target/arm: Fix accidental write to TCG constant

Currently an unpredictable movw such as

  movw pc, 0x123

results in the tinycode

   and_i32 $0x123,$0x123,$0xfffffffc
   mov_i32 pc,$0x123
   exit_tb $0x0

which is clearly a bug: writing to a constant is incorrect and
discards the result of the mask.  Fix this by always doing an and_i32
and trusting the optimizer to turn this into a simple move when the
mask is zero.

Signed-off-by: Anton Johansson <anjo@rev.ng>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Gustavo Romero <gustavo.romero@linaro.org>
Reviewed-by: <gustavo.romero@linaro.org>
Message-id: 20251106144909.533997-1-richard.henderson@linaro.org
[rth: Avoid an extra temp and extra move.]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
[PMM: commit message tweak]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

target/arm/tcg/translate.c

@@ -303,20 +303,23 @@ TCGv_i32 add_reg_for_lit(DisasContext *s, int reg, int ofs)
    marked as dead.  */
 void store_reg(DisasContext *s, int reg, TCGv_i32 var)
 {
+    uint32_t mask = 0;
+
     if (reg == 15) {
-        /* In Thumb mode, we must ignore bit 0.
+        /*
+         * 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);
+        mask = s->thumb ? 1 : 3;
         s->base.is_jmp = DISAS_JUMP;
         s->pc_save = -1;
     } else if (reg == 13 && arm_dc_feature(s, ARM_FEATURE_M)) {
         /* For M-profile SP bits [1:0] are always zero */
-        tcg_gen_andi_i32(var, var, ~3);
+        mask = 3;
     }
-    tcg_gen_mov_i32(cpu_R[reg], var);
+    tcg_gen_andi_i32(cpu_R[reg], var, ~mask);
 }
 
 /*