[llvm] [DA] Add overflow check in ExactSIV (PR #157086)

[Ryotaro Kasuga via llvm-commits]

https://github.com/kasuga-fj updated https://github.com/llvm/llvm-project/pull/157086

>From 38b8bddbb8b10a7e4ed75166c55c88e333e91ed3 Mon Sep 17 00:00:00 2001
From: Ryotaro Kasuga <kasuga.ryotaro at fujitsu.com>
Date: Fri, 5 Sep 2025 11:41:29 +0000
Subject: [PATCH 1/3] [DA] Add overflow check in ExactSIV

---
 llvm/lib/Analysis/DependenceAnalysis.cpp          | 14 +++++++++++++-
 llvm/test/Analysis/DependenceAnalysis/ExactSIV.ll |  2 +-
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Analysis/DependenceAnalysis.cpp b/llvm/lib/Analysis/DependenceAnalysis.cpp
index 0f77a1410e83b..6e576e866b310 100644
--- a/llvm/lib/Analysis/DependenceAnalysis.cpp
+++ b/llvm/lib/Analysis/DependenceAnalysis.cpp
@@ -1170,6 +1170,15 @@ const SCEVConstant *DependenceInfo::collectConstantUpperBound(const Loop *L,
   return nullptr;
 }
 
+/// Returns \p A - \p B if it guaranteed not to signed wrap. Otherwise returns
+/// nullptr. \p A and \p B must have the same integer type.
+static const SCEV *minusSCEVNoSignedOverflow(const SCEV *A, const SCEV *B,
+                                             ScalarEvolution &SE) {
+  if (SE.willNotOverflow(Instruction::Sub, /*Signed=*/true, A, B))
+    return SE.getMinusSCEV(A, B);
+  return nullptr;
+}
+
 // testZIV -
 // When we have a pair of subscripts of the form [c1] and [c2],
 // where c1 and c2 are both loop invariant, we attack it using
@@ -1626,7 +1635,9 @@ bool DependenceInfo::exactSIVtest(const SCEV *SrcCoeff, const SCEV *DstCoeff,
   assert(0 < Level && Level <= CommonLevels && "Level out of range");
   Level--;
   Result.Consistent = false;
-  const SCEV *Delta = SE->getMinusSCEV(DstConst, SrcConst);
+  const SCEV *Delta = minusSCEVNoSignedOverflow(DstConst, SrcConst, *SE);
+  if (!Delta)
+    return false;
   LLVM_DEBUG(dbgs() << "\t    Delta = " << *Delta << "\n");
   NewConstraint.setLine(SrcCoeff, SE->getNegativeSCEV(DstCoeff), Delta,
                         CurLoop);
@@ -1716,6 +1727,7 @@ bool DependenceInfo::exactSIVtest(const SCEV *SrcCoeff, const SCEV *DstCoeff,
   // explore directions
   unsigned NewDirection = Dependence::DVEntry::NONE;
   APInt LowerDistance, UpperDistance;
+  // TODO: Overflow check may be needed.
   if (TA.sgt(TB)) {
     LowerDistance = (TY - TX) + (TA - TB) * TL;
     UpperDistance = (TY - TX) + (TA - TB) * TU;
diff --git a/llvm/test/Analysis/DependenceAnalysis/ExactSIV.ll b/llvm/test/Analysis/DependenceAnalysis/ExactSIV.ll
index 2a809c32d7d21..e8e7cb11bb23e 100644
--- a/llvm/test/Analysis/DependenceAnalysis/ExactSIV.ll
+++ b/llvm/test/Analysis/DependenceAnalysis/ExactSIV.ll
@@ -841,7 +841,7 @@ define void @exact14(ptr %A) {
 ; CHECK-SIV-ONLY-NEXT:  Src: store i8 0, ptr %idx.0, align 1 --> Dst: store i8 0, ptr %idx.0, align 1
 ; CHECK-SIV-ONLY-NEXT:    da analyze - none!
 ; CHECK-SIV-ONLY-NEXT:  Src: store i8 0, ptr %idx.0, align 1 --> Dst: store i8 1, ptr %idx.1, align 1
-; CHECK-SIV-ONLY-NEXT:    da analyze - none!
+; CHECK-SIV-ONLY-NEXT:    da analyze - output [*|<]!
 ; CHECK-SIV-ONLY-NEXT:  Src: store i8 1, ptr %idx.1, align 1 --> Dst: store i8 1, ptr %idx.1, align 1
 ; CHECK-SIV-ONLY-NEXT:    da analyze - none!
 ;

>From a4f0f1f69bfebedde6991c684241e90b714ef378 Mon Sep 17 00:00:00 2001
From: Ryotaro Kasuga <kasuga.ryotaro at fujitsu.com>
Date: Tue, 16 Sep 2025 13:12:16 +0000
Subject: [PATCH 2/3] fix comment

---
 llvm/test/Analysis/DependenceAnalysis/ExactSIV.ll | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/llvm/test/Analysis/DependenceAnalysis/ExactSIV.ll b/llvm/test/Analysis/DependenceAnalysis/ExactSIV.ll
index e8e7cb11bb23e..6f33e2314ffba 100644
--- a/llvm/test/Analysis/DependenceAnalysis/ExactSIV.ll
+++ b/llvm/test/Analysis/DependenceAnalysis/ExactSIV.ll
@@ -815,8 +815,8 @@ for.end:                                          ; preds = %for.body
 ;;     A[3*i - 2] = 1;
 ;; }
 ;;
-;; FIXME: DependencyAnalsysis currently detects no dependency between
-;; `A[-6*i + INT64_MAX]` and `A[3*i - 2]`, but it does exist. For example,
+;; There exists dependency between `A[-6*i + INT64_MAX]` and `A[3*i - 2]`.
+;; For example,
 ;;
 ;; | memory location        | -6*i + INT64_MAX       | 3*i - 2
 ;; |------------------------|------------------------|-----------

>From 146b38e8deef9c30461e57c569b6a0c13b5912ba Mon Sep 17 00:00:00 2001
From: Ryotaro Kasuga <kasuga.ryotaro at fujitsu.com>
Date: Fri, 19 Sep 2025 08:08:35 +0000
Subject: [PATCH 3/3] Revert "fix comment"

This reverts commit a4f0f1f69bfebedde6991c684241e90b714ef378.
---
 llvm/test/Analysis/DependenceAnalysis/ExactSIV.ll | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/llvm/test/Analysis/DependenceAnalysis/ExactSIV.ll b/llvm/test/Analysis/DependenceAnalysis/ExactSIV.ll
index 6f33e2314ffba..e8e7cb11bb23e 100644
--- a/llvm/test/Analysis/DependenceAnalysis/ExactSIV.ll
+++ b/llvm/test/Analysis/DependenceAnalysis/ExactSIV.ll
@@ -815,8 +815,8 @@ for.end:                                          ; preds = %for.body
 ;;     A[3*i - 2] = 1;
 ;; }
 ;;
-;; There exists dependency between `A[-6*i + INT64_MAX]` and `A[3*i - 2]`.
-;; For example,
+;; FIXME: DependencyAnalsysis currently detects no dependency between
+;; `A[-6*i + INT64_MAX]` and `A[3*i - 2]`, but it does exist. For example,
 ;;
 ;; | memory location        | -6*i + INT64_MAX       | 3*i - 2
 ;; |------------------------|------------------------|-----------