cmd/9g: don't use R13

R13 is the C TLS pointer.  Once we're calling to and from C code, if
we clobber R13 in our code, sigtramp won't know whether to get the
current g from REGG or from C TLS.  The simplest solution is for Go
code to preserve the C TLS pointer.  This is equivalent to what other
platforms do, except that on other platforms the TLS pointer is in a
special register.

Change-Id: I076e9cb83fd78843eb68cb07c748c4705c9a4c82
Reviewed-on: https://go-review.googlesource.com/2007Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/9g/gsubr.c

@@ -262,8 +262,15 @@ afunclit(Addr *a, Node *n)
 static	int	resvd[] =
 {
 	REGZERO,
-	REGSP,	// reserved for SP, XXX: not reserved in 9c.
-	30,	// for g
+	REGSP,	// reserved for SP
+	// We need to preserve the C ABI TLS pointer because sigtramp
+	// may happen during C code and needs to access the g.  C
+	// clobbers REGG, so if Go were to clobber REGTLS, sigtramp
+	// won't know which convention to use.  By preserving REGTLS,
+	// we can just retrieve g from TLS when we aren't sure.
+	REGTLS,
+	// TODO(austin): Consolidate REGTLS and REGG?
+	REGG,
 	REGTMP,	// REGTMP
 	FREGCVI+NREG,
 	FREGZERO+NREG,

src/cmd/9g/reg.c

@@ -185,7 +185,7 @@ regopt(Prog *firstp)
 	}
 
 	// Exclude registers with fixed functions
-	regbits = (1<<D_R0)|RtoB(REGSP)|RtoB(REGG);
+	regbits = (1<<D_R0)|RtoB(REGSP)|RtoB(REGG)|RtoB(REGTLS);
 	// Also exclude floating point registers with fixed constants
 	regbits |= FtoB(D_F0+27)|FtoB(D_F0+28)|FtoB(D_F0+29)|FtoB(D_F0+30)|FtoB(D_F0+31);
 	externs = zbits;

src/cmd/9l/9.out.h

@@ -48,6 +48,7 @@ enum
 	REGRT2		= 4,	/* reserved for runtime, duffcopy */
 	REGMIN		= 7,	/* register variables allocated from here to REGMAX */
 	REGENV		= 11,	/* environment for closures */
+	REGTLS		= 13,	/* C ABI TLS base pointer */
 	REGMAX		= 27,
 	REGEXT		= 30,	/* external registers allocated from here down */
 	REGG		= 30,	/* G */