCT:
- o := int64(0)
- var fieldoffset int64
for t1 := t.Type; t1 != nil; t1 = t1.Down {
- fieldoffset = t1.Width
- proggenskip(g, *xoffset, fieldoffset-o)
- *xoffset += fieldoffset - o
- gengcprog1(g, t1.Type, xoffset)
- o = fieldoffset + t1.Type.Width
+ p.emit(t1.Type, offset+t1.Width)
}
-
- proggenskip(g, *xoffset, t.Width-o)
- *xoffset += t.Width - o
-
- default:
- Fatal("gengcprog1: unexpected type, %v", t)
}
}
