[SimplifyCFG] Relax restriction for folding unconditional branches

The commit rL308422 introduces a restriction for folding unconditional
branches. Specifically if empty block with unconditional branch leads to
header of the loop then elimination of this basic block is prohibited.
However it seems this condition is redundantly strict.
If elimination of this basic block does not introduce more back edges
then we can eliminate this block.

The patch implements this relax of restriction.

Reviewers: efriedma, mcrosier, pacxx, hsung, davidxl	
Reviewed By: pacxx
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D42691

llvm-svn: 324208

5 files changed, 29 insertions, 13 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index c3343ed8ecc..bcea927925b 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -5733,9 +5733,12 @@ bool SimplifyCFGOpt::SimplifyUncondBranch(BranchInst *BI,
   // header. (This is for early invocations before loop simplify and
   // vectorization to keep canonical loop forms for nested loops. These blocks
   // can be eliminated when the pass is invoked later in the back-end.)
+  // Note that if BB has only one predecessor then we do not introduce new
+  // backedge, so we can eliminate BB.
   bool NeedCanonicalLoop =
       Options.NeedCanonicalLoop &&
-      (LoopHeaders && (LoopHeaders->count(BB) || LoopHeaders->count(Succ)));
+      (LoopHeaders && !BB->getSinglePredecessor() &&
+       (LoopHeaders->count(BB) || LoopHeaders->count(Succ)));
   BasicBlock::iterator I = BB->getFirstNonPHIOrDbg()->getIterator();
   if (I->isTerminator() && BB != &BB->getParent()->getEntryBlock() &&
       !NeedCanonicalLoop && TryToSimplifyUncondBranchFromEmptyBlock(BB))
diff --git a/llvm/test/Transforms/LoopUnroll/peel-loop.ll b/llvm/test/Transforms/LoopUnroll/peel-loop.ll
index 3f7c64d8154..55e818244a7 100644
--- a/llvm/test/Transforms/LoopUnroll/peel-loop.ll
+++ b/llvm/test/Transforms/LoopUnroll/peel-loop.ll
@@ -19,10 +19,8 @@
 ; CHECK: store i32 2, i32* %[[INC2]], align 4
 ; CHECK: %[[CMP3:.*]] = icmp eq i32 %k, 3
 ; CHECK: br i1 %[[CMP3]], label %for.end, label %[[LOOP_PH:.*]]
-; CHECK: [[LOOP_PH]]:
-; CHECK: br label %[[LOOP:.*]]
-; CHECK: [[LOOP]]:
-; CHECK: %[[IV:.*]] = phi i32 [ 3, %[[LOOP_PH]] ], [ {{.*}}, %[[LOOP]] ]
+; CHECK: for.end:
+; CHECK: ret void
 
 define void @basic(i32* %p, i32 %k) #0 {
 entry:
@@ -68,11 +66,8 @@ for.end:                                          ; preds = %for.cond.for.end_cr
 ; CHECK: store i32 2, i32* %[[INC2]], align 4
 ; CHECK: %[[CMP3:.*]] = icmp eq i32 %k, 3
 ; CHECK: br i1 %[[CMP3]], label %for.end, label %[[LOOP_PH:.*]]
-; CHECK: [[LOOP_PH]]:
-; CHECK: br label %[[LOOP:.*]]
-; CHECK: [[LOOP]]:
-; CHECK: %[[IV:.*]] = phi i32 [ 3, %[[LOOP_PH]] ], [ %[[IV:.*]], %[[LOOP]] ]
-; CHECK: %ret = phi i32 [ 0, %entry ], [ 1, %[[NEXT0]] ], [ 2, %[[NEXT1]] ], [ 3, %[[NEXT2]] ], [ %[[IV]], %[[LOOP]] ]
+; CHECK: for.end:
+; CHECK: %ret = phi i32 [ 0, %entry ], [ 1, %[[NEXT0]] ], [ 2, %[[NEXT1]] ], [ 3, %[[NEXT2]] ], [ %inc, %for.body ]
 ; CHECK: ret i32 %ret
 define i32 @output(i32* %p, i32 %k) #0 {
 entry:
diff --git a/llvm/test/Transforms/LoopUnswitch/2015-06-17-Metadata.ll b/llvm/test/Transforms/LoopUnswitch/2015-06-17-Metadata.ll
index 8652829bc51..a215be9d487 100644
--- a/llvm/test/Transforms/LoopUnswitch/2015-06-17-Metadata.ll
+++ b/llvm/test/Transforms/LoopUnswitch/2015-06-17-Metadata.ll
@@ -16,7 +16,7 @@ for.body:                                         ; preds = %for.inc, %for.body.
   %cmp1 = icmp eq i32 %a, 12345
   br i1 %cmp1, label %if.then, label %if.else, !prof !0
 ; CHECK: %cmp1 = icmp eq i32 %a, 12345
-; CHECK-NEXT: br i1 %cmp1, label %for.body.preheader.split.us, label %for.body.preheader.split, !prof !0
+; CHECK-NEXT: br i1 %cmp1, label %for.body.us, label %for.body, !prof !0
 if.then:                                          ; preds = %for.body
 ; CHECK: for.body.us:
 ; CHECK: add nsw i32 %{{.*}}, 123
@@ -53,7 +53,7 @@ entry:
   br label %for.body
 ;CHECK: entry:
 ;CHECK-NEXT: %cmp1 = icmp eq i32 1, 2
-;CHECK-NEXT: br i1 %cmp1, label %entry.split, label %for.cond.cleanup.split, !prof !1
+;CHECK-NEXT: br i1 %cmp1, label %for.body, label %for.cond.cleanup.split, !prof !1
 ;CHECK: for.body:
 for.body:                                         ; preds = %for.inc, %entry
   %inc.i = phi i32 [ 0, %entry ], [ %inc, %if.then ]
diff --git a/llvm/test/Transforms/LoopUnswitch/infinite-loop.ll b/llvm/test/Transforms/LoopUnswitch/infinite-loop.ll
index af8725b02a1..013355faf35 100644
--- a/llvm/test/Transforms/LoopUnswitch/infinite-loop.ll
+++ b/llvm/test/Transforms/LoopUnswitch/infinite-loop.ll
@@ -16,7 +16,7 @@
 ; CHECK-NEXT: br i1 %a, label %entry.split, label %abort0.split
 
 ; CHECK: entry.split:
-; CHECK-NEXT: br i1 %b, label %entry.split.split, label %abort1.split
+; CHECK-NEXT: br i1 %b, label %for.body, label %abort1.split
 
 ; CHECK: for.body:
 ; CHECK-NEXT: br label %for.body
diff --git a/llvm/test/Transforms/SimplifyCFG/UncondBranchToHeader.ll b/llvm/test/Transforms/SimplifyCFG/UncondBranchToHeader.ll
new file mode 100644
index 00000000000..6a265539e75
--- /dev/null
+++ b/llvm/test/Transforms/SimplifyCFG/UncondBranchToHeader.ll
@@ -0,0 +1,18 @@
+; RUN: opt < %s -simplifycfg -S | FileCheck %s
+
+; Check that we can get rid of empty block leading to header
+; if it does not introduce new edge.
+define i32 @test(i32 %c) {
+entry:
+  br label %header
+header:
+  %i = phi i32 [0, %entry], [%i.1, %backedge]
+  %i.1 = add i32 %i, 1
+  %cmp = icmp slt i32 %i.1, %c
+  br i1 %cmp, label %backedge, label %exit
+; CHECK-NOT: backedge:
+backedge:
+  br label %header
+exit:
+  ret i32 %i
+}