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https://github.com/golang/go
synced 2024-11-18 04:24:47 -07:00
cmd/link: write DWARF PC table without seeking
This per-symbol table was written with the strategy: 1. record offset and write fake header 2. write body 3. seek back to fake header 4. write real header This CL collects the per-symbol body into a []byte, then writes the real header followed by the body to the output file. This saves two seeks per-symbol and overwriting the fake header. Small performance improvement (3.5%) in best-of-ten links of godoc: tip: real 0m1.132s user 0m1.256s this: real 0m1.090s user 0m1.210s I'm not sure if the performance measured here alone justifies it, but I think this is an easier to read style of code. Change-Id: I1663901eb7c2ee330591b8b6550cdff0402ed5dc Reviewed-on: https://go-review.googlesource.com/20074 Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org>
This commit is contained in:
David Crawshaw
parent
021e0c5f1f
commit
3601a1e77d
@ -111,64 +111,47 @@ func addrput(addr int64) {
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}
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}
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func uleb128enc(v uint64, dst []byte) int {
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var c uint8
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length := uint8(0)
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func appendUleb128(b []byte, v uint64) []byte {
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for {
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c = uint8(v & 0x7f)
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c := uint8(v & 0x7f)
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v >>= 7
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if v != 0 {
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c |= 0x80
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}
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if dst != nil {
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dst[0] = byte(c)
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dst = dst[1:]
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}
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length++
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b = append(b, c)
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if c&0x80 == 0 {
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break
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}
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}
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return int(length)
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return b
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}
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func sleb128enc(v int64, dst []byte) int {
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var c uint8
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var s uint8
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length := uint8(0)
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func appendSleb128(b []byte, v int64) []byte {
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for {
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c = uint8(v & 0x7f)
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s = uint8(v & 0x40)
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c := uint8(v & 0x7f)
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s := uint8(v & 0x40)
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v >>= 7
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if (v != -1 || s == 0) && (v != 0 || s != 0) {
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c |= 0x80
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}
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if dst != nil {
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dst[0] = byte(c)
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dst = dst[1:]
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}
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length++
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b = append(b, c)
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if c&0x80 == 0 {
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break
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}
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}
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return int(length)
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return b
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}
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var encbuf [10]byte
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func uleb128put(v int64) {
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n := uleb128enc(uint64(v), encbuf[:])
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Cwrite(encbuf[:n])
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b := appendUleb128(encbuf[:0], uint64(v))
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Cwrite(b)
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}
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func sleb128put(v int64) {
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n := sleb128enc(v, encbuf[:])
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Cwrite(encbuf[:n])
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b := appendSleb128(encbuf[:0], v)
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Cwrite(b)
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}
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/*
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@ -892,12 +875,9 @@ func reversetree(list **DWDie) {
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func newmemberoffsetattr(die *DWDie, offs int32) {
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var block [20]byte
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i := 0
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block[i] = DW_OP_plus_uconst
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i++
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i += uleb128enc(uint64(offs), block[i:])
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newattr(die, DW_AT_data_member_location, DW_CLS_BLOCK, int64(i), block[:i])
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b := append(block[:0], DW_OP_plus_uconst)
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b = appendUleb128(b, uint64(offs))
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newattr(die, DW_AT_data_member_location, DW_CLS_BLOCK, int64(len(b)), b)
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}
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// GDB doesn't like DW_FORM_addr for DW_AT_location, so emit a
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@ -1462,20 +1442,15 @@ func putpclcdelta(delta_pc int64, delta_lc int64) {
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func newcfaoffsetattr(die *DWDie, offs int32) {
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var block [20]byte
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b := append(block[:0], DW_OP_call_frame_cfa)
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i := 0
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block[i] = DW_OP_call_frame_cfa
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i++
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if offs != 0 {
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block[i] = DW_OP_consts
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i++
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i += sleb128enc(int64(offs), block[i:])
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block[i] = DW_OP_plus
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i++
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b = append(b, DW_OP_consts)
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b = appendSleb128(b, int64(offs))
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b = append(b, DW_OP_plus)
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}
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newattr(die, DW_AT_location, DW_CLS_BLOCK, int64(i), block[:i])
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newattr(die, DW_AT_location, DW_CLS_BLOCK, int64(len(b)), b)
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}
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func mkvarname(name string, da int) string {
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@ -1719,22 +1694,25 @@ const (
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DATAALIGNMENTFACTOR = -4
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)
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func putpccfadelta(deltapc int64, cfa int64) {
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Cput(DW_CFA_def_cfa_offset_sf)
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sleb128put(cfa / DATAALIGNMENTFACTOR)
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// appendPCDeltaCFA appends per-PC CFA deltas to b and returns the final slice.
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func appendPCDeltaCFA(b []byte, deltapc, cfa int64) []byte {
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b = append(b, DW_CFA_def_cfa_offset_sf)
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b = appendSleb128(b, cfa/DATAALIGNMENTFACTOR)
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if deltapc < 0x40 {
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Cput(uint8(DW_CFA_advance_loc + deltapc))
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} else if deltapc < 0x100 {
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Cput(DW_CFA_advance_loc1)
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Cput(uint8(deltapc))
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} else if deltapc < 0x10000 {
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Cput(DW_CFA_advance_loc2)
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Thearch.Wput(uint16(deltapc))
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} else {
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Cput(DW_CFA_advance_loc4)
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Thearch.Lput(uint32(deltapc))
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switch {
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case deltapc < 0x40:
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b = append(b, uint8(DW_CFA_advance_loc+deltapc))
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case deltapc < 0x100:
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b = append(b, DW_CFA_advance_loc1)
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b = append(b, uint8(deltapc))
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case deltapc < 0x10000:
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b = append(b, DW_CFA_advance_loc2)
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b = Thearch.Append16(b, uint16(deltapc))
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default:
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b = append(b, DW_CFA_advance_loc4)
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b = Thearch.Append32(b, uint32(deltapc))
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}
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return b
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}
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func writeframes() {
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@ -1779,6 +1757,7 @@ func writeframes() {
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strnput("", int(pad))
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var deltaBuf []byte
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var pcsp Pciter
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for Ctxt.Cursym = Ctxt.Textp; Ctxt.Cursym != nil; Ctxt.Cursym = Ctxt.Cursym.Next {
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s := Ctxt.Cursym
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@ -1786,14 +1765,9 @@ func writeframes() {
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continue
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}
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fdeo := Cpos()
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// Emit a FDE, Section 6.4.1, starting wit a placeholder.
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Thearch.Lput(0) // length, must be multiple of thearch.ptrsize
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Thearch.Lput(0) // Pointer to the CIE above, at offset 0
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addrput(0) // initial location
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addrput(0) // address range
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// Emit a FDE, Section 6.4.1.
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// First build the section contents into a byte buffer.
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deltaBuf = deltaBuf[:0]
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for pciterinit(Ctxt, &pcsp, &s.Pcln.Pcsp); pcsp.done == 0; pciternext(&pcsp) {
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nextpc := pcsp.nextpc
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@ -1807,31 +1781,30 @@ func writeframes() {
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}
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if haslinkregister() {
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putpccfadelta(int64(nextpc)-int64(pcsp.pc), int64(pcsp.value))
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deltaBuf = appendPCDeltaCFA(deltaBuf, int64(nextpc)-int64(pcsp.pc), int64(pcsp.value))
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} else {
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putpccfadelta(int64(nextpc)-int64(pcsp.pc), int64(Thearch.Ptrsize)+int64(pcsp.value))
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deltaBuf = appendPCDeltaCFA(deltaBuf, int64(nextpc)-int64(pcsp.pc), int64(Thearch.Ptrsize)+int64(pcsp.value))
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}
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}
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pad := int(Rnd(int64(len(deltaBuf)), int64(Thearch.Ptrsize))) - len(deltaBuf)
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deltaBuf = append(deltaBuf, zeros[:pad]...)
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fdesize := Cpos() - fdeo - 4 // exclude the length field.
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pad = Rnd(fdesize, int64(Thearch.Ptrsize)) - fdesize
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strnput("", int(pad))
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fdesize += pad
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// Emit the FDE header for real, Section 6.4.1.
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Cseek(fdeo)
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Thearch.Lput(uint32(fdesize))
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// Emit the FDE header, Section 6.4.1.
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// 4 bytes: length, must be multiple of thearch.ptrsize
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// 4 bytes: Pointer to the CIE above, at offset 0
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// ptrsize: initial location
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// ptrsize: address range
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Thearch.Lput(uint32(4 + 2*Thearch.Ptrsize + len(deltaBuf))) // length (excludes itself)
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if Linkmode == LinkExternal {
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adddwarfrel(framesec, framesym, frameo, 4, 0)
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adddwarfrel(framesec, s, frameo, Thearch.Ptrsize, 0)
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adddwarfrel(framesec, framesym, frameo, 4, 0) // CIE offset
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adddwarfrel(framesec, s, frameo, Thearch.Ptrsize, 0) // initial location
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} else {
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Thearch.Lput(0)
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addrput(s.Value)
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Thearch.Lput(0) // CIE offset
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addrput(s.Value) // initial location
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}
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addrput(s.Size) // address range
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addrput(s.Size)
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Cseek(fdeo + 4 + fdesize)
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Cwrite(deltaBuf)
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}
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Cflush()
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