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[InstCombine] Preserve profile data with select instructions and binary operators #158375

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[InstCombine] Preserve profile data with select instructions and binary operators #158375

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5ac86b7
[InstCombine] Preserve profile data with select instructions and bina…
alanzhao1 Sep 12, 2025
cad50aa
fix missing trailing newline
alanzhao1 Sep 12, 2025
938a4d4
fix conditional
alanzhao1 Sep 12, 2025
5d7169e
test fixes
alanzhao1 Sep 12, 2025
701bf31
more test fixes
alanzhao1 Sep 12, 2025
fe929d4
code formatting fixes
alanzhao1 Sep 12, 2025
d2979b4
initialize ProfileData to nullptr
alanzhao1 Sep 12, 2025
52d9c46
assert that LHS and RHS profile are the same
alanzhao1 Sep 12, 2025
d8731d3
pass original select to MDFrom argument of CreateSelect
alanzhao1 Sep 12, 2025
dfbd3f3
use update_test_checks.py
alanzhao1 Sep 13, 2025
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20 changes: 14 additions & 6 deletions llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -144,6 +144,8 @@ static cl::opt<unsigned>
MaxArraySize("instcombine-maxarray-size", cl::init(1024),
cl::desc("Maximum array size considered when doing a combine"));

extern cl::opt<bool> ProfcheckDisableMetadataFixes;
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Unused, should be dropped.


// FIXME: Remove this flag when it is no longer necessary to convert
// llvm.dbg.declare to avoid inaccurate debug info. Setting this to false
// increases variable availability at the cost of accuracy. Variables that
Expand Down Expand Up @@ -1361,6 +1363,8 @@ Value *InstCombinerImpl::SimplifySelectsFeedingBinaryOp(BinaryOperator &I,
if (!LHSIsSelect && !RHSIsSelect)
return nullptr;

SelectInst *SI = cast<SelectInst>(LHSIsSelect ? LHS : RHS);

FastMathFlags FMF;
BuilderTy::FastMathFlagGuard Guard(Builder);
if (isa<FPMathOperator>(&I)) {
Expand All @@ -1381,15 +1385,14 @@ Value *InstCombinerImpl::SimplifySelectsFeedingBinaryOp(BinaryOperator &I,
// We need an 'add' and exactly 1 arm of the select to have been simplified.
if (Opcode != Instruction::Add || (!True && !False) || (True && False))
return nullptr;

Value *N;
if (True && match(FVal, m_Neg(m_Value(N)))) {
Value *Sub = Builder.CreateSub(Z, N);
return Builder.CreateSelect(Cond, True, Sub, I.getName());
return Builder.CreateSelect(Cond, True, Sub, I.getName(), SI);
}
if (False && match(TVal, m_Neg(m_Value(N)))) {
Value *Sub = Builder.CreateSub(Z, N);
return Builder.CreateSelect(Cond, Sub, False, I.getName());
return Builder.CreateSelect(Cond, Sub, False, I.getName(), SI);
}
return nullptr;
};
Expand All @@ -1399,6 +1402,11 @@ Value *InstCombinerImpl::SimplifySelectsFeedingBinaryOp(BinaryOperator &I,
Cond = A;
True = simplifyBinOp(Opcode, B, E, FMF, Q);
False = simplifyBinOp(Opcode, C, F, FMF, Q);
// Profile weights for both LHS and RHS should be the same because they have
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@mtrofin mtrofin Sep 12, 2025 •
edited
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is there something we could assert? like maybe that the fraction is the same... albeit that's float point comparison, ugh... ok, how about a ORE message about what the 2 were, or LLVM_DEBUG, just so we can easily introspect if we ever need to.

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Added an assertion that left and right profiles should be equal.

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there are 2 cases where the pointers won't be the same:

  1. the branch weights are e.g. 2, 4 and 1, 2. So same probability, different expression
  2. the metadata was added as unique, and then you'd have 2 different objects, content-wise identical

IDK, may be safer to LLVM_DEBUG. The assert may get someone on a wild goose chase in the cases above. Or add a comment above it that we don't expect those cases to happen, but if they do, safe to replace the assert w a LLVM_DEBUG or something of the sort.

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There is no IR validity constraint that branch weight metadata has to "make sense", so you also can't assert that here.

// the same idempotent conditional.
assert(cast<SelectInst>(LHS)->getMetadata(LLVMContext::MD_prof) ==
cast<SelectInst>(RHS)->getMetadata(LLVMContext::MD_prof) &&
"LHS and RHS select statements have different metadata!");

if (LHS->hasOneUse() && RHS->hasOneUse()) {
if (False && !True)
Expand All @@ -1425,9 +1433,9 @@ Value *InstCombinerImpl::SimplifySelectsFeedingBinaryOp(BinaryOperator &I,
if (!True || !False)
return nullptr;

Value *SI = Builder.CreateSelect(Cond, True, False);
SI->takeName(&I);
return SI;
Value *NewSI = Builder.CreateSelect(Cond, True, False, I.getName(), SI);
NewSI->takeName(&I);
return NewSI;
}

/// Freely adapt every user of V as-if V was changed to !V.
Expand Down
54 changes: 54 additions & 0 deletions llvm/test/Transforms/InstCombine/preserve-profile.ll
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@@ -0,0 +1,54 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --check-globals all --version 6
; RUN: opt < %s -passes=instcombine -S | FileCheck %s

define i32 @LHSBin(i1 %C) !prof !0 {
; CHECK-LABEL: define i32 @LHSBin(
; CHECK-SAME: i1 [[C:%.*]]) !prof [[PROF0:![0-9]+]] {
; CHECK-NEXT: [[V:%.*]] = select i1 [[C]], i32 1010, i32 20, !prof [[PROF1:![0-9]+]]
; CHECK-NEXT: ret i32 [[V]]
;
%A = select i1 %C, i32 1000, i32 10, !prof !1
%V = add i32 %A, 10
ret i32 %V
}

define i32 @RHSBin(i1 %C) !prof !0 {
; CHECK-LABEL: define i32 @RHSBin(
; CHECK-SAME: i1 [[C:%.*]]) !prof [[PROF0]] {
; CHECK-NEXT: [[V:%.*]] = select i1 [[C]], i32 1010, i32 20, !prof [[PROF1]]
; CHECK-NEXT: ret i32 [[V]]
;
%A = select i1 %C, i32 1000, i32 10, !prof !1
%V = add i32 10, %A
ret i32 %V;
}

define i32 @BothBin(i1 %C) !prof !0 {
; CHECK-LABEL: define i32 @BothBin(
; CHECK-SAME: i1 [[C:%.*]]) !prof [[PROF0]] {
; CHECK-NEXT: [[V:%.*]] = select i1 [[C]], i32 2000, i32 20, !prof [[PROF1]]
; CHECK-NEXT: ret i32 [[V]]
;
%A = select i1 %C, i32 1000, i32 10, !prof !1
%B = select i1 %C, i32 1000, i32 10, !prof !1
%V = add i32 %A, %B
ret i32 %V;
}

define i32 @NegBin(i1 %C) !prof !0 {
; CHECK-LABEL: define i32 @NegBin(
; CHECK-SAME: i1 [[C:%.*]]) !prof [[PROF0]] {
; CHECK-NEXT: [[V:%.*]] = select i1 [[C]], i32 1010, i32 0, !prof [[PROF1]]
; CHECK-NEXT: ret i32 [[V]]
;
%A = select i1 %C, i32 1000, i32 -10, !prof !1
%V = add i32 %A, 10
ret i32 %V
}

!0 = !{!"function_entry_count", i64 1000}
!1 = !{!"branch_weights", i32 2, i32 3}
;.
; CHECK: [[PROF0]] = !{!"function_entry_count", i64 1000}
; CHECK: [[PROF1]] = !{!"branch_weights", i32 2, i32 3}
;.