[llvm] r324572 - [SimplifyCFG] Re-apply Relax restriction for folding unconditional branches
Serguei Katkov via llvm-commits
llvm-commits at lists.llvm.org
Tue Feb 13 09:11:34 PST 2018
Just to clarify (I'm not sure I was clear here :)).
1) The motivation was to remove BBs in a loop.
2) Removing of empty pre-header is a kind of side-effect.
3) It is interesting why removing of pre-headers results in perfromance regressions.
Thank you,
Serguei.
> -----Original Message-----
> From: llvm-commits [mailto:llvm-commits-bounces at lists.llvm.org] On Behalf
> Of Serguei Katkov via llvm-commits
> Sent: Wednesday, February 14, 2018 12:08 AM
> To: David Green <David.Green at arm.com>; llvm-commits <llvm-
> commits at lists.llvm.org>
> Cc: nd <nd at arm.com>
> Subject: RE: [llvm] r324572 - [SimplifyCFG] Re-apply Relax restriction for
> folding unconditional branches
>
> Hello David,
>
> the main idea was removing empty BB in a loop coming to header (latch) but
> not a removing of pre-header actually.
>
> However it is not clear why removing an empty pre-header comes to
> regressions. Jut interesting to know about the reason.
>
> Thank you,
> Serguei.
>
> > -----Original Message-----
> > From: David Green [mailto:David.Green at arm.com]
> > Sent: Tuesday, February 13, 2018 11:40 PM
> > To: Serguei Katkov <serguei.katkov at azul.com>; llvm-commits <llvm-
> > commits at lists.llvm.org>
> > Cc: nd <nd at arm.com>
> > Subject: Re: [llvm] r324572 - [SimplifyCFG] Re-apply Relax restriction
> > for folding unconditional branches
> >
> > Hello
> >
> > First up, sorry for the lateness of this response.
> >
> > Secondly, I am seeing a fair number of small to medium size
> > performance regressions from this patch, without many improvements (on
> > ARM). Is the idea that this should be removing preheaders? Or just empty
> blocks in loops?
> >
> > The only regressions I have analysed so far have turned into "code is
> > in a different order, the backend does something slightly different as a
> result".
> > And many regressions only appear to show up when LTO is enabled.
> >
> > So I don't have any sensible reproducers yet, but any thoughts about
> > it removing preheaders? It doesn't seem like that was the intent.
> > Dave
> >
> >
> >
> > From: llvm-commits <llvm-commits-bounces at lists.llvm.org> on behalf of
> > Serguei Katkov via llvm-commits <llvm-commits at lists.llvm.org>
> > Sent: 08 February 2018 07:16
> > To: llvm-commits at lists.llvm.org
> > Subject: [llvm] r324572 - [SimplifyCFG] Re-apply Relax restriction for
> > folding unconditional branches
> >
> > Author: skatkov
> > Date: Wed Feb 7 23:16:29 2018
> > New Revision: 324572
> >
> > URL: http://llvm.org/viewvc/llvm-project?rev=324572&view=rev
> > Log:
> > [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
> >
> > Added:
> > llvm/trunk/test/Transforms/SimplifyCFG/UncondBranchToHeader.ll
> > Modified:
> > llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp
> > llvm/trunk/test/Transforms/LoopUnroll/peel-loop.ll
> > llvm/trunk/test/Transforms/LoopUnswitch/2015-06-17-Metadata.ll
> > llvm/trunk/test/Transforms/LoopUnswitch/infinite-loop.ll
> >
> > Modified: llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp
> > URL: http://llvm.org/viewvc/llvm-
> > project/llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp?rev=324572&r1=
> > 32
> > 4571&r2=324572&view=diff
> >
> ==========================================================
> > ====================
> > --- llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp (original)
> > +++ llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp Wed Feb 7
> > +++ 23:16:29
> > +++ 2018
> > @@ -5733,9 +5733,12 @@ bool SimplifyCFGOpt::SimplifyUncondBranc
> > // 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))
> >
> > Modified: llvm/trunk/test/Transforms/LoopUnroll/peel-loop.ll
> > URL: http://llvm.org/viewvc/llvm-
> > project/llvm/trunk/test/Transforms/LoopUnroll/peel-
> > loop.ll?rev=324572&r1=324571&r2=324572&view=diff
> >
> ==========================================================
> > ====================
> > --- llvm/trunk/test/Transforms/LoopUnroll/peel-loop.ll (original)
> > +++ llvm/trunk/test/Transforms/LoopUnroll/peel-loop.ll Wed Feb 7
> > +++ 23:16:29 2018
> > @@ -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:
> > ; 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:
> >
> > Modified: llvm/trunk/test/Transforms/LoopUnswitch/2015-06-17-
> > Metadata.ll
> > URL: http://llvm.org/viewvc/llvm-
> > project/llvm/trunk/test/Transforms/LoopUnswitch/2015-06-17-
> > Metadata.ll?rev=324572&r1=324571&r2=324572&view=diff
> >
> ==========================================================
> > ====================
> > --- llvm/trunk/test/Transforms/LoopUnswitch/2015-06-17-Metadata.ll
> > (original)
> > +++ llvm/trunk/test/Transforms/LoopUnswitch/2015-06-17-Metadata.ll
> Wed
> > +++ Feb 7 23:16:29 2018
> > @@ -16,7 +16,7 @@ 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 ]
> >
> > Modified: llvm/trunk/test/Transforms/LoopUnswitch/infinite-loop.ll
> > URL: http://llvm.org/viewvc/llvm-
> > project/llvm/trunk/test/Transforms/LoopUnswitch/infinite-
> > loop.ll?rev=324572&r1=324571&r2=324572&view=diff
> >
> ==========================================================
> > ====================
> > --- llvm/trunk/test/Transforms/LoopUnswitch/infinite-loop.ll
> > (original)
> > +++ llvm/trunk/test/Transforms/LoopUnswitch/infinite-loop.ll Wed Feb
> > +++ 7
> > +++ 23:16:29 2018
> > @@ -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
> >
> > Added:
> llvm/trunk/test/Transforms/SimplifyCFG/UncondBranchToHeader.ll
> > URL: http://llvm.org/viewvc/llvm-
> >
> project/llvm/trunk/test/Transforms/SimplifyCFG/UncondBranchToHeader.ll
> > ?rev=324572&view=auto
> >
> ==========================================================
> > ====================
> > --- llvm/trunk/test/Transforms/SimplifyCFG/UncondBranchToHeader.ll
> > (added)
> > +++ llvm/trunk/test/Transforms/SimplifyCFG/UncondBranchToHeader.ll
> > Wed
> > +++ Feb 7 23:16:29 2018
> > @@ -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
> > +}
> >
> >
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