[llvm] 59a5964 - [SimplifyCFG] Don't speculatively execute BB[s] if they are predictably not taken
Roman Lebedev via llvm-commits
llvm-commits at lists.llvm.org
Mon Jul 26 15:09:37 PDT 2021
On Mon, Jul 26, 2021 at 6:27 PM Philip Reames <listmail at philipreames.com> wrote:
>
> Style wise, could we extract a predictedNotTakenSuccessor interface?
> Seems like a reoccurring pattern which would be nice to extract.
It does seem reoccurring, but i'm not quite sure how it would look best,
so for now i've kept reinventing the weel.
> Philip
Roman
> On 7/25/21 5:01 PM, Roman Lebedev via llvm-commits wrote:
> > Author: Roman Lebedev
> > Date: 2021-07-26T02:55:15+03:00
> > New Revision: 59a5964e033c92f3b4866429552f20979c998e56
> >
> > URL: https://github.com/llvm/llvm-project/commit/59a5964e033c92f3b4866429552f20979c998e56
> > DIFF: https://github.com/llvm/llvm-project/commit/59a5964e033c92f3b4866429552f20979c998e56.diff
> >
> > LOG: [SimplifyCFG] Don't speculatively execute BB[s] if they are predictably not taken
> >
> > Same as D106650, but for `FoldTwoEntryPHINode()`
> >
> > Reviewed By: spatel
> >
> > Differential Revision: https://reviews.llvm.org/D106717
> >
> > Added:
> >
> >
> > Modified:
> > llvm/lib/Transforms/Utils/SimplifyCFG.cpp
> > llvm/test/Transforms/PGOProfile/chr.ll
> > llvm/test/Transforms/SimplifyCFG/fold-two-entry-phi-node-with-one-block-profmd.ll
> > llvm/test/Transforms/SimplifyCFG/fold-two-entry-phi-node-with-two-blocks-profmd.ll
> >
> > Removed:
> >
> >
> >
> > ################################################################################
> > diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
> > index f2f15199ccfd..7a88e1cab73c 100644
> > --- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
> > +++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
> > @@ -2736,7 +2736,33 @@ static bool FoldTwoEntryPHINode(PHINode *PN, const TargetTransformInfo &TTI,
> > PN->blocks(), std::back_inserter(IfBlocks), [](BasicBlock *IfBlock) {
> > return cast<BranchInst>(IfBlock->getTerminator())->isUnconditional();
> > });
> > - assert(!IfBlocks.empty() && "Will have at least one block to speculate.");
> > + assert((IfBlocks.size() == 1 || IfBlocks.size() == 2) &&
> > + "Will have either one or two blocks to speculate.");
> > +
> > + // If the branch is non-unpredictable, see if we either predictably jump to
> > + // the merge bb (if we have only a single 'then' block), or if we predictably
> > + // jump to one specific 'then' block (if we have two of them).
> > + // It isn't beneficial to speculatively execute the code
> > + // from the block that we know is predictably not entered.
> > + if (!DomBI->getMetadata(LLVMContext::MD_unpredictable)) {
> > + uint64_t TWeight, FWeight;
> > + if (DomBI->extractProfMetadata(TWeight, FWeight) &&
> > + (TWeight + FWeight) != 0) {
> > + BranchProbability BITrueProb =
> > + BranchProbability::getBranchProbability(TWeight, TWeight + FWeight);
> > + BranchProbability Likely = TTI.getPredictableBranchThreshold();
> > + BranchProbability BIFalseProb = BITrueProb.getCompl();
> > + if (IfBlocks.size() == 1) {
> > + BranchProbability BIBBProb =
> > + DomBI->getSuccessor(0) == BB ? BITrueProb : BIFalseProb;
> > + if (BIBBProb >= Likely)
> > + return false;
> > + } else {
> > + if (BITrueProb >= Likely || BIFalseProb >= Likely)
> > + return false;
> > + }
> > + }
> > + }
> >
> > // Don't try to fold an unreachable block. For example, the phi node itself
> > // can't be the candidate if-condition for a select that we want to form.
> >
> > diff --git a/llvm/test/Transforms/PGOProfile/chr.ll b/llvm/test/Transforms/PGOProfile/chr.ll
> > index 35fad463ac4a..ec215269e1ae 100644
> > --- a/llvm/test/Transforms/PGOProfile/chr.ll
> > +++ b/llvm/test/Transforms/PGOProfile/chr.ll
> > @@ -2013,7 +2013,8 @@ define i64 @test_chr_22(i1 %i, i64* %j, i64 %v0) !prof !14 {
> > ; CHECK-NEXT: [[C1:%.*]] = icmp slt i64 [[V2]], 100
> > ; CHECK-NEXT: br i1 [[C1]], label [[BB0_SPLIT:%.*]], label [[BB0_SPLIT_NONCHR:%.*]], !prof [[PROF15]]
> > ; CHECK: common.ret:
> > -; CHECK-NEXT: ret i64 99
> > +; CHECK-NEXT: [[COMMON_RET_OP:%.*]] = phi i64 [ 99, [[BB0_SPLIT]] ], [ 99, [[BB0_SPLIT_NONCHR]] ]
> > +; CHECK-NEXT: ret i64 [[COMMON_RET_OP]]
> > ; CHECK: bb0.split:
> > ; CHECK-NEXT: [[V299:%.*]] = mul i64 [[V2]], 7860086430977039991
> > ; CHECK-NEXT: store i64 [[V299]], i64* [[J:%.*]], align 4
> >
> > diff --git a/llvm/test/Transforms/SimplifyCFG/fold-two-entry-phi-node-with-one-block-profmd.ll b/llvm/test/Transforms/SimplifyCFG/fold-two-entry-phi-node-with-one-block-profmd.ll
> > index 18734f658ef0..c81eb4b08620 100644
> > --- a/llvm/test/Transforms/SimplifyCFG/fold-two-entry-phi-node-with-one-block-profmd.ll
> > +++ b/llvm/test/Transforms/SimplifyCFG/fold-two-entry-phi-node-with-one-block-profmd.ll
> > @@ -59,8 +59,12 @@ define i32 @predictably_nontaken(i32 %a, i32 %b, i32 %c, i32 %d) {
> > ; CHECK-NEXT: entry:
> > ; CHECK-NEXT: call void @sideeffect0()
> > ; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[A:%.*]], [[B:%.*]]
> > +; CHECK-NEXT: br i1 [[CMP]], label [[END:%.*]], label [[COND_TRUE:%.*]], !prof [[PROF0]]
> > +; CHECK: cond.true:
> > ; CHECK-NEXT: [[V0:%.*]] = add i32 [[C:%.*]], [[D:%.*]]
> > -; CHECK-NEXT: [[RES:%.*]] = select i1 [[CMP]], i32 0, i32 [[V0]], !prof [[PROF0]]
> > +; CHECK-NEXT: br label [[END]]
> > +; CHECK: end:
> > +; CHECK-NEXT: [[RES:%.*]] = phi i32 [ [[V0]], [[COND_TRUE]] ], [ 0, [[ENTRY:%.*]] ]
> > ; CHECK-NEXT: call void @sideeffect1()
> > ; CHECK-NEXT: ret i32 [[RES]]
> > ;
> >
> > diff --git a/llvm/test/Transforms/SimplifyCFG/fold-two-entry-phi-node-with-two-blocks-profmd.ll b/llvm/test/Transforms/SimplifyCFG/fold-two-entry-phi-node-with-two-blocks-profmd.ll
> > index 22344f141e15..0951162b7e83 100644
> > --- a/llvm/test/Transforms/SimplifyCFG/fold-two-entry-phi-node-with-two-blocks-profmd.ll
> > +++ b/llvm/test/Transforms/SimplifyCFG/fold-two-entry-phi-node-with-two-blocks-profmd.ll
> > @@ -39,9 +39,15 @@ define i32 @predictably_taken(i32 %a, i32 %b, i32 %c, i32 %d) {
> > ; CHECK-NEXT: entry:
> > ; CHECK-NEXT: call void @sideeffect0()
> > ; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[A:%.*]], [[B:%.*]]
> > +; CHECK-NEXT: br i1 [[CMP]], label [[COND_TRUE:%.*]], label [[COND_FALSE:%.*]], !prof [[PROF0:![0-9]+]]
> > +; CHECK: cond.true:
> > ; CHECK-NEXT: [[V0:%.*]] = add i32 [[C:%.*]], [[D:%.*]]
> > +; CHECK-NEXT: br label [[END:%.*]]
> > +; CHECK: cond.false:
> > ; CHECK-NEXT: [[V1:%.*]] = sub i32 [[C]], [[D]]
> > -; CHECK-NEXT: [[RES:%.*]] = select i1 [[CMP]], i32 [[V0]], i32 [[V1]], !prof [[PROF0:![0-9]+]]
> > +; CHECK-NEXT: br label [[END]]
> > +; CHECK: end:
> > +; CHECK-NEXT: [[RES:%.*]] = phi i32 [ [[V0]], [[COND_TRUE]] ], [ [[V1]], [[COND_FALSE]] ]
> > ; CHECK-NEXT: call void @sideeffect1()
> > ; CHECK-NEXT: ret i32 [[RES]]
> > ;
> > @@ -99,9 +105,15 @@ define i32 @predictably_nontaken(i32 %a, i32 %b, i32 %c, i32 %d) {
> > ; CHECK-NEXT: entry:
> > ; CHECK-NEXT: call void @sideeffect0()
> > ; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[A:%.*]], [[B:%.*]]
> > +; CHECK-NEXT: br i1 [[CMP]], label [[COND_FALSE:%.*]], label [[COND_TRUE:%.*]], !prof [[PROF0]]
> > +; CHECK: cond.true:
> > ; CHECK-NEXT: [[V0:%.*]] = add i32 [[C:%.*]], [[D:%.*]]
> > +; CHECK-NEXT: br label [[END:%.*]]
> > +; CHECK: cond.false:
> > ; CHECK-NEXT: [[V1:%.*]] = sub i32 [[C]], [[D]]
> > -; CHECK-NEXT: [[RES:%.*]] = select i1 [[CMP]], i32 [[V1]], i32 [[V0]], !prof [[PROF0]]
> > +; CHECK-NEXT: br label [[END]]
> > +; CHECK: end:
> > +; CHECK-NEXT: [[RES:%.*]] = phi i32 [ [[V0]], [[COND_TRUE]] ], [ [[V1]], [[COND_FALSE]] ]
> > ; CHECK-NEXT: call void @sideeffect1()
> > ; CHECK-NEXT: ret i32 [[RES]]
> > ;
> >
> >
> >
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