target/arm: Add v8M stack checks for Thumb2 LDM/STM

Add the v8M stack checks for:
 * LDM (T2 encoding)
 * STM (T2 encoding)

This includes the 32-bit encodings of the instructions listed
in v8M ARM ARM rule R_YVWT as
 * LDM, LDMIA, LDMFD
 * LDMDB, LDMEA
 * POP (multiple registers)
 * PUSH (muliple registers)
 * STM, STMIA, STMEA
 * STMDB, STMFD

We perform the stack limit before doing any other part
of the load or store.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20181002163556.10279-10-peter.maydell@linaro.org

target/arm/translate.c

@@ -10524,17 +10524,34 @@ static void disas_thumb2_insn(DisasContext *s, uint32_t insn)
             } else {
                 int i, loaded_base = 0;
                 TCGv_i32 loaded_var;
+                bool wback = extract32(insn, 21, 1);
                 /* Load/store multiple.  */
                 addr = load_reg(s, rn);
                 offset = 0;
                 for (i = 0; i < 16; i++) {
                     if (insn & (1 << i))
                         offset += 4;
                 }
+
                 if (insn & (1 << 24)) {
                     tcg_gen_addi_i32(addr, addr, -offset);
                 }
 
+                if (s->v8m_stackcheck && rn == 13 && wback) {
+                    /*
+                     * If the writeback is incrementing SP rather than
+                     * decrementing it, and the initial SP is below the
+                     * stack limit but the final written-back SP would
+                     * be above, then then we must not perform any memory
+                     * accesses, but it is IMPDEF whether we generate
+                     * an exception. We choose to do so in this case.
+                     * At this point 'addr' is the lowest address, so
+                     * either the original SP (if incrementing) or our
+                     * final SP (if decrementing), so that's what we check.
+                     */
+                    gen_helper_v8m_stackcheck(cpu_env, addr);
+                }
+
                 loaded_var = NULL;
                 for (i = 0; i < 16; i++) {
                     if ((insn & (1 << i)) == 0)
@@ -10562,7 +10579,7 @@ static void disas_thumb2_insn(DisasContext *s, uint32_t insn)
                 if (loaded_base) {
                     store_reg(s, rn, loaded_var);
                 }
-                if (insn & (1 << 21)) {
+                if (wback) {
                     /* Base register writeback.  */
                     if (insn & (1 << 24)) {
                         tcg_gen_addi_i32(addr, addr, -offset);