--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/hotspot/src/share/vm/opto/buildOopMap.cpp Sat Dec 01 00:00:00 2007 +0000
@@ -0,0 +1,623 @@
+/*
+ * Copyright 2002-2007 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/_buildOopMap.cpp.incl"
+
+// The functions in this file builds OopMaps after all scheduling is done.
+//
+// OopMaps contain a list of all registers and stack-slots containing oops (so
+// they can be updated by GC). OopMaps also contain a list of derived-pointer
+// base-pointer pairs. When the base is moved, the derived pointer moves to
+// follow it. Finally, any registers holding callee-save values are also
+// recorded. These might contain oops, but only the caller knows.
+//
+// BuildOopMaps implements a simple forward reaching-defs solution. At each
+// GC point we'll have the reaching-def Nodes. If the reaching Nodes are
+// typed as pointers (no offset), then they are oops. Pointers+offsets are
+// derived pointers, and bases can be found from them. Finally, we'll also
+// track reaching callee-save values. Note that a copy of a callee-save value
+// "kills" it's source, so that only 1 copy of a callee-save value is alive at
+// a time.
+//
+// We run a simple bitvector liveness pass to help trim out dead oops. Due to
+// irreducible loops, we can have a reaching def of an oop that only reaches
+// along one path and no way to know if it's valid or not on the other path.
+// The bitvectors are quite dense and the liveness pass is fast.
+//
+// At GC points, we consult this information to build OopMaps. All reaching
+// defs typed as oops are added to the OopMap. Only 1 instance of a
+// callee-save register can be recorded. For derived pointers, we'll have to
+// find and record the register holding the base.
+//
+// The reaching def's is a simple 1-pass worklist approach. I tried a clever
+// breadth-first approach but it was worse (showed O(n^2) in the
+// pick-next-block code).
+//
+// The relevent data is kept in a struct of arrays (it could just as well be
+// an array of structs, but the struct-of-arrays is generally a little more
+// efficient). The arrays are indexed by register number (including
+// stack-slots as registers) and so is bounded by 200 to 300 elements in
+// practice. One array will map to a reaching def Node (or NULL for
+// conflict/dead). The other array will map to a callee-saved register or
+// OptoReg::Bad for not-callee-saved.
+
+
+//------------------------------OopFlow----------------------------------------
+// Structure to pass around
+struct OopFlow : public ResourceObj {
+ short *_callees; // Array mapping register to callee-saved
+ Node **_defs; // array mapping register to reaching def
+ // or NULL if dead/conflict
+ // OopFlow structs, when not being actively modified, describe the _end_ of
+ // this block.
+ Block *_b; // Block for this struct
+ OopFlow *_next; // Next free OopFlow
+
+ OopFlow( short *callees, Node **defs ) : _callees(callees), _defs(defs),
+ _b(NULL), _next(NULL) { }
+
+ // Given reaching-defs for this block start, compute it for this block end
+ void compute_reach( PhaseRegAlloc *regalloc, int max_reg, Dict *safehash );
+
+ // Merge these two OopFlows into the 'this' pointer.
+ void merge( OopFlow *flow, int max_reg );
+
+ // Copy a 'flow' over an existing flow
+ void clone( OopFlow *flow, int max_size);
+
+ // Make a new OopFlow from scratch
+ static OopFlow *make( Arena *A, int max_size );
+
+ // Build an oopmap from the current flow info
+ OopMap *build_oop_map( Node *n, int max_reg, PhaseRegAlloc *regalloc, int* live );
+};
+
+//------------------------------compute_reach----------------------------------
+// Given reaching-defs for this block start, compute it for this block end
+void OopFlow::compute_reach( PhaseRegAlloc *regalloc, int max_reg, Dict *safehash ) {
+
+ for( uint i=0; i<_b->_nodes.size(); i++ ) {
+ Node *n = _b->_nodes[i];
+
+ if( n->jvms() ) { // Build an OopMap here?
+ JVMState *jvms = n->jvms();
+ // no map needed for leaf calls
+ if( n->is_MachSafePoint() && !n->is_MachCallLeaf() ) {
+ int *live = (int*) (*safehash)[n];
+ assert( live, "must find live" );
+ n->as_MachSafePoint()->set_oop_map( build_oop_map(n,max_reg,regalloc, live) );
+ }
+ }
+
+ // Assign new reaching def's.
+ // Note that I padded the _defs and _callees arrays so it's legal
+ // to index at _defs[OptoReg::Bad].
+ OptoReg::Name first = regalloc->get_reg_first(n);
+ OptoReg::Name second = regalloc->get_reg_second(n);
+ _defs[first] = n;
+ _defs[second] = n;
+
+ // Pass callee-save info around copies
+ int idx = n->is_Copy();
+ if( idx ) { // Copies move callee-save info
+ OptoReg::Name old_first = regalloc->get_reg_first(n->in(idx));
+ OptoReg::Name old_second = regalloc->get_reg_second(n->in(idx));
+ int tmp_first = _callees[old_first];
+ int tmp_second = _callees[old_second];
+ _callees[old_first] = OptoReg::Bad; // callee-save is moved, dead in old location
+ _callees[old_second] = OptoReg::Bad;
+ _callees[first] = tmp_first;
+ _callees[second] = tmp_second;
+ } else if( n->is_Phi() ) { // Phis do not mod callee-saves
+ assert( _callees[first] == _callees[regalloc->get_reg_first(n->in(1))], "" );
+ assert( _callees[second] == _callees[regalloc->get_reg_second(n->in(1))], "" );
+ assert( _callees[first] == _callees[regalloc->get_reg_first(n->in(n->req()-1))], "" );
+ assert( _callees[second] == _callees[regalloc->get_reg_second(n->in(n->req()-1))], "" );
+ } else {
+ _callees[first] = OptoReg::Bad; // No longer holding a callee-save value
+ _callees[second] = OptoReg::Bad;
+
+ // Find base case for callee saves
+ if( n->is_Proj() && n->in(0)->is_Start() ) {
+ if( OptoReg::is_reg(first) &&
+ regalloc->_matcher.is_save_on_entry(first) )
+ _callees[first] = first;
+ if( OptoReg::is_reg(second) &&
+ regalloc->_matcher.is_save_on_entry(second) )
+ _callees[second] = second;
+ }
+ }
+ }
+}
+
+//------------------------------merge------------------------------------------
+// Merge the given flow into the 'this' flow
+void OopFlow::merge( OopFlow *flow, int max_reg ) {
+ assert( _b == NULL, "merging into a happy flow" );
+ assert( flow->_b, "this flow is still alive" );
+ assert( flow != this, "no self flow" );
+
+ // Do the merge. If there are any differences, drop to 'bottom' which
+ // is OptoReg::Bad or NULL depending.
+ for( int i=0; i<max_reg; i++ ) {
+ // Merge the callee-save's
+ if( _callees[i] != flow->_callees[i] )
+ _callees[i] = OptoReg::Bad;
+ // Merge the reaching defs
+ if( _defs[i] != flow->_defs[i] )
+ _defs[i] = NULL;
+ }
+
+}
+
+//------------------------------clone------------------------------------------
+void OopFlow::clone( OopFlow *flow, int max_size ) {
+ _b = flow->_b;
+ memcpy( _callees, flow->_callees, sizeof(short)*max_size);
+ memcpy( _defs , flow->_defs , sizeof(Node*)*max_size);
+}
+
+//------------------------------make-------------------------------------------
+OopFlow *OopFlow::make( Arena *A, int max_size ) {
+ short *callees = NEW_ARENA_ARRAY(A,short,max_size+1);
+ Node **defs = NEW_ARENA_ARRAY(A,Node*,max_size+1);
+ debug_only( memset(defs,0,(max_size+1)*sizeof(Node*)) );
+ OopFlow *flow = new (A) OopFlow(callees+1, defs+1);
+ assert( &flow->_callees[OptoReg::Bad] == callees, "Ok to index at OptoReg::Bad" );
+ assert( &flow->_defs [OptoReg::Bad] == defs , "Ok to index at OptoReg::Bad" );
+ return flow;
+}
+
+//------------------------------bit twiddlers----------------------------------
+static int get_live_bit( int *live, int reg ) {
+ return live[reg>>LogBitsPerInt] & (1<<(reg&(BitsPerInt-1))); }
+static void set_live_bit( int *live, int reg ) {
+ live[reg>>LogBitsPerInt] |= (1<<(reg&(BitsPerInt-1))); }
+static void clr_live_bit( int *live, int reg ) {
+ live[reg>>LogBitsPerInt] &= ~(1<<(reg&(BitsPerInt-1))); }
+
+//------------------------------build_oop_map----------------------------------
+// Build an oopmap from the current flow info
+OopMap *OopFlow::build_oop_map( Node *n, int max_reg, PhaseRegAlloc *regalloc, int* live ) {
+ int framesize = regalloc->_framesize;
+ int max_inarg_slot = OptoReg::reg2stack(regalloc->_matcher._new_SP);
+ debug_only( char *dup_check = NEW_RESOURCE_ARRAY(char,OptoReg::stack0());
+ memset(dup_check,0,OptoReg::stack0()) );
+
+ OopMap *omap = new OopMap( framesize, max_inarg_slot );
+ MachCallNode *mcall = n->is_MachCall() ? n->as_MachCall() : NULL;
+ JVMState* jvms = n->jvms();
+
+ // For all registers do...
+ for( int reg=0; reg<max_reg; reg++ ) {
+ if( get_live_bit(live,reg) == 0 )
+ continue; // Ignore if not live
+
+ // %%% C2 can use 2 OptoRegs when the physical register is only one 64bit
+ // register in that case we'll get an non-concrete register for the second
+ // half. We only need to tell the map the register once!
+ //
+ // However for the moment we disable this change and leave things as they
+ // were.
+
+ VMReg r = OptoReg::as_VMReg(OptoReg::Name(reg), framesize, max_inarg_slot);
+
+ if (false && r->is_reg() && !r->is_concrete()) {
+ continue;
+ }
+
+ // See if dead (no reaching def).
+ Node *def = _defs[reg]; // Get reaching def
+ assert( def, "since live better have reaching def" );
+
+ // Classify the reaching def as oop, derived, callee-save, dead, or other
+ const Type *t = def->bottom_type();
+ if( t->isa_oop_ptr() ) { // Oop or derived?
+ assert( !OptoReg::is_valid(_callees[reg]), "oop can't be callee save" );
+#ifdef _LP64
+ // 64-bit pointers record oop-ishness on 2 aligned adjacent registers.
+ // Make sure both are record from the same reaching def, but do not
+ // put both into the oopmap.
+ if( (reg&1) == 1 ) { // High half of oop-pair?
+ assert( _defs[reg-1] == _defs[reg], "both halves from same reaching def" );
+ continue; // Do not record high parts in oopmap
+ }
+#endif
+
+ // Check for a legal reg name in the oopMap and bailout if it is not.
+ if (!omap->legal_vm_reg_name(r)) {
+ regalloc->C->record_method_not_compilable("illegal oopMap register name");
+ continue;
+ }
+ if( t->is_ptr()->_offset == 0 ) { // Not derived?
+ if( mcall ) {
+ // Outgoing argument GC mask responsibility belongs to the callee,
+ // not the caller. Inspect the inputs to the call, to see if
+ // this live-range is one of them.
+ uint cnt = mcall->tf()->domain()->cnt();
+ uint j;
+ for( j = TypeFunc::Parms; j < cnt; j++)
+ if( mcall->in(j) == def )
+ break; // reaching def is an argument oop
+ if( j < cnt ) // arg oops dont go in GC map
+ continue; // Continue on to the next register
+ }
+ omap->set_oop(r);
+ } else { // Else it's derived.
+ // Find the base of the derived value.
+ uint i;
+ // Fast, common case, scan
+ for( i = jvms->oopoff(); i < n->req(); i+=2 )
+ if( n->in(i) == def ) break; // Common case
+ if( i == n->req() ) { // Missed, try a more generous scan
+ // Scan again, but this time peek through copies
+ for( i = jvms->oopoff(); i < n->req(); i+=2 ) {
+ Node *m = n->in(i); // Get initial derived value
+ while( 1 ) {
+ Node *d = def; // Get initial reaching def
+ while( 1 ) { // Follow copies of reaching def to end
+ if( m == d ) goto found; // breaks 3 loops
+ int idx = d->is_Copy();
+ if( !idx ) break;
+ d = d->in(idx); // Link through copy
+ }
+ int idx = m->is_Copy();
+ if( !idx ) break;
+ m = m->in(idx);
+ }
+ }
+ guarantee( 0, "must find derived/base pair" );
+ }
+ found: ;
+ Node *base = n->in(i+1); // Base is other half of pair
+ int breg = regalloc->get_reg_first(base);
+ VMReg b = OptoReg::as_VMReg(OptoReg::Name(breg), framesize, max_inarg_slot);
+
+ // I record liveness at safepoints BEFORE I make the inputs
+ // live. This is because argument oops are NOT live at a
+ // safepoint (or at least they cannot appear in the oopmap).
+ // Thus bases of base/derived pairs might not be in the
+ // liveness data but they need to appear in the oopmap.
+ if( get_live_bit(live,breg) == 0 ) {// Not live?
+ // Flag it, so next derived pointer won't re-insert into oopmap
+ set_live_bit(live,breg);
+ // Already missed our turn?
+ if( breg < reg ) {
+ if (b->is_stack() || b->is_concrete() || true ) {
+ omap->set_oop( b);
+ }
+ }
+ }
+ if (b->is_stack() || b->is_concrete() || true ) {
+ omap->set_derived_oop( r, b);
+ }
+ }
+
+ } else if( OptoReg::is_valid(_callees[reg])) { // callee-save?
+ // It's a callee-save value
+ assert( dup_check[_callees[reg]]==0, "trying to callee save same reg twice" );
+ debug_only( dup_check[_callees[reg]]=1; )
+ VMReg callee = OptoReg::as_VMReg(OptoReg::Name(_callees[reg]));
+ if ( callee->is_concrete() || true ) {
+ omap->set_callee_saved( r, callee);
+ }
+
+ } else {
+ // Other - some reaching non-oop value
+ omap->set_value( r);
+ }
+
+ }
+
+#ifdef ASSERT
+ /* Nice, Intel-only assert
+ int cnt_callee_saves=0;
+ int reg2 = 0;
+ while (OptoReg::is_reg(reg2)) {
+ if( dup_check[reg2] != 0) cnt_callee_saves++;
+ assert( cnt_callee_saves==3 || cnt_callee_saves==5, "missed some callee-save" );
+ reg2++;
+ }
+ */
+#endif
+
+ return omap;
+}
+
+//------------------------------do_liveness------------------------------------
+// Compute backwards liveness on registers
+static void do_liveness( PhaseRegAlloc *regalloc, PhaseCFG *cfg, Block_List *worklist, int max_reg_ints, Arena *A, Dict *safehash ) {
+ int *live = NEW_ARENA_ARRAY(A, int, (cfg->_num_blocks+1) * max_reg_ints);
+ int *tmp_live = &live[cfg->_num_blocks * max_reg_ints];
+ Node *root = cfg->C->root();
+ // On CISC platforms, get the node representing the stack pointer that regalloc
+ // used for spills
+ Node *fp = NodeSentinel;
+ if (UseCISCSpill && root->req() > 1) {
+ fp = root->in(1)->in(TypeFunc::FramePtr);
+ }
+ memset( live, 0, cfg->_num_blocks * (max_reg_ints<<LogBytesPerInt) );
+ // Push preds onto worklist
+ for( uint i=1; i<root->req(); i++ )
+ worklist->push(cfg->_bbs[root->in(i)->_idx]);
+
+ // ZKM.jar includes tiny infinite loops which are unreached from below.
+ // If we missed any blocks, we'll retry here after pushing all missed
+ // blocks on the worklist. Normally this outer loop never trips more
+ // than once.
+ while( 1 ) {
+
+ while( worklist->size() ) { // Standard worklist algorithm
+ Block *b = worklist->rpop();
+
+ // Copy first successor into my tmp_live space
+ int s0num = b->_succs[0]->_pre_order;
+ int *t = &live[s0num*max_reg_ints];
+ for( int i=0; i<max_reg_ints; i++ )
+ tmp_live[i] = t[i];
+
+ // OR in the remaining live registers
+ for( uint j=1; j<b->_num_succs; j++ ) {
+ uint sjnum = b->_succs[j]->_pre_order;
+ int *t = &live[sjnum*max_reg_ints];
+ for( int i=0; i<max_reg_ints; i++ )
+ tmp_live[i] |= t[i];
+ }
+
+ // Now walk tmp_live up the block backwards, computing live
+ for( int k=b->_nodes.size()-1; k>=0; k-- ) {
+ Node *n = b->_nodes[k];
+ // KILL def'd bits
+ int first = regalloc->get_reg_first(n);
+ int second = regalloc->get_reg_second(n);
+ if( OptoReg::is_valid(first) ) clr_live_bit(tmp_live,first);
+ if( OptoReg::is_valid(second) ) clr_live_bit(tmp_live,second);
+
+ MachNode *m = n->is_Mach() ? n->as_Mach() : NULL;
+
+ // Check if m is potentially a CISC alternate instruction (i.e, possibly
+ // synthesized by RegAlloc from a conventional instruction and a
+ // spilled input)
+ bool is_cisc_alternate = false;
+ if (UseCISCSpill && m) {
+ is_cisc_alternate = m->is_cisc_alternate();
+ }
+
+ // GEN use'd bits
+ for( uint l=1; l<n->req(); l++ ) {
+ Node *def = n->in(l);
+ assert(def != 0, "input edge required");
+ int first = regalloc->get_reg_first(def);
+ int second = regalloc->get_reg_second(def);
+ if( OptoReg::is_valid(first) ) set_live_bit(tmp_live,first);
+ if( OptoReg::is_valid(second) ) set_live_bit(tmp_live,second);
+ // If we use the stack pointer in a cisc-alternative instruction,
+ // check for use as a memory operand. Then reconstruct the RegName
+ // for this stack location, and set the appropriate bit in the
+ // live vector 4987749.
+ if (is_cisc_alternate && def == fp) {
+ const TypePtr *adr_type = NULL;
+ intptr_t offset;
+ const Node* base = m->get_base_and_disp(offset, adr_type);
+ if (base == NodeSentinel) {
+ // Machnode has multiple memory inputs. We are unable to reason
+ // with these, but are presuming (with trepidation) that not any of
+ // them are oops. This can be fixed by making get_base_and_disp()
+ // look at a specific input instead of all inputs.
+ assert(!def->bottom_type()->isa_oop_ptr(), "expecting non-oop mem input");
+ } else if (base != fp || offset == Type::OffsetBot) {
+ // Do nothing: the fp operand is either not from a memory use
+ // (base == NULL) OR the fp is used in a non-memory context
+ // (base is some other register) OR the offset is not constant,
+ // so it is not a stack slot.
+ } else {
+ assert(offset >= 0, "unexpected negative offset");
+ offset -= (offset % jintSize); // count the whole word
+ int stack_reg = regalloc->offset2reg(offset);
+ if (OptoReg::is_stack(stack_reg)) {
+ set_live_bit(tmp_live, stack_reg);
+ } else {
+ assert(false, "stack_reg not on stack?");
+ }
+ }
+ }
+ }
+
+ if( n->jvms() ) { // Record liveness at safepoint
+
+ // This placement of this stanza means inputs to calls are
+ // considered live at the callsite's OopMap. Argument oops are
+ // hence live, but NOT included in the oopmap. See cutout in
+ // build_oop_map. Debug oops are live (and in OopMap).
+ int *n_live = NEW_ARENA_ARRAY(A, int, max_reg_ints);
+ for( int l=0; l<max_reg_ints; l++ )
+ n_live[l] = tmp_live[l];
+ safehash->Insert(n,n_live);
+ }
+
+ }
+
+ // Now at block top, see if we have any changes. If so, propagate
+ // to prior blocks.
+ int *old_live = &live[b->_pre_order*max_reg_ints];
+ int l;
+ for( l=0; l<max_reg_ints; l++ )
+ if( tmp_live[l] != old_live[l] )
+ break;
+ if( l<max_reg_ints ) { // Change!
+ // Copy in new value
+ for( l=0; l<max_reg_ints; l++ )
+ old_live[l] = tmp_live[l];
+ // Push preds onto worklist
+ for( l=1; l<(int)b->num_preds(); l++ )
+ worklist->push(cfg->_bbs[b->pred(l)->_idx]);
+ }
+ }
+
+ // Scan for any missing safepoints. Happens to infinite loops
+ // ala ZKM.jar
+ uint i;
+ for( i=1; i<cfg->_num_blocks; i++ ) {
+ Block *b = cfg->_blocks[i];
+ uint j;
+ for( j=1; j<b->_nodes.size(); j++ )
+ if( b->_nodes[j]->jvms() &&
+ (*safehash)[b->_nodes[j]] == NULL )
+ break;
+ if( j<b->_nodes.size() ) break;
+ }
+ if( i == cfg->_num_blocks )
+ break; // Got 'em all
+#ifndef PRODUCT
+ if( PrintOpto && Verbose )
+ tty->print_cr("retripping live calc");
+#endif
+ // Force the issue (expensively): recheck everybody
+ for( i=1; i<cfg->_num_blocks; i++ )
+ worklist->push(cfg->_blocks[i]);
+ }
+
+}
+
+//------------------------------BuildOopMaps-----------------------------------
+// Collect GC mask info - where are all the OOPs?
+void Compile::BuildOopMaps() {
+ NOT_PRODUCT( TracePhase t3("bldOopMaps", &_t_buildOopMaps, TimeCompiler); )
+ // Can't resource-mark because I need to leave all those OopMaps around,
+ // or else I need to resource-mark some arena other than the default.
+ // ResourceMark rm; // Reclaim all OopFlows when done
+ int max_reg = _regalloc->_max_reg; // Current array extent
+
+ Arena *A = Thread::current()->resource_area();
+ Block_List worklist; // Worklist of pending blocks
+
+ int max_reg_ints = round_to(max_reg, BitsPerInt)>>LogBitsPerInt;
+ Dict *safehash = NULL; // Used for assert only
+ // Compute a backwards liveness per register. Needs a bitarray of
+ // #blocks x (#registers, rounded up to ints)
+ safehash = new Dict(cmpkey,hashkey,A);
+ do_liveness( _regalloc, _cfg, &worklist, max_reg_ints, A, safehash );
+ OopFlow *free_list = NULL; // Free, unused
+
+ // Array mapping blocks to completed oopflows
+ OopFlow **flows = NEW_ARENA_ARRAY(A, OopFlow*, _cfg->_num_blocks);
+ memset( flows, 0, _cfg->_num_blocks*sizeof(OopFlow*) );
+
+
+ // Do the first block 'by hand' to prime the worklist
+ Block *entry = _cfg->_blocks[1];
+ OopFlow *rootflow = OopFlow::make(A,max_reg);
+ // Initialize to 'bottom' (not 'top')
+ memset( rootflow->_callees, OptoReg::Bad, max_reg*sizeof(short) );
+ memset( rootflow->_defs , 0, max_reg*sizeof(Node*) );
+ flows[entry->_pre_order] = rootflow;
+
+ // Do the first block 'by hand' to prime the worklist
+ rootflow->_b = entry;
+ rootflow->compute_reach( _regalloc, max_reg, safehash );
+ for( uint i=0; i<entry->_num_succs; i++ )
+ worklist.push(entry->_succs[i]);
+
+ // Now worklist contains blocks which have some, but perhaps not all,
+ // predecessors visited.
+ while( worklist.size() ) {
+ // Scan for a block with all predecessors visited, or any randoms slob
+ // otherwise. All-preds-visited order allows me to recycle OopFlow
+ // structures rapidly and cut down on the memory footprint.
+ // Note: not all predecessors might be visited yet (must happen for
+ // irreducible loops). This is OK, since every live value must have the
+ // SAME reaching def for the block, so any reaching def is OK.
+ uint i;
+
+ Block *b = worklist.pop();
+ // Ignore root block
+ if( b == _cfg->_broot ) continue;
+ // Block is already done? Happens if block has several predecessors,
+ // he can get on the worklist more than once.
+ if( flows[b->_pre_order] ) continue;
+
+ // If this block has a visited predecessor AND that predecessor has this
+ // last block as his only undone child, we can move the OopFlow from the
+ // pred to this block. Otherwise we have to grab a new OopFlow.
+ OopFlow *flow = NULL; // Flag for finding optimized flow
+ Block *pred = (Block*)0xdeadbeef;
+ uint j;
+ // Scan this block's preds to find a done predecessor
+ for( j=1; j<b->num_preds(); j++ ) {
+ Block *p = _cfg->_bbs[b->pred(j)->_idx];
+ OopFlow *p_flow = flows[p->_pre_order];
+ if( p_flow ) { // Predecessor is done
+ assert( p_flow->_b == p, "cross check" );
+ pred = p; // Record some predecessor
+ // If all successors of p are done except for 'b', then we can carry
+ // p_flow forward to 'b' without copying, otherwise we have to draw
+ // from the free_list and clone data.
+ uint k;
+ for( k=0; k<p->_num_succs; k++ )
+ if( !flows[p->_succs[k]->_pre_order] &&
+ p->_succs[k] != b )
+ break;
+
+ // Either carry-forward the now-unused OopFlow for b's use
+ // or draw a new one from the free list
+ if( k==p->_num_succs ) {
+ flow = p_flow;
+ break; // Found an ideal pred, use him
+ }
+ }
+ }
+
+ if( flow ) {
+ // We have an OopFlow that's the last-use of a predecessor.
+ // Carry it forward.
+ } else { // Draw a new OopFlow from the freelist
+ if( !free_list )
+ free_list = OopFlow::make(A,max_reg);
+ flow = free_list;
+ assert( flow->_b == NULL, "oopFlow is not free" );
+ free_list = flow->_next;
+ flow->_next = NULL;
+
+ // Copy/clone over the data
+ flow->clone(flows[pred->_pre_order], max_reg);
+ }
+
+ // Mark flow for block. Blocks can only be flowed over once,
+ // because after the first time they are guarded from entering
+ // this code again.
+ assert( flow->_b == pred, "have some prior flow" );
+ flow->_b = NULL;
+
+ // Now push flow forward
+ flows[b->_pre_order] = flow;// Mark flow for this block
+ flow->_b = b;
+ flow->compute_reach( _regalloc, max_reg, safehash );
+
+ // Now push children onto worklist
+ for( i=0; i<b->_num_succs; i++ )
+ worklist.push(b->_succs[i]);
+
+ }
+}