cmd/link, runtime: use offset for _func.entry

The first field of the func data stored by the linker is the
entry PC for the function. Prior to this change, this was stored
as a relocation to the function. Change this to be an offset
relative to runtime.text.

This reduces the number of relocations on darwin/arm64 by about 10%.
It also slightly shrinks binaries:

file      before    after     Δ       %
addr2line 3803058   3791298   -11760  -0.309%
api       5140114   5104242   -35872  -0.698%
asm       4886850   4840626   -46224  -0.946%
buildid   2512466   2503042   -9424   -0.375%
cgo       4374770   4342274   -32496  -0.743%
compile   22920530  22769202  -151328 -0.660%
cover     4624626   4588242   -36384  -0.787%
dist      3217570   3205522   -12048  -0.374%
doc       3715026   3684498   -30528  -0.822%
fix       3148226   3119266   -28960  -0.920%
link      6350226   6313362   -36864  -0.581%
nm        3768850   3757106   -11744  -0.312%
objdump   4140594   4127618   -12976  -0.313%
pack      2227474   2218818   -8656   -0.389%
pprof     13598706  13506786  -91920  -0.676%
test2json 2497234   2487426   -9808   -0.393%
trace     10198066  10118498  -79568  -0.780%
vet       6930658   6889074   -41584  -0.600%
total     108055044 107366900 -688144 -0.637%

It should also incrementally speed up binary launching.

This is the first step towards removing enough relocations
that pages that were previously dirtied by the loader may remain clean,
which will offer memory savings useful in constrained environments.

Change-Id: Icfba55e696ba2f9c99c4f179125ba5a3ba4369c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/351463
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

1 files changed, 38 insertions, 5 deletions

diff --git a/src/debug/gosym/pclntab.go b/src/debug/gosym/pclntab.go
index bf97976b3c..fdaa47a917 100644
--- a/src/debug/gosym/pclntab.go
+++ b/src/debug/gosym/pclntab.go
@@ -22,6 +22,7 @@ const (
 	ver11
 	ver12
 	ver116
+	ver118
 )
 
 // A LineTable is a data structure mapping program counters to line numbers.
@@ -48,10 +49,11 @@ type LineTable struct {
 	// Contains the version of the pclntab section.
 	version version
 
-	// Go 1.2/1.16 state
+	// Go 1.2/1.16/1.18 state
 	binary      binary.ByteOrder
 	quantum     uint32
 	ptrsize     uint32
+	textStart   uintptr // address of runtime.text symbol (1.18+)
 	funcnametab []byte
 	cutab       []byte
 	funcdata    []byte
@@ -166,8 +168,11 @@ func (t *LineTable) isGo12() bool {
 	return t.version >= ver12
 }
 
-const go12magic = 0xfffffffb
-const go116magic = 0xfffffffa
+const (
+	go12magic  = 0xfffffffb
+	go116magic = 0xfffffffa
+	go118magic = 0xfffffff0
+)
 
 // uintptr returns the pointer-sized value encoded at b.
 // The pointer size is dictated by the table being read.
@@ -219,11 +224,15 @@ func (t *LineTable) parsePclnTab() {
 		t.binary, possibleVersion = binary.LittleEndian, ver116
 	case beMagic == go116magic:
 		t.binary, possibleVersion = binary.BigEndian, ver116
+	case leMagic == go118magic:
+		t.binary, possibleVersion = binary.LittleEndian, ver118
+	case beMagic == go118magic:
+		t.binary, possibleVersion = binary.BigEndian, ver118
 	default:
 		return
 	}
 
-	// quantum and ptrSize are the same between 1.2 and 1.16
+	// quantum and ptrSize are the same between 1.2, 1.16, and 1.18
 	t.quantum = uint32(t.Data[6])
 	t.ptrsize = uint32(t.Data[7])
 
@@ -235,6 +244,18 @@ func (t *LineTable) parsePclnTab() {
 	}
 
 	switch possibleVersion {
+	case ver118:
+		t.nfunctab = uint32(offset(0))
+		t.nfiletab = uint32(offset(1))
+		t.textStart = uintptr(offset(2))
+		t.funcnametab = data(3)
+		t.cutab = data(4)
+		t.filetab = data(5)
+		t.pctab = data(6)
+		t.funcdata = data(7)
+		t.functab = data(7)
+		functabsize := t.nfunctab*2*t.ptrsize + t.ptrsize
+		t.functab = t.functab[:functabsize]
 	case ver116:
 		t.nfunctab = uint32(offset(0))
 		t.nfiletab = uint32(offset(1))
@@ -380,7 +401,14 @@ func (f funcData) IsZero() bool {
 }
 
 // entryPC returns the func's entry PC.
-func (f funcData) entryPC() uint64 {
+func (f *funcData) entryPC() uint64 {
+	// In Go 1.18, the first field of _func changed
+	// from a uintptr entry PC to a uint32 entry offset.
+	if f.t.version >= ver118 {
+		// TODO: support multiple text sections.
+		// See runtime/symtab.go:(*moduledata).textAddr.
+		return uint64(f.t.binary.Uint32(f.data)) + uint64(f.t.textStart)
+	}
 	return f.t.uintptr(f.data)
 }
 
@@ -397,7 +425,12 @@ func (f funcData) field(n uint32) uint32 {
 	if n == 0 || n > 9 {
 		panic("bad funcdata field")
 	}
+	// In Go 1.18, the first field of _func changed
+	// from a uintptr entry PC to a uint32 entry offset.
 	sz0 := f.t.ptrsize
+	if f.t.version >= ver118 {
+		sz0 = 4
+	}
 	off := sz0 + (n-1)*4 // subsequent fields are 4 bytes each
 	data := f.data[off:]
 	return f.t.binary.Uint32(data)