target/arm: Implement MVE LETP insn

Implement the MVE LETP insn.  This is like the existing LE loop-end
insn, but it must perform an FPU-enabled check, and on loop-exit it
resets LTPSIZE to 4.

To accommodate the requirement to do something on loop-exit, we drop
the use of condlabel and instead manage both the TB exits manually,
in the same way we already do in trans_WLS().

The other MVE-specific change to the LE insn is that we must raise an
INVSTATE UsageFault insn if LTPSIZE is not 4.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210614151007.4545-10-peter.maydell@linaro.org

target/arm/t32.decode

@@ -674,7 +674,7 @@ BL               1111 0. .......... 11.1 ............         @branch24
     DLS          1111 0 0000 100     rn:4 1110 0000 0000 0001 size=4
     WLS          1111 0 0000 100     rn:4 1100 . .......... 1 imm=%lob_imm size=4
     {
-      LE         1111 0 0000 0 f:1 0 1111 1100 . .......... 1 imm=%lob_imm
+      LE         1111 0 0000 0 f:1 tp:1 1111 1100 . .......... 1 imm=%lob_imm
       # This is WLSTP
       WLS        1111 0 0000 0 size:2 rn:4 1100 . .......... 1 imm=%lob_imm
     }

target/arm/translate.c

@@ -8223,25 +8223,113 @@ static bool trans_LE(DisasContext *s, arg_LE *a)
      * any faster.
      */
     TCGv_i32 tmp;
+    TCGLabel *loopend;
+    bool fpu_active;
 
     if (!dc_isar_feature(aa32_lob, s)) {
         return false;
     }
+    if (a->f && a->tp) {
+        return false;
+    }
+    if (s->condexec_mask) {
+        /*
+         * LE in an IT block is CONSTRAINED UNPREDICTABLE;
+         * we choose to UNDEF, because otherwise our use of
+         * gen_goto_tb(1) would clash with the use of TB exit 1
+         * in the dc->condjmp condition-failed codepath in
+         * arm_tr_tb_stop() and we'd get an assertion.
+         */
+        return false;
+    }
+    if (a->tp) {
+        /* LETP */
+        if (!dc_isar_feature(aa32_mve, s)) {
+            return false;
+        }
+        if (!vfp_access_check(s)) {
+            s->eci_handled = true;
+            return true;
+        }
+    }
 
     /* LE/LETP is OK with ECI set and leaves it untouched */
     s->eci_handled = true;
 
-    if (!a->f) {
+    /*
-        /* Not loop-forever. If LR <= 1 this is the last loop: do nothing. */
+     * With MVE, LTPSIZE might not be 4, and we must emit an INVSTATE
-        arm_gen_condlabel(s);
+     * UsageFault exception for the LE insn in that case. Note that we
-        tcg_gen_brcondi_i32(TCG_COND_LEU, cpu_R[14], 1, s->condlabel);
+     * are not directly checking FPSCR.LTPSIZE but instead check the
-        /* Decrement LR */
+     * pseudocode LTPSIZE() function, which returns 4 if the FPU is
-        tmp = load_reg(s, 14);
+     * not currently active (ie ActiveFPState() returns false). We
-        tcg_gen_addi_i32(tmp, tmp, -1);
+     * can identify not-active purely from our TB state flags, as the
-        store_reg(s, 14, tmp);
+     * FPU is active only if:
+     *  the FPU is enabled
+     *  AND lazy state preservation is not active
+     *  AND we do not need a new fp context (this is the ASPEN/FPCA check)
+     *
+     * Usually we don't need to care about this distinction between
+     * LTPSIZE and FPSCR.LTPSIZE, because the code in vfp_access_check()
+     * will either take an exception or clear the conditions that make
+     * the FPU not active. But LE is an unusual case of a non-FP insn
+     * that looks at LTPSIZE.
+     */
+    fpu_active = !s->fp_excp_el && !s->v7m_lspact && !s->v7m_new_fp_ctxt_needed;
+
+    if (!a->tp && dc_isar_feature(aa32_mve, s) && fpu_active) {
+        /* Need to do a runtime check for LTPSIZE != 4 */
+        TCGLabel *skipexc = gen_new_label();
+        tmp = load_cpu_field(v7m.ltpsize);
+        tcg_gen_brcondi_i32(TCG_COND_EQ, tmp, 4, skipexc);
+        tcg_temp_free_i32(tmp);
+        gen_exception_insn(s, s->pc_curr, EXCP_INVSTATE, syn_uncategorized(),
+                           default_exception_el(s));
+        gen_set_label(skipexc);
+    }
+
+    if (a->f) {
+        /* Loop-forever: just jump back to the loop start */
+        gen_jmp(s, read_pc(s) - a->imm);
+        return true;
+    }
+
+    /*
+     * Not loop-forever. If LR <= loop-decrement-value this is the last loop.
+     * For LE, we know at this point that LTPSIZE must be 4 and the
+     * loop decrement value is 1. For LETP we need to calculate the decrement
+     * value from LTPSIZE.
+     */
+    loopend = gen_new_label();
+    if (!a->tp) {
+        tcg_gen_brcondi_i32(TCG_COND_LEU, cpu_R[14], 1, loopend);
+        tcg_gen_addi_i32(cpu_R[14], cpu_R[14], -1);
+    } else {
+        /*
+         * Decrement by 1 << (4 - LTPSIZE). We need to use a TCG local
+         * so that decr stays live after the brcondi.
+         */
+        TCGv_i32 decr = tcg_temp_local_new_i32();
+        TCGv_i32 ltpsize = load_cpu_field(v7m.ltpsize);
+        tcg_gen_sub_i32(decr, tcg_constant_i32(4), ltpsize);
+        tcg_gen_shl_i32(decr, tcg_constant_i32(1), decr);
+        tcg_temp_free_i32(ltpsize);
+
+        tcg_gen_brcond_i32(TCG_COND_LEU, cpu_R[14], decr, loopend);
+
+        tcg_gen_sub_i32(cpu_R[14], cpu_R[14], decr);
+        tcg_temp_free_i32(decr);
     }
     /* Jump back to the loop start */
     gen_jmp(s, read_pc(s) - a->imm);
+
+    gen_set_label(loopend);
+    if (a->tp) {
+        /* Exits from tail-pred loops must reset LTPSIZE to 4 */
+        tmp = tcg_const_i32(4);
+        store_cpu_field(tmp, v7m.ltpsize);
+    }
+    /* End TB, continuing to following insn */
+    gen_jmp_tb(s, s->base.pc_next, 1);
     return true;
 }