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[SimplfyCFG] Set MD_prof for select used for certain conditional simplifications #154426

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Sep 12, 2025
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[SimplfyCFG] Set MD_prof for select used for certain conditional simplifications #154426

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32 changes: 31 additions & 1 deletion llvm/lib/Transforms/Utils/SimplifyCFG.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -332,6 +332,16 @@ class SimplifyCFGOpt {
}
};

// we synthesize a || b as select a, true, b
// we synthesize a && b as select a, b, false
// this function determines if SI is playing one of those roles.
bool isSelectInRoleOfConjunctionOrDisjunction(const SelectInst *SI) {
return ((isa<ConstantInt>(SI->getTrueValue()) &&
(dyn_cast<ConstantInt>(SI->getTrueValue())->isOne())) ||
(isa<ConstantInt>(SI->getFalseValue()) &&
(dyn_cast<ConstantInt>(SI->getFalseValue())->isNullValue())));
}

} // end anonymous namespace

/// Return true if all the PHI nodes in the basic block \p BB
Expand Down Expand Up @@ -4033,6 +4043,7 @@ static bool performBranchToCommonDestFolding(BranchInst *BI, BranchInst *PBI,

// Try to update branch weights.
uint64_t PredTrueWeight, PredFalseWeight, SuccTrueWeight, SuccFalseWeight;
SmallVector<uint32_t, 2> MDWeights;
if (extractPredSuccWeights(PBI, BI, PredTrueWeight, PredFalseWeight,
SuccTrueWeight, SuccFalseWeight)) {
SmallVector<uint64_t, 8> NewWeights;
Expand Down Expand Up @@ -4063,7 +4074,7 @@ static bool performBranchToCommonDestFolding(BranchInst *BI, BranchInst *PBI,
// Halve the weights if any of them cannot fit in an uint32_t
fitWeights(NewWeights);

SmallVector<uint32_t, 8> MDWeights(NewWeights.begin(), NewWeights.end());
append_range(MDWeights, NewWeights);
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Why this change?

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I need MDWeights in the newly introduced code.

setBranchWeights(PBI, MDWeights[0], MDWeights[1], /*IsExpected=*/false);

// TODO: If BB is reachable from all paths through PredBlock, then we
Expand Down Expand Up @@ -4100,6 +4111,13 @@ static bool performBranchToCommonDestFolding(BranchInst *BI, BranchInst *PBI,
Value *BICond = VMap[BI->getCondition()];
PBI->setCondition(
createLogicalOp(Builder, Opc, PBI->getCondition(), BICond, "or.cond"));
if (!ProfcheckDisableMetadataFixes)
if (auto *SI = dyn_cast<SelectInst>(PBI->getCondition()))
if (!MDWeights.empty()) {
assert(isSelectInRoleOfConjunctionOrDisjunction(SI));
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Note: I preferred assert-ing here (and below) because the expectation is the condition comes from createLogicalOp and the only reason we get a select is because of how or or and get lowered there.

setBranchWeights(SI, MDWeights[0], MDWeights[1],
/*IsExpected=*/false);
}

++NumFoldBranchToCommonDest;
return true;
Expand Down Expand Up @@ -4812,6 +4830,18 @@ static bool SimplifyCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI,
fitWeights(NewWeights);

setBranchWeights(PBI, NewWeights[0], NewWeights[1], /*IsExpected=*/false);
// Cond may be a select instruction with the first operand set to "true", or
// the second to "false" (see how createLogicalOp works for `and` and `or`)
if (!ProfcheckDisableMetadataFixes)
if (auto *SI = dyn_cast<SelectInst>(Cond)) {
assert(isSelectInRoleOfConjunctionOrDisjunction(SI));
// The select is predicated on PBICond
assert(dyn_cast<SelectInst>(SI)->getCondition() == PBICond);
// The corresponding probabilities are what was referred to above as
// PredCommon and PredOther.
setBranchWeights(SI, PredCommon, PredOther,
/*IsExpected=*/false);
}
}

// OtherDest may have phi nodes. If so, add an entry from PBI's
Expand Down
29 changes: 20 additions & 9 deletions llvm/test/Transforms/SimplifyCFG/branch-fold-threshold.ll
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@@ -1,4 +1,4 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --check-globals all --version 5
; RUN: opt %s -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 -S | FileCheck %s --check-prefixes=NORMAL,BASELINE
; RUN: opt %s -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 -S -bonus-inst-threshold=2 | FileCheck %s --check-prefixes=NORMAL,AGGRESSIVE
; RUN: opt %s -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 -S -bonus-inst-threshold=4 | FileCheck %s --check-prefixes=WAYAGGRESSIVE
Expand All @@ -11,12 +11,12 @@ define i32 @foo(i32 %a, i32 %b, i32 %c, i32 %d, ptr %input) {
; BASELINE-SAME: i32 [[A:%.*]], i32 [[B:%.*]], i32 [[C:%.*]], i32 [[D:%.*]], ptr [[INPUT:%.*]]) {
; BASELINE-NEXT: [[ENTRY:.*]]:
; BASELINE-NEXT: [[CMP:%.*]] = icmp sgt i32 [[D]], 3
; BASELINE-NEXT: br i1 [[CMP]], label %[[COND_END:.*]], label %[[LOR_LHS_FALSE:.*]]
; BASELINE-NEXT: br i1 [[CMP]], label %[[COND_END:.*]], label %[[LOR_LHS_FALSE:.*]], !prof [[PROF0:![0-9]+]]
; BASELINE: [[LOR_LHS_FALSE]]:
; BASELINE-NEXT: [[MUL:%.*]] = shl i32 [[C]], 1
; BASELINE-NEXT: [[ADD:%.*]] = add nsw i32 [[MUL]], [[A]]
; BASELINE-NEXT: [[CMP1:%.*]] = icmp slt i32 [[ADD]], [[B]]
; BASELINE-NEXT: br i1 [[CMP1]], label %[[COND_FALSE:.*]], label %[[COND_END]]
; BASELINE-NEXT: br i1 [[CMP1]], label %[[COND_FALSE:.*]], label %[[COND_END]], !prof [[PROF1:![0-9]+]]
; BASELINE: [[COND_FALSE]]:
; BASELINE-NEXT: [[TMP0:%.*]] = load i32, ptr [[INPUT]], align 4
; BASELINE-NEXT: br label %[[COND_END]]
Expand All @@ -31,8 +31,8 @@ define i32 @foo(i32 %a, i32 %b, i32 %c, i32 %d, ptr %input) {
; AGGRESSIVE-NEXT: [[MUL:%.*]] = shl i32 [[C]], 1
; AGGRESSIVE-NEXT: [[ADD:%.*]] = add nsw i32 [[MUL]], [[A]]
; AGGRESSIVE-NEXT: [[CMP1:%.*]] = icmp slt i32 [[ADD]], [[B]]
; AGGRESSIVE-NEXT: [[OR_COND:%.*]] = select i1 [[CMP]], i1 [[CMP1]], i1 false
; AGGRESSIVE-NEXT: br i1 [[OR_COND]], label %[[COND_FALSE:.*]], label %[[COND_END:.*]]
; AGGRESSIVE-NEXT: [[OR_COND:%.*]] = select i1 [[CMP]], i1 [[CMP1]], i1 false, !prof [[PROF0:![0-9]+]]
; AGGRESSIVE-NEXT: br i1 [[OR_COND]], label %[[COND_FALSE:.*]], label %[[COND_END:.*]], !prof [[PROF0]]
; AGGRESSIVE: [[COND_FALSE]]:
; AGGRESSIVE-NEXT: [[TMP0:%.*]] = load i32, ptr [[INPUT]], align 4
; AGGRESSIVE-NEXT: br label %[[COND_END]]
Expand All @@ -47,8 +47,8 @@ define i32 @foo(i32 %a, i32 %b, i32 %c, i32 %d, ptr %input) {
; WAYAGGRESSIVE-NEXT: [[MUL:%.*]] = shl i32 [[C]], 1
; WAYAGGRESSIVE-NEXT: [[ADD:%.*]] = add nsw i32 [[MUL]], [[A]]
; WAYAGGRESSIVE-NEXT: [[CMP1:%.*]] = icmp slt i32 [[ADD]], [[B]]
; WAYAGGRESSIVE-NEXT: [[OR_COND:%.*]] = select i1 [[CMP]], i1 [[CMP1]], i1 false
; WAYAGGRESSIVE-NEXT: br i1 [[OR_COND]], label %[[COND_FALSE:.*]], label %[[COND_END:.*]]
; WAYAGGRESSIVE-NEXT: [[OR_COND:%.*]] = select i1 [[CMP]], i1 [[CMP1]], i1 false, !prof [[PROF0:![0-9]+]]
; WAYAGGRESSIVE-NEXT: br i1 [[OR_COND]], label %[[COND_FALSE:.*]], label %[[COND_END:.*]], !prof [[PROF0]]
; WAYAGGRESSIVE: [[COND_FALSE]]:
; WAYAGGRESSIVE-NEXT: [[TMP0:%.*]] = load i32, ptr [[INPUT]], align 4
; WAYAGGRESSIVE-NEXT: br label %[[COND_END]]
Expand All @@ -58,13 +58,13 @@ define i32 @foo(i32 %a, i32 %b, i32 %c, i32 %d, ptr %input) {
;
entry:
%cmp = icmp sgt i32 %d, 3
br i1 %cmp, label %cond.end, label %lor.lhs.false
br i1 %cmp, label %cond.end, label %lor.lhs.false, !prof !0

lor.lhs.false:
%mul = shl i32 %c, 1
%add = add nsw i32 %mul, %a
%cmp1 = icmp slt i32 %add, %b
br i1 %cmp1, label %cond.false, label %cond.end
br i1 %cmp1, label %cond.false, label %cond.end, !prof !1

cond.false:
%0 = load i32, ptr %input, align 4
Expand Down Expand Up @@ -160,3 +160,14 @@ cond.end:
%cond = phi i32 [ %0, %cond.false ], [ 0, %lor.lhs.false ],[ 0, %pred_a ],[ 0, %pred_b ]
ret i32 %cond
}

!0 = !{!"branch_weights", i32 7, i32 11}
!1 = !{!"branch_weights", i32 13, i32 5}
;.
; BASELINE: [[PROF0]] = !{!"branch_weights", i32 7, i32 11}
; BASELINE: [[PROF1]] = !{!"branch_weights", i32 13, i32 5}
;.
; AGGRESSIVE: [[PROF0]] = !{!"branch_weights", i32 143, i32 181}
;.
; WAYAGGRESSIVE: [[PROF0]] = !{!"branch_weights", i32 143, i32 181}
;.
19 changes: 14 additions & 5 deletions llvm/test/Transforms/SimplifyCFG/branch-fold.ll
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Original file line number Diff line number Diff line change
@@ -1,12 +1,12 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --check-globals
; RUN: opt < %s -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 -S | FileCheck %s

define void @test(ptr %P, ptr %Q, i1 %A, i1 %B) {
; CHECK-LABEL: @test(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[A_NOT:%.*]] = xor i1 [[A:%.*]], true
; CHECK-NEXT: [[BRMERGE:%.*]] = select i1 [[A_NOT]], i1 true, i1 [[B:%.*]]
; CHECK-NEXT: br i1 [[BRMERGE]], label [[B:%.*]], label [[COMMON_RET:%.*]]
; CHECK-NEXT: [[BRMERGE:%.*]] = select i1 [[A_NOT]], i1 true, i1 [[B:%.*]], !prof [[PROF0:![0-9]+]]
; CHECK-NEXT: br i1 [[BRMERGE]], label [[B:%.*]], label [[COMMON_RET:%.*]], !prof [[PROF1:![0-9]+]]
; CHECK: common.ret:
; CHECK-NEXT: ret void
; CHECK: b:
Expand All @@ -15,9 +15,9 @@ define void @test(ptr %P, ptr %Q, i1 %A, i1 %B) {
;

entry:
br i1 %A, label %a, label %b
br i1 %A, label %a, label %b, !prof !0
a:
br i1 %B, label %b, label %c
br i1 %B, label %b, label %c, !prof !1
b:
store i32 123, ptr %P
ret void
Expand Down Expand Up @@ -146,3 +146,12 @@ Succ:
}

declare void @dummy()

!0 = !{!"branch_weights", i32 3, i32 7}
!1 = !{!"branch_weights", i32 11, i32 4}
;.
; CHECK: attributes #[[ATTR0:[0-9]+]] = { nounwind ssp memory(read) uwtable }
;.
; CHECK: [[PROF0]] = !{!"branch_weights", i32 7, i32 3}
; CHECK: [[PROF1]] = !{!"branch_weights", i32 138, i32 12}
;.
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