[Bug tree-optimization/122635] RISC-V: -mrvv-vector-bits=zvl introducing unoptimal vsetivli zero,0 and insns depending on AVL nops

[rdapp at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122635

--- Comment #11 from Robin Dapp <rdapp at gcc dot gnu.org> ---
As mentioned in #2 dse can do it.  It probably can be done in other places as
well but it doesn't feel very out of place in dse either:

diff --git a/gcc/tree-ssa-dse.cc b/gcc/tree-ssa-dse.cc
index 51a572316cd..119b5c6c319 100644
--- a/gcc/tree-ssa-dse.cc
+++ b/gcc/tree-ssa-dse.cc
@@ -164,7 +164,7 @@ initialize_ao_ref_for_dse (gimple *stmt, ao_ref *write,
bool may_def_ok = false)
            internal_fn ifn = gimple_call_internal_fn (stmt);
            int stored_value_index = internal_fn_stored_value_index (ifn);
            int len_index = internal_fn_len_index (ifn);
-           if (ifn == IFN_LEN_STORE)
+           if (ifn == IFN_LEN_STORE || ifn == IFN_MASK_LEN_STORE)
              {
                tree len = gimple_call_arg (stmt, len_index);
                tree bias = gimple_call_arg (stmt, len_index + 1);
@@ -1568,8 +1568,29 @@ dse_optimize_stmt (function *fun, gimple_stmt_iterator
*gsi, sbitmap live_bytes)
       switch (gimple_call_internal_fn (stmt))
        {
        case IFN_LEN_STORE:
-       case IFN_MASK_STORE:
        case IFN_MASK_LEN_STORE:
+         {
+           internal_fn ifn = gimple_call_internal_fn (stmt);
+           int len_index = internal_fn_len_index (ifn);
+           tree len = gimple_call_arg (stmt, len_index);
+           tree bias = gimple_call_arg (stmt, len_index + 1);
+
+           if (tree_fits_uhwi_p (len) && tree_fits_shwi_p (bias))
+             {
+               HOST_WIDE_INT len_val = tree_to_shwi (len);
+               HOST_WIDE_INT bias_val = tree_to_uhwi (bias);
+               if (len_val + bias_val <= 0)
+                 {
+                   delete_dead_or_redundant_call (gsi, "dead");
+                   return;
+                 }
+             }
+           /* FALLTHRU */
+         }
+       case IFN_MASK_STORE:
          {
            enum dse_store_status store_status;
            store_status = dse_classify_store (&ref, stmt, false, live_bytes);