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

alanzhao1 · 2025-09-12T22:07:00Z

Tracking issue: #147390

…ry operators Tracking issue: llvm#147390

llvmbot · 2025-09-12T22:07:30Z

@llvm/pr-subscribers-llvm-transforms

Author: Alan Zhao (alanzhao1)

Changes

Tracking issue: #147390

Full diff: https://github.com/llvm/llvm-project/pull/158375.diff

2 Files Affected:

(modified) llvm/lib/Transforms/InstCombine/InstructionCombining.cpp (+16-5)
(added) llvm/test/Transforms/InstCombine/preserve-profile.ll (+40)

diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index c2f045a2ab02e..4f4d533262006 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -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;      
+
 // 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
@@ -1372,6 +1374,7 @@ Value *InstCombinerImpl::SimplifySelectsFeedingBinaryOp(BinaryOperator &I,
   SimplifyQuery Q = SQ.getWithInstruction(&I);
 
   Value *Cond, *True = nullptr, *False = nullptr;
+  MDNode *ProfileData;
 
   // Special-case for add/negate combination. Replace the zero in the negation
   // with the trailing add operand:
@@ -1381,17 +1384,18 @@ 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;
+    Value *N, *SI = nullptr;
     if (True && match(FVal, m_Neg(m_Value(N)))) {
       Value *Sub = Builder.CreateSub(Z, N);
-      return Builder.CreateSelect(Cond, True, Sub, I.getName());
+      SI = Builder.CreateSelect(Cond, True, Sub, I.getName());
     }
     if (False && match(TVal, m_Neg(m_Value(N)))) {
       Value *Sub = Builder.CreateSub(Z, N);
-      return Builder.CreateSelect(Cond, Sub, False, I.getName());
+      SI = Builder.CreateSelect(Cond, Sub, False, I.getName());
     }
-    return nullptr;
+    if (!ProfcheckDisableMetadataFixes && !SI)
+      cast<SelectInst>(SI)->setMetadata(LLVMContext::MD_prof, ProfileData);
+    return SI;
   };
 
   if (LHSIsSelect && RHSIsSelect && A == D) {
@@ -1399,6 +1403,9 @@ 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
+    // the same idempotent conditional.
+    ProfileData = cast<SelectInst>(LHS)->getMetadata(LLVMContext::MD_prof);
 
     if (LHS->hasOneUse() && RHS->hasOneUse()) {
       if (False && !True)
@@ -1408,6 +1415,7 @@ Value *InstCombinerImpl::SimplifySelectsFeedingBinaryOp(BinaryOperator &I,
     }
   } else if (LHSIsSelect && LHS->hasOneUse()) {
     // (A ? B : C) op Y -> A ? (B op Y) : (C op Y)
+    ProfileData = cast<SelectInst>(LHS)->getMetadata(LLVMContext::MD_prof);
     Cond = A;
     True = simplifyBinOp(Opcode, B, RHS, FMF, Q);
     False = simplifyBinOp(Opcode, C, RHS, FMF, Q);
@@ -1415,6 +1423,7 @@ Value *InstCombinerImpl::SimplifySelectsFeedingBinaryOp(BinaryOperator &I,
       return NewSel;
   } else if (RHSIsSelect && RHS->hasOneUse()) {
     // X op (D ? E : F) -> D ? (X op E) : (X op F)
+    ProfileData = cast<SelectInst>(RHS)->getMetadata(LLVMContext::MD_prof);
     Cond = D;
     True = simplifyBinOp(Opcode, LHS, E, FMF, Q);
     False = simplifyBinOp(Opcode, LHS, F, FMF, Q);
@@ -1427,6 +1436,8 @@ Value *InstCombinerImpl::SimplifySelectsFeedingBinaryOp(BinaryOperator &I,
 
   Value *SI = Builder.CreateSelect(Cond, True, False);
   SI->takeName(&I);
+  if (!ProfcheckDisableMetadataFixes)
+    cast<SelectInst>(SI)->setMetadata(LLVMContext::MD_prof, ProfileData);
   return SI;
 }
 
diff --git a/llvm/test/Transforms/InstCombine/preserve-profile.ll b/llvm/test/Transforms/InstCombine/preserve-profile.ll
new file mode 100644
index 0000000000000..47254a88b1ed1
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/preserve-profile.ll
@@ -0,0 +1,40 @@
+; RUN: opt < %s -passes=instcombine -S | FileCheck %s
+
+; CHECK: define i32 @LHSBin(i1 %C) !prof ![[PROF0:[0-9]]]
+; CHECK: %V = select i1 %C, i32 1010, i32 20, !prof ![[PROF1:[0-9]]]
+define i32 @LHSBin(i1 %C) !prof !0 {
+  %A = select i1 %C, i32 1000, i32 10, !prof !1
+  %V = add i32 %A, 10
+  ret i32 %V
+}
+
+; CHECK: define i32 @RHSBin(i1 %C) !prof ![[PROF0]]
+; CHECK: %V = select i1 %C, i32 1010, i32 20, !prof ![[PROF1]]
+define i32 @RHSBin(i1 %C) !prof !0 {
+  %A = select i1 %C, i32 1000, i32 10, !prof !1
+  %V = add i32 10, %A
+  ret i32 %V;
+}
+
+; CHECK: define i32 @BothBin(i1 %C) !prof ![[PROF0]]
+; CHECK: %V = select i1 %C, i32 2000, i32 20, !prof ![[PROF1]]
+define i32 @BothBin(i1 %C) !prof !0 {
+  %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;
+}
+
+; CHECK: define i32 @NegBin(i1 %C) !prof ![[PROF0:[0-9]]]
+; CHECK: %V = select i1 %C, i32 1010, i32 0, !prof ![[PROF1:[0-9]]]
+define i32 @NegBin(i1 %C) !prof !0 {
+  %A = select i1 %C, i32 1000, i32 -10, !prof !1
+  %V = add i32 %A, 10
+  ret i32 %V
+}
+
+
+; CHECK: ![[PROF0]] = !{!"function_entry_count", i64 1000}
+; CHECK: ![[PROF1]] = !{!"branch_weights", i32 2, i32 3}
+!0 = !{!"function_entry_count", i64 1000}
+!1 = !{!"branch_weights", i32 2, i32 3}

github-actions · 2025-09-12T22:13:59Z

✅ With the latest revision this PR passed the C/C++ code formatter.

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

llvm/test/Transforms/InstCombine/preserve-profile.ll

mtrofin · 2025-09-12T22:19:59Z

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

  };

  if (LHSIsSelect && RHSIsSelect && A == D) {
    // (A ? B : C) op (A ? E : F) -> A ? (B op E) : (C op F)
    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


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.

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

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

the branch weights are e.g. 2, 4 and 1, 2. So same probability, different expression

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.

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

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

mtrofin

lgtm - ptal my note about asserting on metadata equality

nikic · 2025-09-13T09:05:59Z

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -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;


Unused, should be dropped.

[InstCombine] Preserve profile data with select instructions and bina…

5ac86b7

…ry operators Tracking issue: llvm#147390

alanzhao1 requested a review from mtrofin September 12, 2025 22:07

alanzhao1 requested a review from nikic as a code owner September 12, 2025 22:07

llvmbot added llvm:instcombine Covers the InstCombine, InstSimplify and AggressiveInstCombine passes llvm:transforms labels Sep 12, 2025

fix missing trailing newline

cad50aa

alanzhao1 added 3 commits September 12, 2025 15:08

fix conditional

938a4d4

test fixes

5d7169e

more test fixes

701bf31

code formatting fixes

fe929d4

mtrofin reviewed Sep 12, 2025

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llvm/lib/Transforms/InstCombine/InstructionCombining.cpp Outdated Show resolved Hide resolved

initialize ProfileData to nullptr

d2979b4

boomanaiden154 reviewed Sep 12, 2025

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llvm/test/Transforms/InstCombine/preserve-profile.ll Show resolved Hide resolved

mtrofin reviewed Sep 12, 2025

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mtrofin mentioned this pull request Sep 12, 2025

[InstCombine] Set !prof metadata on selects #157599

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nikic reviewed Sep 12, 2025

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llvm/lib/Transforms/InstCombine/InstructionCombining.cpp Outdated Show resolved Hide resolved

alanzhao1 added 3 commits September 12, 2025 15:32

assert that LHS and RHS profile are the same

52d9c46

pass original select to MDFrom argument of CreateSelect

d8731d3

use update_test_checks.py

dfbd3f3

mtrofin approved these changes Sep 13, 2025

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nikic reviewed Sep 13, 2025

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

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

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