[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
Sun Jul 25 17:01:29 PDT 2021
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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