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[SimplifyCFG] Prefer a simplification based on a dominating condition. 

Rather than merge two branches with a common destination.
Differential Revision: http://reviews.llvm.org/D19743

llvm-svn: 268735
This commit is contained in:
Chad Rosier 2016-05-06 14:25:14 +00:00
parent c7e4239fab
commit 4ab37c0037
2 changed files with 72 additions and 20 deletions
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44
llvm/lib/Transforms/Utils/SimplifyCFG.cpp
View File

@ -2706,26 +2706,6 @@ static bool SimplifyCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI,
if (CE->canTrap())
return false;
// If BI is reached from the true path of PBI and PBI's condition implies
// BI's condition, we know the direction of the BI branch.
if ((PBI->getSuccessor(0) == BI->getParent() ||
PBI->getSuccessor(1) == BI->getParent()) &&
PBI->getSuccessor(0) != PBI->getSuccessor(1) &&
BB->getSinglePredecessor()) {
bool FalseDest = PBI->getSuccessor(1) == BI->getParent();
Optional<bool> Implication = isImpliedCondition(
PBI->getCondition(), BI->getCondition(), DL, FalseDest);
if (Implication) {
// Turn this into a branch on constant.
auto *OldCond = BI->getCondition();
ConstantInt *CI = *Implication ? ConstantInt::getTrue(BB->getContext())
: ConstantInt::getFalse(BB->getContext());
BI->setCondition(CI);
RecursivelyDeleteTriviallyDeadInstructions(OldCond);
return true; // Nuke the branch on constant.
}
}
// If both branches are conditional and both contain stores to the same
// address, remove the stores from the conditionals and create a conditional
// merged store at the end.
@ -5149,6 +5129,30 @@ bool SimplifyCFGOpt::SimplifyCondBranch(BranchInst *BI, IRBuilder<> &Builder) {
if (SimplifyBranchOnICmpChain(BI, Builder, DL))
return true;
// If this basic block has a single dominating predecessor block and the
// dominating block's condition implies BI's condition, we know the direction
// of the BI branch.
if (BasicBlock *Dom = BB->getSinglePredecessor()) {
auto *PBI = dyn_cast_or_null<BranchInst>(Dom->getTerminator());
if (PBI && PBI->isConditional() &&
PBI->getSuccessor(0) != PBI->getSuccessor(1) &&
(PBI->getSuccessor(0) == BB || PBI->getSuccessor(1) == BB)) {
bool CondIsFalse = PBI->getSuccessor(1) == BB;
Optional<bool> Implication = isImpliedCondition(
PBI->getCondition(), BI->getCondition(), DL, CondIsFalse);
if (Implication) {
// Turn this into a branch on constant.
auto *OldCond = BI->getCondition();
ConstantInt *CI = *Implication
? ConstantInt::getTrue(BB->getContext())
: ConstantInt::getFalse(BB->getContext());
BI->setCondition(CI);
RecursivelyDeleteTriviallyDeadInstructions(OldCond);
return SimplifyCFG(BB, TTI, BonusInstThreshold, AC) | true;
}
}
}
// If this basic block is ONLY a compare and a branch, and if a predecessor
// branches to us and one of our successors, fold the comparison into the
// predecessor and use logical operations to pick the right destination.

48
llvm/test/Transforms/SimplifyCFG/basictest.ll
View File

@ -25,6 +25,54 @@ define void @test3(i1 %T) {
; CHECK-NEXT: ret void
}
; Folding branch to a common destination.
; CHECK-LABEL: @test4_fold
; CHECK: %cmp1 = icmp eq i32 %a, %b
; CHECK: %cmp2 = icmp ugt i32 %a, 0
; CHECK: %or.cond = and i1 %cmp1, %cmp2
; CHECK: br i1 %or.cond, label %else, label %untaken
; CHECK-NOT: taken:
; CHECK: ret void
define void @test4_fold(i32 %a, i32 %b) {
%cmp1 = icmp eq i32 %a, %b
br i1 %cmp1, label %taken, label %untaken
taken:
%cmp2 = icmp ugt i32 %a, 0
br i1 %cmp2, label %else, label %untaken
else:
call void @foo()
ret void
untaken:
ret void
}
; Prefer a simplification based on a dominating condition rather than folding a
; branch to a common destination.
; CHECK-LABEL: @test4
; CHECK-NOT: br
; CHECK-NOT: br
; CHECK-NOT: call
; CHECK: ret void
define void @test4_no_fold(i32 %a, i32 %b) {
%cmp1 = icmp eq i32 %a, %b
br i1 %cmp1, label %taken, label %untaken
taken:
%cmp2 = icmp ugt i32 %a, %b
br i1 %cmp2, label %else, label %untaken
else:
call void @foo()
ret void
untaken:
ret void
}
declare void @foo()
; PR5795
define void @test5(i32 %A) {