[BUG] Top rounding is incorrect when length rounding causes exp to increase.

[joncapltd]

Is there an existing CVA6 bug for this?

• I have searched the existing bug issues

Bug Description

--- a/cva6_cheri_pkg.sv
+++ b/cva6_cheri_pkg.sv
@@ -832,12 +832,17 @@ package cva6_cheri_pkg;
addrwe2_t lmsk_m_bits = ~(-1 << (exp + 3 + CAP_M_WIDTH - 4)) & ~lmsk_exp_bits;
//addrwe2_t lmsk_m_less_1_bits = ~(-1 << (exp + 3 + CAP_M_WIDTH - 3)) & ~lmsk_exp_bits;
addrwe2_t lmsk_m_less_1_bits = (-1 << (exp + 4)) & lmsk_m_bits;

•    // Creates mask to check all bits bellow msb(l) = exp + CAP_MW_WIDTH + 1;
  

•    // in case length_over increases exp.
  

•    addrwe2_t lmsk_exp_bits_over = ~(-1 << (exp + 4));
     // Check if any of the lsb of len, base and top were lost, i.e., [Einitial+2:0]
     // are all non-zero
     bool_t lost_lsb_len = (new_len & lmsk_exp_bits) != 0 && int_e;
     bool_t lost_lsb_base = (new_base & lmsk_exp_bits) != 0 && int_e;
     bool_t lost_lsb_top = (new_top & lmsk_exp_bits) != 0 && int_e;
     bool_t is_exact = !(lost_lsb_base || lost_lsb_top);
  

•    // Check lsb of Top for length_over case
  

•    bool_t lost_lsb_top_over = (new_top & lmsk_exp_bits_over) != 0 && int_e;
     // Check if all mantissa bits above the Einitial+3 are all ones (i.e., length is max)
     bool_t is_len_max = (new_len & (lmsk_m_bits)) == (lmsk_m_bits);
     bool_t is_len_max_less_one = (new_len & (lmsk_m_bits)) == (lmsk_m_less_1_bits);
  

@@ -860,7 +865,7 @@ package cva6_cheri_pkg;
// C = , we need to increase the E
exp = exp + 1;
// Sum one to T if there was a overflow and we lost the 3 lsb bits of T

•        ret.cap.bounds.top_bits = lost_lsb_top ? new_top_bits_over + 14'b00000000001000
  

•        ret.cap.bounds.top_bits = lost_lsb_top_over ? new_top_bits_over + 14'b00000000001000
                                               : new_top_bits_over;
         ret.cap.bounds.base_bits = new_base_bits_over;
     end else begin