[SimplifyCFG] Re-apply 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.

The test profile/Linux/counter_promo_nest.c in compiler-rt project
is updated to meet this change.

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

llvm-svn: 324572
This commit is contained in:
Serguei Katkov 2018-02-08 07:16:29 +00:00
parent d535c4d9d2
commit 66182d6c38
5 changed files with 29 additions and 13 deletions
llvm
lib/Transforms/Utils
test/Transforms

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@ -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 && std::distance(pred_begin(BB), pred_end(BB)) > 1 &&
(LoopHeaders->count(BB) || LoopHeaders->count(Succ)));
BasicBlock::iterator I = BB->getFirstNonPHIOrDbg()->getIterator();
if (I->isTerminator() && BB != &BB->getParent()->getEntryBlock() &&
!NeedCanonicalLoop && TryToSimplifyUncondBranchFromEmptyBlock(BB))

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@ -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:

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@ -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 ]

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@ -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

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@ -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
}