liblink, cmd/5a, cmd/5l: restore cgo on older ARM processors

CL 56120043 fixed TLS handling on ARM after the introduction of
liblink but left older ARM processors broken.

Before liblink, the MRC instruction was replaced with a fallback
on older ARMs. CL 56120043 removed that, because the rewrite matched
bit patterns on the AWORD pseudo-instruction and could therefore change
unrelated AWORDs that happened to match.

This CL adds an AMRC instruction to encode both MRC and MCR previously
encoded as AWORDs. Then, in liblink, the AMRC instructions are either
rewritten to AWORD, or, on goarm < 7, replaced with a branch to the
fallback.

./all.bash completes successfully on an ARMv7 with either GOARM=7 or
GOARM=5. I have verified that the fallback is indeed present in both
runtime.save_gm and runtime.load_gm when GOARM=5 but not when GOARM=7.

If all goes well, this should fix the armv5 builders.

LGTM=iant
R=iant, rsc
CC=golang-codereviews
https://golang.org/cl/55540044

src/cmd/5a/a.y

@@ -294,7 +294,7 @@ inst:
 			(($11 & 15) << 0) |	/* Crm */
 			(($12 & 7) << 5) |	/* coprocessor information */
 			(1<<4);			/* must be set */
-		outcode(AWORD, Always, &nullgen, NREG, &g);
+		outcode(AMRC, Always, &nullgen, NREG, &g);
 	}
 /*
  * MULL r1,r2,(hi,lo)

src/cmd/5a/y.tab.h

-/* A Bison parser, made by GNU Bison 2.3.  */
+/* A Bison parser, made by GNU Bison 2.7.12-4996.  */
 
-/* Skeleton interface for Bison's Yacc-like parsers in C
+/* Bison interface for Yacc-like parsers in C
    
-   Copyright (C) 1984, 1989, 1990, 2000, 2001, 2002, 2003, 2004, 2005, 2006
-   Free Software Foundation, Inc.
+      Copyright (C) 1984, 1989-1990, 2000-2013 Free Software Foundation, Inc.
    
-   This program is free software; you can redistribute it and/or modify
+   This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
-   the Free Software Foundation; either version 2, or (at your option)
-   any later version.
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
    
    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ -16,9 +15,7 @@
    GNU General Public License for more details.
    
    You should have received a copy of the GNU General Public License
-   along with this program; if not, write to the Free Software
-   Foundation, Inc., 51 Franklin Street, Fifth Floor,
-   Boston, MA 02110-1301, USA.  */
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
 /* As a special exception, you may create a larger work that contains
    part or all of the Bison parser skeleton and distribute that work
@@ -33,6 +30,16 @@
    This special exception was added by the Free Software Foundation in
    version 2.2 of Bison.  */
 
+#ifndef YY_YY_Y_TAB_H_INCLUDED
+# define YY_YY_Y_TAB_H_INCLUDED
+/* Enabling traces.  */
+#ifndef YYDEBUG
+# define YYDEBUG 0
+#endif
+#if YYDEBUG
+extern int yydebug;
+#endif
+
 /* Tokens.  */
 #ifndef YYTOKENTYPE
 # define YYTOKENTYPE
@@ -141,24 +148,41 @@
 
 
 
-
 #if ! defined YYSTYPE && ! defined YYSTYPE_IS_DECLARED
 typedef union YYSTYPE
-#line 39 "a.y"
 {
+/* Line 2053 of yacc.c  */
+#line 39 "a.y"
+
 	Sym	*sym;
 	int32	lval;
 	double	dval;
 	char	sval[8];
 	Addr	addr;
-}
-/* Line 1529 of yacc.c.  */
-#line 157 "y.tab.h"
-	YYSTYPE;
+
+
+/* Line 2053 of yacc.c  */
+#line 166 "y.tab.h"
+} YYSTYPE;
+# define YYSTYPE_IS_TRIVIAL 1
 # define yystype YYSTYPE /* obsolescent; will be withdrawn */
 # define YYSTYPE_IS_DECLARED 1
-# define YYSTYPE_IS_TRIVIAL 1
 #endif
 
 extern YYSTYPE yylval;
 
+#ifdef YYPARSE_PARAM
+#if defined __STDC__ || defined __cplusplus
+int yyparse (void *YYPARSE_PARAM);
+#else
+int yyparse ();
+#endif
+#else /* ! YYPARSE_PARAM */
+#if defined __STDC__ || defined __cplusplus
+int yyparse (void);
+#else
+int yyparse ();
+#endif
+#endif /* ! YYPARSE_PARAM */
+
+#endif /* !YY_YY_Y_TAB_H_INCLUDED  */

src/cmd/5l/5.out.h

@@ -200,6 +200,8 @@ enum	as
 	ACHECKNIL,
 	AFATVARDEF,
 
+	AMRC, // MRC/MCR
+
 	ALAST,
 };
 

src/liblink/obj5.c

@@ -92,6 +92,7 @@ progedit(Link *ctxt, Prog *p)
 {
 	char literal[64];
 	LSym *s;
+	LSym *tlsfallback;
 
 	p->from.class = 0;
 	p->to.class = 0;
@@ -105,6 +106,25 @@ progedit(Link *ctxt, Prog *p)
 		break;
 	}
 
+	// Replace TLS register fetches on older ARM procesors.
+	switch(p->as) {
+	case AMRC:
+		// If the instruction matches MRC 15, 0, <reg>, C13, C0, 3, replace it.
+		if(ctxt->goarm < 7 && (p->to.offset & 0xffff0fff) == 0xee1d0f70) {
+			tlsfallback = linklookup(ctxt, "runtime.read_tls_fallback", 0);
+
+			// BL runtime.read_tls_fallback(SB)
+			p->as = ABL;
+			p->to.type = D_BRANCH;
+			p->to.sym = tlsfallback;
+			p->to.offset = 0;
+		} else {
+			// Otherwise, MRC/MCR instructions need no further treatment.
+			p->as = AWORD;
+		}
+		break;
+	}
+
 	// Rewrite float constants to values stored in memory.
 	switch(p->as) {
 	case AMOVF: