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Fix problems with loop invariant removal optimization.

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These mainly related to situations where the optimization of multiple natural loops (including those created by continue statements) could interact to generate invalid results. Invalid optimizations could also be performed in certain other cases where there were multiple goto statements targeting a single label and at least one of them formed a loop.

These issues are addressed by appropriately adjusting the control flow and updating various data structures after each loop is processed during loop invariant removal.

This fixes #18 (compca18.c).
This commit is contained in:
Stephen Heumann 2017-12-11 23:20:51 -06:00
parent 89f4257742
commit 4e7a7e67e7
1 changed files with 178 additions and 71 deletions
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235
DAG.pas
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@ -3224,18 +3224,7 @@ var
end; {DepthFirstOrder}
procedure Dominators;
{ Find a list of dominators for each node }
var
bb: blockPtr; {used to trace the block list}
change: boolean; {for loop termination test}
i, j: integer; {loop variables}
maxdfn, mindfn: integer; {max and min dfn values used}
procedure Add (var dom: blockListPtr; dfn: integer);
procedure AddDominator (var dom: blockListPtr; dfn: integer);
{ Add dfn to the list of dominators }
{ }
@ -3246,14 +3235,25 @@ var
var
dp: blockListPtr; {new node}
begin {Add}
begin {AddDominator}
new(dp);
dp^.last := nil;
dp^.next := dom;
dom^.last := dp;
dom := dp;
dp^.dfn := dfn;
end; {Add}
end; {AddDominator}
procedure Dominators;
{ Find a list of dominators for each node }
var
bb: blockPtr; {used to trace the block list}
change: boolean; {for loop termination test}
i, j: integer; {loop variables}
maxdfn, mindfn: integer; {max and min dfn values used}
procedure CheckPredecessors (bb: blockPtr; bl: dftptr);
@ -3305,13 +3305,13 @@ var
maxdfn := bb^.dfn;
bb := bb^.next;
end; {while}
Add(DAGBlocks^.dom, DAGBlocks^.dfn); {the first node is it's own dominator}
AddDominator(DAGBlocks^.dom, DAGBlocks^.dfn); {the first node is it's own dominator}
mindfn := DAGBlocks^.dfn; {assume all other nodes are dominated by every other node}
for i := mindfn+1 to maxdfn do begin
bb := DFN(i);
if bb <> nil then
for j := mindfn to maxdfn do
Add(bb^.dom, j);
AddDominator(bb^.dom, j);
end; {for}
repeat {iterate to the true set of dominators}
change := false;
@ -3595,8 +3595,31 @@ var
loops: loopListPtr; {list of loops}
lp: loopPtr; {used to trace loop lists}
llp: loopListPtr; {used to trace the list of loops}
fakeDFN: integer; {to uniquely number newly-created blocks}
function InLoop (blk: blockPtr; lp: loopPtr): boolean;
{ See if the block is in the loop }
{ }
{ parameters: }
{ blk - block to check for }
{ lp - loop list }
{ }
{ Returns: True if blk is in the list, else false }
begin {InLoop}
InLoop := false;
while lp <> nil do begin
if lp^.block = blk then begin
lp := nil;
InLoop := true;
end {if}
else
lp := lp^.next;
end; {while}
end; {InLoop}
procedure FindLoops;
@ -3630,29 +3653,6 @@ var
end; {Add}
function InLoop (blk: blockPtr; lp: loopPtr): boolean;
{ See if the block is in the loop }
{ }
{ parameters: }
{ blk - block to check for }
{ lp - loop list }
{ }
{ Returns: True if blk is in the list, else false }
begin {InLoop}
InLoop := false;
while lp <> nil do begin
if lp^.block = blk then begin
lp := nil;
InLoop := true;
end {if}
else
lp := lp^.next;
end; {while}
end; {InLoop}
procedure Insert (block: blockPtr);
{ Insert a block into the loop list }
@ -3679,28 +3679,6 @@ var
end; {AddPredecessors}
function InLoop (block: blockPtr; lp: loopPtr): boolean;
{ See if a block is in the loop }
{ }
{ parameters: }
{ block - block to check }
{ lp - list of blocks in the loop }
{ }
{ Returns: True if the block is in the loop, else false }
begin {InLoop}
InLoop := false;
while lp <> nil do
if lp^.block = block then begin
InLoop := true;
lp := nil;
end {if}
else
lp := lp^.next;
end; {InLoop}
begin {Insert}
if not InLoop(block, llp^.loop) then begin
Add(block);
@ -4243,7 +4221,8 @@ var
var
icount, oldIcount: integer; {invariant order counters}
nhp: blockPtr; {new loop hedaer pointer}
nhp: blockPtr; {new loop header pointer}
ohp: blockPtr; {old loop header pointer}
op1, op2, op3: icptr; {used to reverse the code list}
@ -4252,11 +4231,10 @@ var
{ Create the new loop header }
{ }
{ Notes: As a side effect, CreateHeader sets nhp to point to }
{ the new loop header. }
{ the new loop header, and ohp to point to the old header. }
var
lp: loopPtr; {new loop list entry}
ohp: blockPtr; {old loop hedaer pointer}
begin {CreateHeader}
nhp := pointer(Calloc(sizeof(block))); {create the new block}
@ -4266,6 +4244,8 @@ var
nhp^.last^.next := nhp;
nhp^.next := ohp;
ohp^.last := nhp;
nhp^.dfn := fakeDFN; {just a unique number, not a real DFN}
fakeDFN := fakeDFN - 1;
new(lp); {add it to the loop list}
lp^.next := llp^.loop;
llp^.loop := lp;
@ -4465,6 +4445,125 @@ var
end; {RemoveInvariant}
procedure AdjustControlFlow;
{ Adjust control flow to account for loop invariant removal. }
{ The current loop's back edges should go to the old header }
{ block, bypassing removed invariant computations. Any other }
{ jumps to the start of the loop should go to the new header }
{ block so that those computations are performed. }
var
lp: loopPtr; {used to trace loop list}
op, op1: icptr; {used to trace code list}
begin {AdjustControlFlow}
{move old header label to new header}
{(for any jumps to it from outside loop)}
if (ohp^.code = nil) or (ohp^.code^.opcode <> dc_lab) then
TermError(3); {shouldn't happen, but let's be sure}
op1 := pointer(Calloc(sizeof(intermediate_code)));
op1^.opcode := dc_lab;
op1^.q := ohp^.code^.q;
op1^.next := nhp^.code;
nhp^.code := op1;
ohp^.code^.q := GenLabel; {make new label for old header &}
lp := llp^.loop; {adjust loop back edges to go to it}
while (lp <> nil) do begin
op := lp^.block^.code;
while op <> nil do begin
if op^.opcode in [pc_ujp,pc_fjp,pc_tjp,pc_add] then
if op^.q = op1^.q then begin
op^.q := ohp^.code^.q;
end;
op := op^.next;
end; {while}
lp := lp^.next;
end; {while}
end; {AdjustControlFlow}
procedure UpdateLoopLists;
{ Update not-yet-processed loops to include the new header }
{ block if appropriate. Also update any additional loops with }
{ the same original header to now include all the nodes of the }
{ loop just processed, since their back edges will now go to }
{ the new header, which dominates the original header. }
var
lp, lp2, lp3: loopPtr; {used to trace loop list}
begin {UpdateLoopLists}
loops := llp^.next;
while loops <> nil do begin
if loops^.loop^.block = ohp then begin
{Another loop with the same header.}
{Nodes of llp^.loop must be added to it.}
{They go after the original header.}
lp3 := loops^.loop;
lp := llp^.loop;
while lp <> nil do begin
if lp^.block <> nhp then
if not InLoop(lp^.block, loops^.loop) then begin
new(lp2);
lp2^.next := lp3^.next;
lp2^.block := lp^.block;
lp2^.exit := lp^.exit;
lp3^.next := lp2;
lp3 := lp2;
end; {if}
lp := lp^.next;
end; {while}
end; {if}
lp := loops^.loop; {Add nhp to other loops containing ohp}
while lp <> nil do begin
if lp^.block = ohp then begin
new(lp2);
lp2^.next := lp^.next;
lp2^.block := lp^.block;
lp2^.exit := lp^.exit;
lp^.next := lp2;
lp^.block := nhp;
lp^.exit := false;
lp := nil;
end {if}
else
lp := lp^.next;
end; {while}
loops := loops^.next;
end; {while}
end; {UpdateLoopLists}
procedure UpdateDominators;
{ Set dominators of the new header block, and update }
{ dominators of other blocks to include it where appropriate. }
var
bb: blockPtr; {used to trace list of basic blocks}
dom: blockListPtr; {used to trace dominator list}
begin {UpdateDominators}
dom := ohp^.dom; {Set dominators of new header block}
while dom <> nil do begin
if dom^.dfn <> ohp^.dfn then
AddDominator(nhp^.dom, dom^.dfn);
dom := dom^.next;
end; {while}
AddDominator(nhp^.dom, nhp^.dfn);
bb := DAGBlocks; {Add nhp to other loops' dominators}
while bb <> nil do begin
if MemberDFNList(ohp^.dfn, bb^.dom) then
AddDominator(bb^.dom, nhp^.dfn);
bb := bb^.next;
end; {while}
end; {UpdateDominators}
begin {RemoveInvariants}
CreateHeader; {create a loop header block}
icount := 0; {find & remove all invariants}
@ -4483,6 +4582,13 @@ var
op2 := op3;
end; {while}
nhp^.code := op2;
{adjust things to account for changes}
if nhp^.code <> nil then begin
Spin;
AdjustControlFlow;
UpdateLoopLists;
UpdateDominators;
end; {if}
end; {RemoveInvariants}
@ -4529,6 +4635,7 @@ var
begin {LoopInvariantRemoval}
Spin;
FindLoops; {find a list of natural loops}
fakeDFN := -1;
llp := loops; {scan the loops...}
icount := 1;