target-arm: Correctly handle 'sub pc, pc, 1' for ARMv6

In the ARM v6 architecture, 'sub pc, pc, 1' is not an interworking
branch, so the computed new value is written to r15 as a normal
value. The architecture says that in this case, bits [1:0] of
the value written must be ignored if we are in ARM mode (or
bit [0] ignored if in Thumb mode); this is a change from the
ARMv4/v5 specification that behaviour is UNPREDICTABLE.
Use the correct mask on the PC value when doing a non-interworking
store to PC.

A popular library used on RaspberryPi uses this instruction
as part of a trick to determine whether it is running on
ARMv6 or ARMv7, and we were mishandling the sequence.

Fixes bug: https://bugs.launchpad.net/bugs/1625295

Reported-by: <stu.axon@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1474380941-4730-1-git-send-email-peter.maydell@linaro.org

target-arm/translate.c

@@ -180,7 +180,12 @@ static inline TCGv_i32 load_reg(DisasContext *s, int reg)
 static void store_reg(DisasContext *s, int reg, TCGv_i32 var)
 {
     if (reg == 15) {
-        tcg_gen_andi_i32(var, var, ~1);
+        /* In Thumb mode, we must ignore bit 0.
+         * In ARM mode, for ARMv4 and ARMv5, it is UNPREDICTABLE if bits [1:0]
+         * are not 0b00, but for ARMv6 and above, we must ignore bits [1:0].
+         * We choose to ignore [1:0] in ARM mode for all architecture versions.
+         */
+        tcg_gen_andi_i32(var, var, s->thumb ? ~1 : ~3);
         s->is_jmp = DISAS_JUMP;
     }
     tcg_gen_mov_i32(cpu_R[reg], var);