Mercurial > jdk8-mips64-public > hotspot / file revision

 /*

  * Copyright 1998-2007 Sun Microsystems, Inc.  All Rights Reserved.

  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

  *

  * This code is free software; you can redistribute it and/or modify it

  * under the terms of the GNU General Public License version 2 only, as

  * published by the Free Software Foundation.

  *

  * This code is distributed in the hope that it will be useful, but WITHOUT

  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

  * version 2 for more details (a copy is included in the LICENSE file that

  * accompanied this code).

  *

  * You should have received a copy of the GNU General Public License version

  * 2 along with this work; if not, write to the Free Software Foundation,

  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

  *

  * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,

  * CA 95054 USA or visit www.sun.com if you need additional information or

  * have any questions.

  *

  */

 #include "incls/_precompiled.incl"

 #include "incls/_postaloc.cpp.incl"

 // see if this register kind does not requires two registers

 static bool is_single_register(uint x) {

 #ifdef _LP64

   return (x != Op_RegD && x != Op_RegL && x != Op_RegP);

 #else

   return (x != Op_RegD && x != Op_RegL);

 #endif

 }

 //------------------------------may_be_copy_of_callee-----------------------------

 // Check to see if we can possibly be a copy of a callee-save value.

 bool PhaseChaitin::may_be_copy_of_callee( Node *def ) const {

   // Short circuit if there are no callee save registers

   if (_matcher.number_of_saved_registers() == 0) return false;

   // Expect only a spill-down and reload on exit for callee-save spills.

   // Chains of copies cannot be deep.

   // 5008997 - This is wishful thinking. Register allocator seems to

   // be splitting live ranges for callee save registers to such

   // an extent that in large methods the chains can be very long

   // (50+). The conservative answer is to return true if we don't

   // know as this prevents optimizations from occuring.

   const int limit = 60;

   int i;

   for( i=0; i < limit; i++ ) {

     if( def->is_Proj() && def->in(0)->is_Start() &&

         _matcher.is_save_on_entry(lrgs(n2lidx(def)).reg()) )

       return true;              // Direct use of callee-save proj

     if( def->is_Copy() )        // Copies carry value through

       def = def->in(def->is_Copy());

     else if( def->is_Phi() )    // Phis can merge it from any direction

       def = def->in(1);

     else

       break;

     guarantee(def != NULL, "must not resurrect dead copy");

   }

   // If we reached the end and didn't find a callee save proj

   // then this may be a callee save proj so we return true

   // as the conservative answer. If we didn't reach then end

   // we must have discovered that it was not a callee save

   // else we would have returned.

   return i == limit;

 }

 //------------------------------yank_if_dead-----------------------------------

 // Removed an edge from 'old'.  Yank if dead.  Return adjustment counts to

 // iterators in the current block.

 int PhaseChaitin::yank_if_dead( Node *old, Block *current_block, Node_List *value, Node_List *regnd ) {

   int blk_adjust=0;

   while (old->outcnt() == 0 && old != C->top()) {

     Block *oldb = _cfg._bbs[old->_idx];

     oldb->find_remove(old);

     // Count 1 if deleting an instruction from the current block

     if( oldb == current_block ) blk_adjust++;

     _cfg._bbs.map(old->_idx,NULL);

     OptoReg::Name old_reg = lrgs(n2lidx(old)).reg();

     if( regnd && (*regnd)[old_reg]==old ) { // Instruction is currently available?

       value->map(old_reg,NULL);  // Yank from value/regnd maps

       regnd->map(old_reg,NULL);  // This register's value is now unknown

     }

     Node *tmp = old->req() > 1 ? old->in(1) : NULL;

     old->disconnect_inputs(NULL);

     if( !tmp ) break;

     old = tmp;

   }

   return blk_adjust;

 }

 //------------------------------use_prior_register-----------------------------

 // Use the prior value instead of the current value, in an effort to make

 // the current value go dead.  Return block iterator adjustment, in case

 // we yank some instructions from this block.

 int PhaseChaitin::use_prior_register( Node *n, uint idx, Node *def, Block *current_block, Node_List &value, Node_List &regnd ) {

   // No effect?

   if( def == n->in(idx) ) return 0;

   // Def is currently dead and can be removed?  Do not resurrect

   if( def->outcnt() == 0 ) return 0;

   // Not every pair of physical registers are assignment compatible,

   // e.g. on sparc floating point registers are not assignable to integer

   // registers.

   const LRG &def_lrg = lrgs(n2lidx(def));

   OptoReg::Name def_reg = def_lrg.reg();

   const RegMask &use_mask = n->in_RegMask(idx);

   bool can_use = ( RegMask::can_represent(def_reg) ? (use_mask.Member(def_reg) != 0)

                                                    : (use_mask.is_AllStack() != 0));

   // Check for a copy to or from a misaligned pair.

   can_use = can_use && !use_mask.is_misaligned_Pair() && !def_lrg.mask().is_misaligned_Pair();

   if (!can_use)

     return 0;

   // Capture the old def in case it goes dead...

   Node *old = n->in(idx);

   // Save-on-call copies can only be elided if the entire copy chain can go

   // away, lest we get the same callee-save value alive in 2 locations at

   // once.  We check for the obvious trivial case here.  Although it can

   // sometimes be elided with cooperation outside our scope, here we will just

   // miss the opportunity.  :-(

   if( may_be_copy_of_callee(def) ) {

     if( old->outcnt() > 1 ) return 0; // We're the not last user

     int idx = old->is_Copy();

     assert( idx, "chain of copies being removed" );

     Node *old2 = old->in(idx);  // Chain of copies

     if( old2->outcnt() > 1 ) return 0; // old is not the last user

     int idx2 = old2->is_Copy();

     if( !idx2 ) return 0;       // Not a chain of 2 copies

     if( def != old2->in(idx2) ) return 0; // Chain of exactly 2 copies

   }

   // Use the new def

   n->set_req(idx,def);

   _post_alloc++;

   // Is old def now dead?  We successfully yanked a copy?

   return yank_if_dead(old,current_block,&value,&regnd);

 }

 //------------------------------skip_copies------------------------------------

 // Skip through any number of copies (that don't mod oop-i-ness)

 Node *PhaseChaitin::skip_copies( Node *c ) {

   int idx = c->is_Copy();

   uint is_oop = lrgs(n2lidx(c))._is_oop;

   while (idx != 0) {

     guarantee(c->in(idx) != NULL, "must not resurrect dead copy");

     if (lrgs(n2lidx(c->in(idx)))._is_oop != is_oop)

       break;  // casting copy, not the same value

     c = c->in(idx);

     idx = c->is_Copy();

   }

   return c;

 }

 //------------------------------elide_copy-------------------------------------

 // Remove (bypass) copies along Node n, edge k.

 int PhaseChaitin::elide_copy( Node *n, int k, Block *current_block, Node_List &value, Node_List &regnd, bool can_change_regs ) {

   int blk_adjust = 0;

   uint nk_idx = n2lidx(n->in(k));

   OptoReg::Name nk_reg = lrgs(nk_idx ).reg();

   // Remove obvious same-register copies

   Node *x = n->in(k);

   int idx;

   while( (idx=x->is_Copy()) != 0 ) {

     Node *copy = x->in(idx);

     guarantee(copy != NULL, "must not resurrect dead copy");

     if( lrgs(n2lidx(copy)).reg() != nk_reg ) break;

     blk_adjust += use_prior_register(n,k,copy,current_block,value,regnd);

     if( n->in(k) != copy ) break; // Failed for some cutout?

     x = copy;                   // Progress, try again

   }

   // Phis and 2-address instructions cannot change registers so easily - their

   // outputs must match their input.

   if( !can_change_regs )

     return blk_adjust;          // Only check stupid copies!

   // Loop backedges won't have a value-mapping yet

   if( &value == NULL ) return blk_adjust;

   // Skip through all copies to the _value_ being used.  Do not change from

   // int to pointer.  This attempts to jump through a chain of copies, where

   // intermediate copies might be illegal, i.e., value is stored down to stack

   // then reloaded BUT survives in a register the whole way.

   Node *val = skip_copies(n->in(k));

   if( val == x ) return blk_adjust; // No progress?

   bool single = is_single_register(val->ideal_reg());

   uint val_idx = n2lidx(val);

   OptoReg::Name val_reg = lrgs(val_idx).reg();

   // See if it happens to already be in the correct register!

   // (either Phi's direct register, or the common case of the name

   // never-clobbered original-def register)

   if( value[val_reg] == val &&

       // Doubles check both halves

       ( single || value[val_reg-1] == val ) ) {

     blk_adjust += use_prior_register(n,k,regnd[val_reg],current_block,value,regnd);

     if( n->in(k) == regnd[val_reg] ) // Success!  Quit trying

       return blk_adjust;

   }

   // See if we can skip the copy by changing registers.  Don't change from

   // using a register to using the stack unless we know we can remove a

   // copy-load.  Otherwise we might end up making a pile of Intel cisc-spill

   // ops reading from memory instead of just loading once and using the

   // register.

   // Also handle duplicate copies here.

   const Type *t = val->is_Con() ? val->bottom_type() : NULL;

   // Scan all registers to see if this value is around already

   for( uint reg = 0; reg < (uint)_max_reg; reg++ ) {

     Node *vv = value[reg];

     if( !single ) {             // Doubles check for aligned-adjacent pair

       if( (reg&1)==0 ) continue;  // Wrong half of a pair

       if( vv != value[reg-1] ) continue; // Not a complete pair

     }

     if( vv == val ||            // Got a direct hit?

         (t && vv && vv->bottom_type() == t && vv->is_Mach() &&

          vv->as_Mach()->rule() == val->as_Mach()->rule()) ) { // Or same constant?

       assert( !n->is_Phi(), "cannot change registers at a Phi so easily" );

       if( OptoReg::is_stack(nk_reg) || // CISC-loading from stack OR

           OptoReg::is_reg(reg) || // turning into a register use OR

           regnd[reg]->outcnt()==1 ) { // last use of a spill-load turns into a CISC use

         blk_adjust += use_prior_register(n,k,regnd[reg],current_block,value,regnd);

         if( n->in(k) == regnd[reg] ) // Success!  Quit trying

           return blk_adjust;

       } // End of if not degrading to a stack

     } // End of if found value in another register

   } // End of scan all machine registers

   return blk_adjust;

 }

 //

 // Check if nreg already contains the constant value val.  Normal copy

 // elimination doesn't doesn't work on constants because multiple

 // nodes can represent the same constant so the type and rule of the

 // MachNode must be checked to ensure equivalence.

 //

 bool PhaseChaitin::eliminate_copy_of_constant(Node* val, Node* n,

                                               Block *current_block,

                                               Node_List& value, Node_List& regnd,

                                               OptoReg::Name nreg, OptoReg::Name nreg2) {

   if (value[nreg] != val && val->is_Con() &&

       value[nreg] != NULL && value[nreg]->is_Con() &&

       (nreg2 == OptoReg::Bad || value[nreg] == value[nreg2]) &&

       value[nreg]->bottom_type() == val->bottom_type() &&

       value[nreg]->as_Mach()->rule() == val->as_Mach()->rule()) {

     // This code assumes that two MachNodes representing constants

     // which have the same rule and the same bottom type will produce

     // identical effects into a register.  This seems like it must be

     // objectively true unless there are hidden inputs to the nodes

     // but if that were to change this code would need to updated.

     // Since they are equivalent the second one if redundant and can

     // be removed.

     //

     // n will be replaced with the old value but n might have

     // kills projections associated with it so remove them now so that

     // yank_if_dead will be able to elminate the copy once the uses

     // have been transferred to the old[value].

     for (DUIterator_Fast imax, i = n->fast_outs(imax); i < imax; i++) {

       Node* use = n->fast_out(i);

       if (use->is_Proj() && use->outcnt() == 0) {

         // Kill projections have no users and one input

         use->set_req(0, C->top());

         yank_if_dead(use, current_block, &value, &regnd);

         --i; --imax;

       }

     }

     _post_alloc++;

     return true;

   }

   return false;

 }

 //------------------------------post_allocate_copy_removal---------------------

 // Post-Allocation peephole copy removal.  We do this in 1 pass over the

 // basic blocks.  We maintain a mapping of registers to Nodes (an  array of

 // Nodes indexed by machine register or stack slot number).  NULL means that a

 // register is not mapped to any Node.  We can (want to have!) have several

 // registers map to the same Node.  We walk forward over the instructions

 // updating the mapping as we go.  At merge points we force a NULL if we have

 // to merge 2 different Nodes into the same register.  Phi functions will give

 // us a new Node if there is a proper value merging.  Since the blocks are

 // arranged in some RPO, we will visit all parent blocks before visiting any

 // successor blocks (except at loops).

 //

 // If we find a Copy we look to see if the Copy's source register is a stack

 // slot and that value has already been loaded into some machine register; if

 // so we use machine register directly.  This turns a Load into a reg-reg

 // Move.  We also look for reloads of identical constants.

 //

 // When we see a use from a reg-reg Copy, we will attempt to use the copy's

 // source directly and make the copy go dead.

 void PhaseChaitin::post_allocate_copy_removal() {

   NOT_PRODUCT( Compile::TracePhase t3("postAllocCopyRemoval", &_t_postAllocCopyRemoval, TimeCompiler); )

   ResourceMark rm;

   // Need a mapping from basic block Node_Lists.  We need a Node_List to

   // map from register number to value-producing Node.

   Node_List **blk2value = NEW_RESOURCE_ARRAY( Node_List *, _cfg._num_blocks+1);

   memset( blk2value, 0, sizeof(Node_List*)*(_cfg._num_blocks+1) );

   // Need a mapping from basic block Node_Lists.  We need a Node_List to

   // map from register number to register-defining Node.

   Node_List **blk2regnd = NEW_RESOURCE_ARRAY( Node_List *, _cfg._num_blocks+1);

   memset( blk2regnd, 0, sizeof(Node_List*)*(_cfg._num_blocks+1) );

   // We keep unused Node_Lists on a free_list to avoid wasting

   // memory.

   GrowableArray<Node_List*> free_list = GrowableArray<Node_List*>(16);

   // For all blocks

   for( uint i = 0; i < _cfg._num_blocks; i++ ) {

     uint j;

     Block *b = _cfg._blocks[i];

     // Count of Phis in block

     uint phi_dex;

     for( phi_dex = 1; phi_dex < b->_nodes.size(); phi_dex++ ) {

       Node *phi = b->_nodes[phi_dex];

       if( !phi->is_Phi() )

         break;

     }

     // If any predecessor has not been visited, we do not know the state

     // of registers at the start.  Check for this, while updating copies

     // along Phi input edges

     bool missing_some_inputs = false;

     Block *freed = NULL;

     for( j = 1; j < b->num_preds(); j++ ) {

       Block *pb = _cfg._bbs[b->pred(j)->_idx];

       // Remove copies along phi edges

       for( uint k=1; k<phi_dex; k++ )

         elide_copy( b->_nodes[k], j, b, *blk2value[pb->_pre_order], *blk2regnd[pb->_pre_order], false );

       if( blk2value[pb->_pre_order] ) { // Have a mapping on this edge?

         // See if this predecessor's mappings have been used by everybody

         // who wants them.  If so, free 'em.

         uint k;

         for( k=0; k<pb->_num_succs; k++ ) {

           Block *pbsucc = pb->_succs[k];

           if( !blk2value[pbsucc->_pre_order] && pbsucc != b )

             break;              // Found a future user

         }

         if( k >= pb->_num_succs ) { // No more uses, free!

           freed = pb;           // Record last block freed

           free_list.push(blk2value[pb->_pre_order]);

           free_list.push(blk2regnd[pb->_pre_order]);

         }

       } else {                  // This block has unvisited (loopback) inputs

         missing_some_inputs = true;

       }

     }

     // Extract Node_List mappings.  If 'freed' is non-zero, we just popped

     // 'freed's blocks off the list

     Node_List &regnd = *(free_list.is_empty() ? new Node_List() : free_list.pop());

     Node_List &value = *(free_list.is_empty() ? new Node_List() : free_list.pop());

     assert( !freed || blk2value[freed->_pre_order] == &value, "" );

     value.map(_max_reg,NULL);

     regnd.map(_max_reg,NULL);

     // Set mappings as OUR mappings

     blk2value[b->_pre_order] = &value;

     blk2regnd[b->_pre_order] = &regnd;

     // Initialize value & regnd for this block

     if( missing_some_inputs ) {

       // Some predecessor has not yet been visited; zap map to empty

       for( uint k = 0; k < (uint)_max_reg; k++ ) {

         value.map(k,NULL);

         regnd.map(k,NULL);

       }

     } else {

       if( !freed ) {            // Didn't get a freebie prior block

         // Must clone some data

         freed = _cfg._bbs[b->pred(1)->_idx];

         Node_List &f_value = *blk2value[freed->_pre_order];

         Node_List &f_regnd = *blk2regnd[freed->_pre_order];

         for( uint k = 0; k < (uint)_max_reg; k++ ) {

           value.map(k,f_value[k]);

           regnd.map(k,f_regnd[k]);

         }

       }

       // Merge all inputs together, setting to NULL any conflicts.

       for( j = 1; j < b->num_preds(); j++ ) {

         Block *pb = _cfg._bbs[b->pred(j)->_idx];

         if( pb == freed ) continue; // Did self already via freelist

         Node_List &p_regnd = *blk2regnd[pb->_pre_order];

         for( uint k = 0; k < (uint)_max_reg; k++ ) {

           if( regnd[k] != p_regnd[k] ) { // Conflict on reaching defs?

             value.map(k,NULL); // Then no value handy

             regnd.map(k,NULL);

           }

         }

       }

     }

     // For all Phi's

     for( j = 1; j < phi_dex; j++ ) {

       uint k;

       Node *phi = b->_nodes[j];

       uint pidx = n2lidx(phi);

       OptoReg::Name preg = lrgs(n2lidx(phi)).reg();

       // Remove copies remaining on edges.  Check for junk phi.

       Node *u = NULL;

       for( k=1; k<phi->req(); k++ ) {

         Node *x = phi->in(k);

         if( phi != x && u != x ) // Found a different input

           u = u ? NodeSentinel : x; // Capture unique input, or NodeSentinel for 2nd input

       }

       if( u != NodeSentinel ) {    // Junk Phi.  Remove

         b->_nodes.remove(j--); phi_dex--;

         _cfg._bbs.map(phi->_idx,NULL);

         phi->replace_by(u);

         phi->disconnect_inputs(NULL);

         continue;

       }

       // Note that if value[pidx] exists, then we merged no new values here

       // and the phi is useless.  This can happen even with the above phi

       // removal for complex flows.  I cannot keep the better known value here

       // because locally the phi appears to define a new merged value.  If I

       // keep the better value then a copy of the phi, being unable to use the

       // global flow analysis, can't "peek through" the phi to the original

       // reaching value and so will act like it's defining a new value.  This

       // can lead to situations where some uses are from the old and some from

       // the new values.  Not illegal by itself but throws the over-strong

       // assert in scheduling.

       if( pidx ) {

         value.map(preg,phi);

         regnd.map(preg,phi);

         OptoReg::Name preg_lo = OptoReg::add(preg,-1);

         if( !is_single_register(phi->ideal_reg()) ) {

           value.map(preg_lo,phi);

           regnd.map(preg_lo,phi);

         }

       }

     }

     // For all remaining instructions

     for( j = phi_dex; j < b->_nodes.size(); j++ ) {

       Node *n = b->_nodes[j];

       if( n->outcnt() == 0 &&   // Dead?

           n != C->top() &&      // (ignore TOP, it has no du info)

           !n->is_Proj() ) {     // fat-proj kills

         j -= yank_if_dead(n,b,&value,&regnd);

         continue;

       }

       // Improve reaching-def info.  Occasionally post-alloc's liveness gives

       // up (at loop backedges, because we aren't doing a full flow pass).

       // The presence of a live use essentially asserts that the use's def is

       // alive and well at the use (or else the allocator fubar'd).  Take

       // advantage of this info to set a reaching def for the use-reg.

       uint k;

       for( k = 1; k < n->req(); k++ ) {

         Node *def = n->in(k);   // n->in(k) is a USE; def is the DEF for this USE

         guarantee(def != NULL, "no disconnected nodes at this point");

         uint useidx = n2lidx(def); // useidx is the live range index for this USE

         if( useidx ) {

           OptoReg::Name ureg = lrgs(useidx).reg();

           if( !value[ureg] ) {

             int idx;            // Skip occasional useless copy

             while( (idx=def->is_Copy()) != 0 &&

                    def->in(idx) != NULL &&  // NULL should not happen

                    ureg == lrgs(n2lidx(def->in(idx))).reg() )

               def = def->in(idx);

             Node *valdef = skip_copies(def); // tighten up val through non-useless copies

             value.map(ureg,valdef); // record improved reaching-def info

             regnd.map(ureg,   def);

             // Record other half of doubles

             OptoReg::Name ureg_lo = OptoReg::add(ureg,-1);

             if( !is_single_register(def->ideal_reg()) &&

                 ( !RegMask::can_represent(ureg_lo) ||

                   lrgs(useidx).mask().Member(ureg_lo) ) && // Nearly always adjacent

                 !value[ureg_lo] ) {

               value.map(ureg_lo,valdef); // record improved reaching-def info

               regnd.map(ureg_lo,   def);

             }

           }

         }

       }

       const uint two_adr = n->is_Mach() ? n->as_Mach()->two_adr() : 0;

       // Remove copies along input edges

       for( k = 1; k < n->req(); k++ )

         j -= elide_copy( n, k, b, value, regnd, two_adr!=k );

       // Unallocated Nodes define no registers

       uint lidx = n2lidx(n);

       if( !lidx ) continue;

       // Update the register defined by this instruction

       OptoReg::Name nreg = lrgs(lidx).reg();

       // Skip through all copies to the _value_ being defined.

       // Do not change from int to pointer

       Node *val = skip_copies(n);

       uint n_ideal_reg = n->ideal_reg();

       if( is_single_register(n_ideal_reg) ) {

         // If Node 'n' does not change the value mapped by the register,

         // then 'n' is a useless copy.  Do not update the register->node

         // mapping so 'n' will go dead.

         if( value[nreg] != val ) {

           if (eliminate_copy_of_constant(val, n, b, value, regnd, nreg, OptoReg::Bad)) {

             n->replace_by(regnd[nreg]);

             j -= yank_if_dead(n,b,&value,&regnd);

           } else {

             // Update the mapping: record new Node defined by the register

             regnd.map(nreg,n);

             // Update mapping for defined *value*, which is the defined

             // Node after skipping all copies.

             value.map(nreg,val);

           }

         } else if( !may_be_copy_of_callee(n) && regnd[nreg]->outcnt() != 0 ) {

           assert( n->is_Copy(), "" );

           n->replace_by(regnd[nreg]);

           j -= yank_if_dead(n,b,&value,&regnd);

         }

       } else {

         // If the value occupies a register pair, record same info

         // in both registers.

         OptoReg::Name nreg_lo = OptoReg::add(nreg,-1);

         if( RegMask::can_represent(nreg_lo) &&     // Either a spill slot, or

             !lrgs(lidx).mask().Member(nreg_lo) ) { // Nearly always adjacent

           // Sparc occasionally has non-adjacent pairs.

           // Find the actual other value

           RegMask tmp = lrgs(lidx).mask();

           tmp.Remove(nreg);

           nreg_lo = tmp.find_first_elem();

         }

         if( value[nreg] != val || value[nreg_lo] != val ) {

           if (eliminate_copy_of_constant(val, n, b, value, regnd, nreg, nreg_lo)) {

             n->replace_by(regnd[nreg]);

             j -= yank_if_dead(n,b,&value,&regnd);

           } else {

             regnd.map(nreg   , n );

             regnd.map(nreg_lo, n );

             value.map(nreg   ,val);

             value.map(nreg_lo,val);

           }

         } else if( !may_be_copy_of_callee(n) && regnd[nreg]->outcnt() != 0 ) {

           assert( n->is_Copy(), "" );

           n->replace_by(regnd[nreg]);

           j -= yank_if_dead(n,b,&value,&regnd);

         }

       }

       // Fat projections kill many registers

       if( n_ideal_reg == MachProjNode::fat_proj ) {

         RegMask rm = n->out_RegMask();

         // wow, what an expensive iterator...

         nreg = rm.find_first_elem();

         while( OptoReg::is_valid(nreg)) {

           rm.Remove(nreg);

           value.map(nreg,n);

           regnd.map(nreg,n);

           nreg = rm.find_first_elem();

         }

       }

     } // End of for all instructions in the block

   } // End for all blocks

 }