[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
This commit is contained in:
Roman Lebedev 2021-07-26 02:38:40 +03:00
parent e58ce35f7b
commit 59a5964e03
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GPG Key ID: 083C3EBB4A1689E0
4 changed files with 48 additions and 5 deletions

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@ -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.

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@ -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

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@ -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]]
;

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@ -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]]
;