tile: avoid undefined behavior with regs[TREG_TP] etc

Recent compilers warn about accesses to arrays past the end,
which is supported for pt_regs and sigcontext with the
intended alias past the end of regs[] to sp, tp, and lr
using TREG_SP, TREG_TP, and TREG_LR.

Make the intended usage explict by providing an anonymous
union of regs[56] on the one hand, and a short __regs[53]
on the other followed by the sp, tp, and lr fields, so the
aliasing is done explicitly and is visible to the compiler.
Reviewed-by: Jeff Epler <jepler@unpythonic.net>
Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>

arch/tile/include/uapi/asm/ptrace.h

@@ -52,12 +52,16 @@ typedef uint_reg_t pt_reg_t;
  * system call or exception.  "struct sigcontext" has the same shape.
  */
 struct pt_regs {
-	/* Saved main processor registers; 56..63 are special. */
-	/* tp, sp, and lr must immediately follow regs[] for aliasing. */
-	pt_reg_t regs[53];
-	pt_reg_t tp;		/* aliases regs[TREG_TP] */
-	pt_reg_t sp;		/* aliases regs[TREG_SP] */
-	pt_reg_t lr;		/* aliases regs[TREG_LR] */
+	union {
+		/* Saved main processor registers; 56..63 are special. */
+		pt_reg_t regs[56];
+		struct {
+			pt_reg_t __regs[53];
+			pt_reg_t tp;		/* aliases regs[TREG_TP] */
+			pt_reg_t sp;		/* aliases regs[TREG_SP] */
+			pt_reg_t lr;		/* aliases regs[TREG_LR] */
+		};
+	};
 
 	/* Saved special registers. */
 	pt_reg_t pc;		/* stored in EX_CONTEXT_K_0 */

arch/tile/include/uapi/asm/sigcontext.h

@@ -24,10 +24,16 @@
  * but is simplified since we know the fault is from userspace.
  */
 struct sigcontext {
-	__uint_reg_t gregs[53];	/* General-purpose registers.  */
-	__uint_reg_t tp;	/* Aliases gregs[TREG_TP].  */
-	__uint_reg_t sp;	/* Aliases gregs[TREG_SP].  */
-	__uint_reg_t lr;	/* Aliases gregs[TREG_LR].  */
+	__extension__ union {
+		/* General-purpose registers.  */
+		__uint_reg_t gregs[56];
+		__extension__ struct {
+			__uint_reg_t __gregs[53];
+			__uint_reg_t tp;	/* Aliases gregs[TREG_TP].  */
+			__uint_reg_t sp;	/* Aliases gregs[TREG_SP].  */
+			__uint_reg_t lr;	/* Aliases gregs[TREG_LR].  */
+		};
+	};
 	__uint_reg_t pc;	/* Program counter.  */
 	__uint_reg_t ics;	/* In Interrupt Critical Section?  */
 	__uint_reg_t faultnum;	/* Fault number.  */