[DA] Set Distance to zero when Direction is EQ #147966

kasuga-fj · 2025-07-10T13:51:21Z

A Dependence object has two entries: Distance and Direction. The former represents the distance of the dependence, while the latter characterizes the distance by whether the value of it is positive, negative, or zero (especially, zero is represented by EQ in DA). So it is expected that the Distance equals zero iff the Direction is EQ. However, this condition was not satisfied in some cases.
This patch adds a logic to set the Distance to zero if the Direction is EQ. Although it is ideal that the Distance is updated to zero simultaneously when the Direction is set to EQ, achieving it would require changing the entire code in DA.

llvmbot · 2025-07-10T13:51:58Z

@llvm/pr-subscribers-llvm-analysis

Author: Ryotaro Kasuga (kasuga-fj)

Changes

A Dependence object has two entries: Distance and Direction. The former represents the distance of the dependence, while the latter characterizes the distance by whether the value of it is positive, negative, or zero (especially, zero is represented by EQ in DA). So it is expected that the Distance equals zero iff the Direction is EQ. However, this condition was not satisfied in some cases.
This patch adds a logic to set the Distance to zero if the Direction is EQ. Although it is ideal that the Distance is updated to zero simultaneously when the Direction is set to EQ, achieving it would require changing the entire code in DA.

Full diff: https://github.com/llvm/llvm-project/pull/147966.diff

4 Files Affected:

(modified) llvm/lib/Analysis/DependenceAnalysis.cpp (+29)
(modified) llvm/test/Analysis/DependenceAnalysis/Banerjee.ll (+9-9)
(modified) llvm/test/Analysis/DependenceAnalysis/Coupled.ll (+3-3)
(modified) llvm/test/Analysis/DependenceAnalysis/NonCanonicalizedSubscript.ll (+1-1)

diff --git a/llvm/lib/Analysis/DependenceAnalysis.cpp b/llvm/lib/Analysis/DependenceAnalysis.cpp
index 5b85060f9caa1..df5fd0e938707 100644
--- a/llvm/lib/Analysis/DependenceAnalysis.cpp
+++ b/llvm/lib/Analysis/DependenceAnalysis.cpp
@@ -187,6 +187,18 @@ static void dumpExampleDependence(raw_ostream &OS, DependenceInfo *DA,
           OS << "  da analyze - ";
           if (auto D = DA->depends(&*SrcI, &*DstI,
                                    /*UnderRuntimeAssumptions=*/true)) {
+
+            // Verify that the distance begin zero is equivalent to the
+            // direction being EQ.
+            for (unsigned Level = 1; Level <= D->getLevels(); Level++) {
+              const SCEV *Distance = D->getDistance(Level);
+              bool IsDistanceZero = Distance && Distance->isZero();
+              bool IsDirectionEQ =
+                  D->getDirection(Level) == Dependence::DVEntry::EQ;
+              assert(IsDistanceZero == IsDirectionEQ &&
+                     "Inconsistent distance and direction.");
+            }
+
             // Normalize negative direction vectors if required by clients.
             if (NormalizeResults && D->normalize(&SE))
                 OS << "normalized - ";
@@ -3991,6 +4003,23 @@ DependenceInfo::depends(Instruction *Src, Instruction *Dst,
     if (CompleteLoops[II])
       Result.DV[II - 1].Scalar = false;
 
+  // Set the distance to zero if the direction is EQ.
+  // TODO: Ideally, the distance should be set to 0 immediately simultaneously
+  // with the corresponding direction being set to EQ.
+  for (unsigned II = 1; II <= Result.getLevels(); ++II) {
+    if (Result.getDirection(II) == Dependence::DVEntry::EQ)
+      Result.DV[II - 1].Distance = SE->getZero(SrcSCEV->getType());
+
+    LLVM_DEBUG({
+      // Check that the converse (i.e., if the distance is zero, then the
+      // direction is EQ) holds.
+      const SCEV *Distance = Result.getDistance(II);
+      if (Distance && Distance->isZero())
+        assert(Result.getDirection(II) == Dependence::DVEntry::EQ &&
+               "Distance is zero, but direction is not EQ");
+    });
+  }
+
   if (PossiblyLoopIndependent) {
     // Make sure the LoopIndependent flag is set correctly.
     // All directions must include equal, otherwise no
diff --git a/llvm/test/Analysis/DependenceAnalysis/Banerjee.ll b/llvm/test/Analysis/DependenceAnalysis/Banerjee.ll
index 6768e9067dca3..d3301520fd107 100644
--- a/llvm/test/Analysis/DependenceAnalysis/Banerjee.ll
+++ b/llvm/test/Analysis/DependenceAnalysis/Banerjee.ll
@@ -802,7 +802,7 @@ define void @banerjee9(ptr %A, ptr %B, i64 %m, i64 %n) nounwind uwtable ssp {
 ; CHECK-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: store i64 0, ptr %arrayidx, align 8
 ; CHECK-NEXT:    da analyze - output [* *]!
 ; CHECK-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: %1 = load i64, ptr %arrayidx7, align 8
-; CHECK-NEXT:    da analyze - flow [<= =|<]!
+; CHECK-NEXT:    da analyze - flow [<= 0|<]!
 ; CHECK-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: store i64 %1, ptr %B.addr.11, align 8
 ; CHECK-NEXT:    da analyze - confused!
 ; CHECK-NEXT:  Src: %1 = load i64, ptr %arrayidx7, align 8 --> Dst: %1 = load i64, ptr %arrayidx7, align 8
@@ -816,7 +816,7 @@ define void @banerjee9(ptr %A, ptr %B, i64 %m, i64 %n) nounwind uwtable ssp {
 ; NORMALIZE-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: store i64 0, ptr %arrayidx, align 8
 ; NORMALIZE-NEXT:    da analyze - output [* *]!
 ; NORMALIZE-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: %1 = load i64, ptr %arrayidx7, align 8
-; NORMALIZE-NEXT:    da analyze - flow [<= =|<]!
+; NORMALIZE-NEXT:    da analyze - flow [<= 0|<]!
 ; NORMALIZE-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: store i64 %1, ptr %B.addr.11, align 8
 ; NORMALIZE-NEXT:    da analyze - confused!
 ; NORMALIZE-NEXT:  Src: %1 = load i64, ptr %arrayidx7, align 8 --> Dst: %1 = load i64, ptr %arrayidx7, align 8
@@ -830,7 +830,7 @@ define void @banerjee9(ptr %A, ptr %B, i64 %m, i64 %n) nounwind uwtable ssp {
 ; DELIN-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: store i64 0, ptr %arrayidx, align 8
 ; DELIN-NEXT:    da analyze - output [* *]!
 ; DELIN-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: %1 = load i64, ptr %arrayidx7, align 8
-; DELIN-NEXT:    da analyze - flow [<= =|<]!
+; DELIN-NEXT:    da analyze - flow [<= 0|<]!
 ; DELIN-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: store i64 %1, ptr %B.addr.11, align 8
 ; DELIN-NEXT:    da analyze - confused!
 ; DELIN-NEXT:  Src: %1 = load i64, ptr %arrayidx7, align 8 --> Dst: %1 = load i64, ptr %arrayidx7, align 8
@@ -888,7 +888,7 @@ define void @banerjee10(ptr %A, ptr %B, i64 %m, i64 %n) nounwind uwtable ssp {
 ; CHECK-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: store i64 0, ptr %arrayidx, align 8
 ; CHECK-NEXT:    da analyze - none!
 ; CHECK-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: %1 = load i64, ptr %arrayidx6, align 8
-; CHECK-NEXT:    da analyze - flow [<> =]!
+; CHECK-NEXT:    da analyze - flow [<> 0]!
 ; CHECK-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: store i64 %1, ptr %B.addr.11, align 8
 ; CHECK-NEXT:    da analyze - confused!
 ; CHECK-NEXT:  Src: %1 = load i64, ptr %arrayidx6, align 8 --> Dst: %1 = load i64, ptr %arrayidx6, align 8
@@ -902,7 +902,7 @@ define void @banerjee10(ptr %A, ptr %B, i64 %m, i64 %n) nounwind uwtable ssp {
 ; NORMALIZE-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: store i64 0, ptr %arrayidx, align 8
 ; NORMALIZE-NEXT:    da analyze - none!
 ; NORMALIZE-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: %1 = load i64, ptr %arrayidx6, align 8
-; NORMALIZE-NEXT:    da analyze - flow [<> =]!
+; NORMALIZE-NEXT:    da analyze - flow [<> 0]!
 ; NORMALIZE-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: store i64 %1, ptr %B.addr.11, align 8
 ; NORMALIZE-NEXT:    da analyze - confused!
 ; NORMALIZE-NEXT:  Src: %1 = load i64, ptr %arrayidx6, align 8 --> Dst: %1 = load i64, ptr %arrayidx6, align 8
@@ -916,7 +916,7 @@ define void @banerjee10(ptr %A, ptr %B, i64 %m, i64 %n) nounwind uwtable ssp {
 ; DELIN-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: store i64 0, ptr %arrayidx, align 8
 ; DELIN-NEXT:    da analyze - none!
 ; DELIN-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: %1 = load i64, ptr %arrayidx6, align 8
-; DELIN-NEXT:    da analyze - flow [<> =]!
+; DELIN-NEXT:    da analyze - flow [<> 0]!
 ; DELIN-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: store i64 %1, ptr %B.addr.11, align 8
 ; DELIN-NEXT:    da analyze - confused!
 ; DELIN-NEXT:  Src: %1 = load i64, ptr %arrayidx6, align 8 --> Dst: %1 = load i64, ptr %arrayidx6, align 8
@@ -1058,7 +1058,7 @@ define void @banerjee12(ptr %A, ptr %B, i64 %m, i64 %n) nounwind uwtable ssp {
 ; CHECK-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: store i64 0, ptr %arrayidx, align 8
 ; CHECK-NEXT:    da analyze - none!
 ; CHECK-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: %0 = load i64, ptr %arrayidx6, align 8
-; CHECK-NEXT:    da analyze - flow [= <>]!
+; CHECK-NEXT:    da analyze - flow [0 <>]!
 ; CHECK-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: store i64 %0, ptr %B.addr.11, align 8
 ; CHECK-NEXT:    da analyze - confused!
 ; CHECK-NEXT:  Src: %0 = load i64, ptr %arrayidx6, align 8 --> Dst: %0 = load i64, ptr %arrayidx6, align 8
@@ -1072,7 +1072,7 @@ define void @banerjee12(ptr %A, ptr %B, i64 %m, i64 %n) nounwind uwtable ssp {
 ; NORMALIZE-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: store i64 0, ptr %arrayidx, align 8
 ; NORMALIZE-NEXT:    da analyze - none!
 ; NORMALIZE-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: %0 = load i64, ptr %arrayidx6, align 8
-; NORMALIZE-NEXT:    da analyze - flow [= <>]!
+; NORMALIZE-NEXT:    da analyze - flow [0 <>]!
 ; NORMALIZE-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: store i64 %0, ptr %B.addr.11, align 8
 ; NORMALIZE-NEXT:    da analyze - confused!
 ; NORMALIZE-NEXT:  Src: %0 = load i64, ptr %arrayidx6, align 8 --> Dst: %0 = load i64, ptr %arrayidx6, align 8
@@ -1086,7 +1086,7 @@ define void @banerjee12(ptr %A, ptr %B, i64 %m, i64 %n) nounwind uwtable ssp {
 ; DELIN-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: store i64 0, ptr %arrayidx, align 8
 ; DELIN-NEXT:    da analyze - none!
 ; DELIN-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: %0 = load i64, ptr %arrayidx6, align 8
-; DELIN-NEXT:    da analyze - flow [= <>]!
+; DELIN-NEXT:    da analyze - flow [0 <>]!
 ; DELIN-NEXT:  Src: store i64 0, ptr %arrayidx, align 8 --> Dst: store i64 %0, ptr %B.addr.11, align 8
 ; DELIN-NEXT:    da analyze - confused!
 ; DELIN-NEXT:  Src: %0 = load i64, ptr %arrayidx6, align 8 --> Dst: %0 = load i64, ptr %arrayidx6, align 8
diff --git a/llvm/test/Analysis/DependenceAnalysis/Coupled.ll b/llvm/test/Analysis/DependenceAnalysis/Coupled.ll
index ff9f393f88152..06bfc5d2e8573 100644
--- a/llvm/test/Analysis/DependenceAnalysis/Coupled.ll
+++ b/llvm/test/Analysis/DependenceAnalysis/Coupled.ll
@@ -285,7 +285,7 @@ define void @couple6(ptr %A, ptr %B, i32 %n) nounwind uwtable ssp {
 ; CHECK-NEXT:  Src: store i32 %conv, ptr %arrayidx1, align 4 --> Dst: store i32 %conv, ptr %arrayidx1, align 4
 ; CHECK-NEXT:    da analyze - none!
 ; CHECK-NEXT:  Src: store i32 %conv, ptr %arrayidx1, align 4 --> Dst: %0 = load i32, ptr %arrayidx3, align 4
-; CHECK-NEXT:    da analyze - flow [=|<]!
+; CHECK-NEXT:    da analyze - flow [0|<]!
 ; CHECK-NEXT:  Src: store i32 %conv, ptr %arrayidx1, align 4 --> Dst: store i32 %0, ptr %B.addr.01, align 4
 ; CHECK-NEXT:    da analyze - confused!
 ; CHECK-NEXT:  Src: %0 = load i32, ptr %arrayidx3, align 4 --> Dst: %0 = load i32, ptr %arrayidx3, align 4
@@ -503,7 +503,7 @@ define void @couple11(ptr %A, ptr %B, i32 %n) nounwind uwtable ssp {
 ; CHECK-NEXT:  Src: store i32 %conv, ptr %arrayidx2, align 4 --> Dst: store i32 %conv, ptr %arrayidx2, align 4
 ; CHECK-NEXT:    da analyze - none!
 ; CHECK-NEXT:  Src: store i32 %conv, ptr %arrayidx2, align 4 --> Dst: %0 = load i32, ptr %arrayidx4, align 4
-; CHECK-NEXT:    da analyze - flow [=|<] splitable!
+; CHECK-NEXT:    da analyze - flow [0|<] splitable!
 ; CHECK-NEXT:    da analyze - split level = 1, iteration = 9!
 ; CHECK-NEXT:  Src: store i32 %conv, ptr %arrayidx2, align 4 --> Dst: store i32 %0, ptr %B.addr.01, align 4
 ; CHECK-NEXT:    da analyze - confused!
@@ -636,7 +636,7 @@ define void @couple14(ptr %A, ptr %B, i32 %n) nounwind uwtable ssp {
 ; CHECK-NEXT:  Src: store i32 %conv, ptr %arrayidx3, align 4 --> Dst: store i32 %conv, ptr %arrayidx3, align 4
 ; CHECK-NEXT:    da analyze - none!
 ; CHECK-NEXT:  Src: store i32 %conv, ptr %arrayidx3, align 4 --> Dst: %0 = load i32, ptr %arrayidx6, align 4
-; CHECK-NEXT:    da analyze - flow [=|<]!
+; CHECK-NEXT:    da analyze - flow [0|<]!
 ; CHECK-NEXT:  Src: store i32 %conv, ptr %arrayidx3, align 4 --> Dst: store i32 %0, ptr %B.addr.01, align 4
 ; CHECK-NEXT:    da analyze - confused!
 ; CHECK-NEXT:  Src: %0 = load i32, ptr %arrayidx6, align 4 --> Dst: %0 = load i32, ptr %arrayidx6, align 4
diff --git a/llvm/test/Analysis/DependenceAnalysis/NonCanonicalizedSubscript.ll b/llvm/test/Analysis/DependenceAnalysis/NonCanonicalizedSubscript.ll
index f0cd2fd4cd930..e5d5d21e365a1 100644
--- a/llvm/test/Analysis/DependenceAnalysis/NonCanonicalizedSubscript.ll
+++ b/llvm/test/Analysis/DependenceAnalysis/NonCanonicalizedSubscript.ll
@@ -18,7 +18,7 @@ define void @i32_subscript(ptr %a, ptr %b) {
 ; CHECK-NEXT:  Src: %0 = load i32, ptr %a.addr, align 4 --> Dst: %0 = load i32, ptr %a.addr, align 4
 ; CHECK-NEXT:    da analyze - none!
 ; CHECK-NEXT:  Src: %0 = load i32, ptr %a.addr, align 4 --> Dst: store i32 %1, ptr %a.addr.2, align 4
-; CHECK-NEXT:    da analyze - anti [=|<]!
+; CHECK-NEXT:    da analyze - anti [0|<]!
 ; CHECK-NEXT:  Src: store i32 %1, ptr %a.addr.2, align 4 --> Dst: store i32 %1, ptr %a.addr.2, align 4
 ; CHECK-NEXT:    da analyze - none!
 ;

llvm/lib/Analysis/DependenceAnalysis.cpp

Meinersbur · 2025-07-10T14:17:15Z

llvm/lib/Analysis/DependenceAnalysis.cpp

+  // TODO: Ideally, the distance should be set to 0 immediately simultaneously
+  // with the corresponding direction being set to EQ.
+  for (unsigned II = 1; II <= Result.getLevels(); ++II) {
+    if (Result.getDirection(II) == Dependence::DVEntry::EQ)


Is it only that Distance is nullptr, or are there cases where it is non-zero but direction is still EQ?

In the former case, could you set the direction only when Distance is nullptr and assert if not SCEV::isZero?

In the latter case, the would be something quite wrong. Why prioritize the Direction analysis result over the Distance?

The former. I noticed it when I changed the code a bit and saw that some of the dumped results in the tests changed from = to 0. Added assert for the latter case.

madhur13490 · 2025-07-10T14:47:56Z

llvm/lib/Analysis/DependenceAnalysis.cpp

          if (auto D = DA->depends(&*SrcI, &*DstI,
                                   /*UnderRuntimeAssumptions=*/true)) {
+
+            // Verify that the distance begin zero is equivalent to the


s/begin/being?

Yes, thanks

Co-authored-by: Michael Kruse <[email protected]>

github-actions · 2025-07-10T15:52:17Z

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

Meinersbur

LGTM

kasuga-fj · 2025-07-11T00:22:11Z

Thanks for the reviews!

[DA] Set Distance to zero when Direction is EQ

ad3f5e9

llvmbot added the llvm:analysis Includes value tracking, cost tables and constant folding label Jul 10, 2025

kasuga-fj requested review from Meinersbur, sebpop and sjoerdmeijer July 10, 2025 13:52

Meinersbur reviewed Jul 10, 2025

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madhur13490 reviewed Jul 10, 2025

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Apply suggestions from code review

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Co-authored-by: Michael Kruse <[email protected]>

kasuga-fj added 3 commits July 11, 2025 01:00

Add assert for the cases where distance is non-zero and direction is EQ

20d5661

Fix typo

2a8f9c8

Apply format

dcb6679

Meinersbur approved these changes Jul 10, 2025

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kasuga-fj merged commit 7d510b7 into llvm:main Jul 11, 2025
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kasuga-fj deleted the da-distance-zero branch September 2, 2025 11:16

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