cmd/link: implement and test automatic symbols

Related changes included in this CL:

 - Add explicit start symbol to Prog.
 - Add omitRuntime bool to Prog.
 - Introduce p.Packages[""] to hold automatic symbols
 - Add SymOrder to Prog to preserve symbol order.
 - Add layout test (and fix bug that was putting everything in text section).

R=iant
CC=golang-codereviews
https://golang.org/cl/51260045

6 files changed, 119 insertions, 0 deletions

diff --git a/src/cmd/link/testdata/autosection.6 b/src/cmd/link/testdata/autosection.6
new file mode 100644
index 0000000000..62619a7ea3
--- /dev/null
+++ b/src/cmd/link/testdata/autosection.6
diff --git a/src/cmd/link/testdata/autosection.s b/src/cmd/link/testdata/autosection.s
new file mode 100644
index 0000000000..8a579b7027
--- /dev/null
+++ b/src/cmd/link/testdata/autosection.s
@@ -0,0 +1,60 @@
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test of section-named symbols.
+
+#include "../../ld/textflag.h"
+
+TEXT start(SB),7,$0
+	MOVQ $autotab(SB),AX
+	MOVQ $autoptr(SB),AX
+	RET
+
+GLOBL zero(SB), $8
+
+GLOBL zeronoptr(SB), NOPTR, $16
+
+// text
+DATA autotab+0x00(SB)/8, $text(SB)
+DATA autotab+0x08(SB)/8, $start(SB)
+DATA autotab+0x10(SB)/8, $etext(SB)
+DATA autotab+0x18(SB)/8, $start+16(SB)
+
+// data
+DATA autotab+0x20(SB)/8, $data(SB)
+DATA autotab+0x28(SB)/8, $autotab(SB)
+DATA autotab+0x30(SB)/8, $edata(SB)
+DATA autotab+0x38(SB)/8, $nonzero+4(SB)
+
+// bss
+DATA autotab+0x40(SB)/8, $bss(SB)
+DATA autotab+0x48(SB)/8, $zero(SB)
+DATA autotab+0x50(SB)/8, $ebss(SB)
+DATA autotab+0x58(SB)/8, $zero+8(SB)
+
+// noptrdata
+DATA autotab+0x60(SB)/8, $noptrdata(SB)
+DATA autotab+0x68(SB)/8, $nonzeronoptr(SB)
+DATA autotab+0x70(SB)/8, $enoptrdata(SB)
+DATA autotab+0x78(SB)/8, $nonzeronoptr+8(SB)
+
+// noptrbss
+DATA autotab+0x80(SB)/8, $noptrbss(SB)
+DATA autotab+0x88(SB)/8, $zeronoptr(SB)
+DATA autotab+0x90(SB)/8, $enoptrbss(SB)
+DATA autotab+0x98(SB)/8, $zeronoptr+16(SB)
+
+// end
+DATA autotab+0xa0(SB)/8, $end(SB)
+DATA autotab+0xa8(SB)/8, $zeronoptr+16(SB)
+
+GLOBL autotab(SB), $0xb0
+
+DATA nonzero(SB)/4, $1
+GLOBL nonzero(SB), $4
+
+DATA nonzeronoptr(SB)/8, $2
+GLOBL nonzeronoptr(SB), NOPTR, $8
+
+GLOBL autoptr(SB), $0
diff --git a/src/cmd/link/testdata/autoweak.6 b/src/cmd/link/testdata/autoweak.6
new file mode 100644
index 0000000000..f7e9e69713
--- /dev/null
+++ b/src/cmd/link/testdata/autoweak.6
diff --git a/src/cmd/link/testdata/autoweak.s b/src/cmd/link/testdata/autoweak.s
new file mode 100644
index 0000000000..2d11330db2
--- /dev/null
+++ b/src/cmd/link/testdata/autoweak.s
@@ -0,0 +1,30 @@
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test of go.weak symbols.
+
+TEXT start(SB),7,$0
+	MOVQ $autotab(SB),AX
+	MOVQ $autoptr(SB),AX
+	RET
+
+// go.weak.sym should resolve to sym, because sym is in the binary.
+DATA autotab+0(SB)/8, $go·weak·sym(SB)
+DATA autotab+8(SB)/8, $sym(SB)
+
+// go.weak.missingsym should resolve to 0, because missingsym is not in the binary.
+DATA autotab+16(SB)/8, $go·weak·missingsym(SB)
+DATA autotab+24(SB)/8, $0
+
+// go.weak.deadsym should resolve to 0, because deadsym is discarded during dead code removal
+DATA autotab+32(SB)/8, $go·weak·deadsym(SB)
+DATA autotab+40(SB)/8, $0
+
+GLOBL autotab(SB), $48
+
+GLOBL sym(SB), $1
+
+GLOBL deadsym(SB), $1
+
+GLOBL autoptr(SB), $0
diff --git a/src/cmd/link/testdata/layout.6 b/src/cmd/link/testdata/layout.6
new file mode 100644
index 0000000000..b19491efc5
--- /dev/null
+++ b/src/cmd/link/testdata/layout.6
diff --git a/src/cmd/link/testdata/layout.s b/src/cmd/link/testdata/layout.s
new file mode 100644
index 0000000000..0d492c5af2
--- /dev/null
+++ b/src/cmd/link/testdata/layout.s
@@ -0,0 +1,29 @@
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test of section assignment in layout.go.
+// Each symbol should end up in the section named by the symbol name prefix (up to the underscore).
+
+#include "../../ld/textflag.h"
+
+TEXT text_start(SB),7,$0
+	MOVQ $rodata_sym(SB), AX
+	MOVQ $noptrdata_sym(SB), AX
+	MOVQ $data_sym(SB), AX
+	MOVQ $bss_sym(SB), AX
+	MOVQ $noptrbss_sym(SB), AX
+	RET
+
+DATA rodata_sym(SB)/4, $1
+GLOBL rodata_sym(SB), RODATA, $4
+
+DATA noptrdata_sym(SB)/4, $1
+GLOBL noptrdata_sym(SB), NOPTR, $4
+
+DATA data_sym(SB)/4, $1
+GLOBL data_sym(SB), $4
+
+GLOBL bss_sym(SB), $4
+
+GLOBL noptrbss_sym(SB), NOPTR, $4