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[SimplifyCFG] Fix for "endless" loop after dead code removal (Alternative to 

D16251)

Summary:
This is a simpler fix to the problem than the dominator approach in
http://reviews.llvm.org/D16251. It adds only values into the gather() while loop
that have been seen before.

The actual endless loop is in the constant compare gather() routine in
Utils/SimplifyCFG.cpp. The same value ret.0.off0.i is pushed back into the
queue:
%.ret.0.off0.i = or i1 %.ret.0.off0.i, %cmp10.i

Here is what happens at the IR level:

for.cond.i:                                       ; preds = %if.end6.i,
%if.end.i54
%ix.0.i = phi i32 [ 0, %if.end.i54 ], [ %inc.i55, %if.end6.i ]
%ret.0.off0.i = phi i1 [false, %if.end.i54], [%.ret.0.off0.i, %if.end6.i] <<<
%cmp2.i = icmp ult i32 %ix.0.i, %11
br i1 %cmp2.i, label %for.body.i, label %LBJ_TmpSimpleNeedExt.exit

if.end6.i:                                        ; preds = %for.body.i
%cmp10.i = icmp ugt i32 %conv.i, %add9.i
%.ret.0.off0.i = or i1 %ret.0.off0.i, %cmp10.i <<<

When if.end.i54 gets eliminated which removes the definition of ret.0.off0.i.
The result is the expression %.ret.0.off0.i = or i1 %.ret.0.off0.i, %cmp10.i
(Note the first ‘or’ operand is now %.ret.0.off0.i, and *NOT* %ret.0.off0.i).
And
now there is use of .ret.0.off0.i before a definition which triggers the
“endless” loop in gather():

while(!DFT.empty()) {

    V = DFT.pop_back_val();   // V is .ret.0.off0.i

    if (Instruction *I = dyn_cast<Instruction>(V)) {
      // If it is a || (or && depending on isEQ), process the operands.
      if (I->getOpcode() == (isEQ ? Instruction::Or : Instruction::And)) {
        DFT.push_back(I->getOperand(1));  // This is now .ret.0.off0.i also
        DFT.push_back(I->getOperand(0));

        continue; // “endless loop” for .ret.0.off0.i
      }

Reviewers: reames, ahatanak

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D16839

llvm-svn: 259730
This commit is contained in:
Gerolf Hoflehner 2016-02-03 23:54:25 +00:00
parent 451ec4b68e
commit 2432bd0ddd
2 changed files with 107 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
llvm/lib/Transforms/Utils/SimplifyCFG.cpp
View File

@ -508,8 +508,10 @@ private:
// Keep a stack (SmallVector for efficiency) for depth-first traversal
SmallVector<Value *, 8> DFT;
SmallPtrSet<Value *, 8> Visited;
// Initialize
Visited.insert(V);
DFT.push_back(V);
while(!DFT.empty()) {
@ -518,8 +520,10 @@ private:
if (Instruction *I = dyn_cast<Instruction>(V)) {
// If it is a || (or && depending on isEQ), process the operands.
if (I->getOpcode() == (isEQ ? Instruction::Or : Instruction::And)) {
DFT.push_back(I->getOperand(1));
DFT.push_back(I->getOperand(0));
if (Visited.insert(I->getOperand(1)).second)
DFT.push_back(I->getOperand(1));
if (Visited.insert(I->getOperand(0)).second)
DFT.push_back(I->getOperand(0));
continue;
}

101
llvm/test/Transforms/SimplifyCFG/InfLoop.ll Normal file
View File

@ -0,0 +1,101 @@
; RUN: opt < %s -simplifycfg -disable-output
; END.
target datalayout = "e-m:o-p:32:32-f64:32:64-v64:32:64-v128:32:128-a:0:32-n32-S32"
target triple = "thumbv7-apple-ios9.0.0"
%struct.anon = type { %struct.anon.0, i32, i32, %union.T1 }
%struct.anon.0 = type { i32, [256 x i32], [256 x i8] }
%union.T1 = type { %struct.F}
%struct.F = type { i32 }
@U = internal global %struct.anon zeroinitializer, align 4
define void @main() {
entry:
%0 = load i32, i32* getelementptr inbounds (%struct.anon, %struct.anon* @U, i32 0, i32 2), align 4
%cmp.i = icmp eq i32 %0, -1
br i1 %cmp.i, label %if.then, label %if.end
if.then: ; preds = %entry
br label %if.end
if.end: ; preds = %entry, %if.then
%1 = load i32, i32* getelementptr inbounds (%struct.anon, %struct.anon* @U, i32 0, i32 2), align 4
%bf.load = load i32, i32* getelementptr inbounds (%struct.anon, %struct.anon* @U, i32 0, i32 3, i32 0, i32 0), align 4
%cmp = icmp slt i32 %0, 0
br i1 %cmp, label %if.end7, label %cond.false
cond.false: ; preds = %if.end
%add = and i32 %bf.load, 30
%shl = add nuw nsw i32 %add, 2
br label %if.end7
if.end7: ; preds = %if.end, %cond.false
%2 = icmp eq i32 %0, 1
br i1 %2, label %if.then9, label %if.else10
if.then9: ; preds = %if.end7
br label %if.end29
if.else10: ; preds = %if.end7
%cmp11 = icmp ugt i32 %0, 13
br i1 %cmp11, label %if.then12, label %if.else14
if.then12: ; preds = %if.else10
br label %if.end26
if.else14: ; preds = %if.else10
%tobool = icmp eq i1 %2, 0
br i1 %tobool, label %lor.rhs, label %if.then18
lor.rhs: ; preds = %if.else14
%tobool.not.i = icmp eq i1 %2, 0
br i1 %tobool.not.i, label %if.else21, label %if.end.i54
if.end.i54: ; preds = %lor.rhs
br label %for.cond.i
for.cond.i: ; preds = %if.end6.i, %if.end.i54
%ix.0.i = phi i32 [ 0, %if.end.i54 ], [ %inc.i55, %if.end6.i ]
%ret.0.off0.i = phi i1 [ false, %if.end.i54 ], [ %.ret.0.off0.i, %if.end6.i ]
%cmp2.i = icmp ult i32 %ix.0.i, 2
br i1 %cmp2.i, label %for.body.i, label %TmpSimpleNeedExt.exit
for.body.i: ; preds = %for.cond.i
%arrayidx.i = getelementptr inbounds %struct.anon, %struct.anon* @U, i32 0, i32 0, i32 2, i32 %ix.0.i
%elt = load i8, i8* %arrayidx.i, align 1
%cmp3.i = icmp sgt i8 %elt, 7
br i1 %cmp3.i, label %if.else21, label %if.end6.i
if.end6.i: ; preds = %for.body.i
%cmp10.i = icmp ugt i8 %elt, 59
%.ret.0.off0.i = or i1 %ret.0.off0.i, %cmp10.i
%inc.i55 = add i32 %ix.0.i, 1
br label %for.cond.i
TmpSimpleNeedExt.exit: ; preds = %for.body.i
br i1 %ret.0.off0.i, label %if.then18, label %if.else21
if.then18: ; preds = %if.else14, %TmpSimpleNeedExt.exit
br label %if.end26
if.else21: ; preds = %for.body.i, %lor.rhs, %TmpSimpleNeedExt.exit
br label %if.end26
if.end26: ; preds = %if.then18, %if.else21, %if.then12
%cmp.i51 = icmp slt i32 %0, 7
br i1 %cmp.i51, label %if.then.i, label %if.end.i
if.then.i: ; preds = %if.end26
br label %if.end.i
if.end.i: ; preds = %if.then.i, %if.end26
br label %if.end29
if.then2.i: ; preds = %if.end.i
br label %if.end29
if.end29: ; preds = %if.end.i, %if.then2.i, %if.then9
ret void
}