[llvm] [LAA] Fix incorrect dependency classification. (PR #70819)

[Alexandros Lamprineas via llvm-commits]

https://github.com/labrinea updated https://github.com/llvm/llvm-project/pull/70819

>From 8753d7254ce4c6fd16d4f204b051f1f16e7e9eb7 Mon Sep 17 00:00:00 2001
From: Alexandros Lamprineas <alexandros.lamprineas at arm.com>
Date: Tue, 31 Oct 2023 15:23:40 +0000
Subject: [PATCH] [LAA] Fix incorrect dependency classification.

As shown in #70473, the following loop was not considered safe to vectorize.
When determining the memory access dependencies in a loop which has negative
iteration step, we invert the source and sink of the dependence. Perhaps we
should just invert the operands to getMinusSCEV(). This way the dependency
is not regarded to be true.

void vectorizable_Read_Write(int *A) {
  for (unsigned i = 1022; i >= 0; i--)
    A[i+1] = A[i] + 1;
}
---
 llvm/lib/Analysis/LoopAccessAnalysis.cpp      | 14 ++--------
 .../forward-negative-step.ll                  | 28 +++++++++++--------
 2 files changed, 20 insertions(+), 22 deletions(-)

diff --git a/llvm/lib/Analysis/LoopAccessAnalysis.cpp b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
index 3d1edd5f038a25e..b9018e9b1b07fc9 100644
--- a/llvm/lib/Analysis/LoopAccessAnalysis.cpp
+++ b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
@@ -1884,18 +1884,10 @@ MemoryDepChecker::isDependent(const MemAccessInfo &A, unsigned AIdx,
   const SCEV *Sink = PSE.getSCEV(BPtr);
 
   // If the induction step is negative we have to invert source and sink of the
-  // dependence.
-  if (StrideAPtr < 0) {
-    std::swap(APtr, BPtr);
-    std::swap(ATy, BTy);
-    std::swap(Src, Sink);
-    std::swap(AIsWrite, BIsWrite);
-    std::swap(AIdx, BIdx);
-    std::swap(StrideAPtr, StrideBPtr);
-  }
-
+  // dependence when measuring the distance between them.
   ScalarEvolution &SE = *PSE.getSE();
-  const SCEV *Dist = SE.getMinusSCEV(Sink, Src);
+  const SCEV *Dist =
+      StrideAPtr < 0 ? SE.getMinusSCEV(Src, Sink) : SE.getMinusSCEV(Sink, Src);
 
   LLVM_DEBUG(dbgs() << "LAA: Src Scev: " << *Src << "Sink Scev: " << *Sink
                     << "(Induction step: " << StrideAPtr << ")\n");
diff --git a/llvm/test/Analysis/LoopAccessAnalysis/forward-negative-step.ll b/llvm/test/Analysis/LoopAccessAnalysis/forward-negative-step.ll
index 89c1737fb730513..e73b6ef340556b2 100644
--- a/llvm/test/Analysis/LoopAccessAnalysis/forward-negative-step.ll
+++ b/llvm/test/Analysis/LoopAccessAnalysis/forward-negative-step.ll
@@ -1,24 +1,30 @@
+; NOTE: Assertions have been autogenerated by utils/update_analyze_test_checks.py UTC_ARGS: --version 3
 ; RUN: opt -passes='print<access-info>' -disable-output  < %s 2>&1 | FileCheck %s
 
 target datalayout = "e-m:e-i64:64-i128:128-n32:64-S128"
 
-; FIXME: This should be vectorizable
-
 ; void vectorizable_Read_Write(int *A) {
 ;  for (unsigned i = 1022; i >= 0; i--)
 ;    A[i+1] = A[i] + 1;
 ; }
 
-; CHECK: function 'vectorizable_Read_Write':
-; CHECK-NEXT:   for.body:
-; CHECK-NEXT:     Report: unsafe dependent memory operations in loop
-; CHECK-NEXT:     Forward loop carried data dependence that prevents store-to-load forwarding.
-; CHECK-NEXT:     Dependences:
-; CHECK-NEXT:       ForwardButPreventsForwarding:
-; CHECK-NEXT:           %0 = load i32, ptr %arrayidx, align 4 ->
-; CHECK-NEXT:           store i32 %add, ptr %gep, align 4
-
 define void @vectorizable_Read_Write(ptr nocapture %A) {
+; CHECK-LABEL: 'vectorizable_Read_Write'
+; CHECK-NEXT:    for.body:
+; CHECK-NEXT:      Memory dependences are safe
+; CHECK-NEXT:      Dependences:
+; CHECK-NEXT:        Forward:
+; CHECK-NEXT:            %0 = load i32, ptr %arrayidx, align 4 ->
+; CHECK-NEXT:            store i32 %add, ptr %gep, align 4
+; CHECK-EMPTY:
+; CHECK-NEXT:      Run-time memory checks:
+; CHECK-NEXT:      Grouped accesses:
+; CHECK-EMPTY:
+; CHECK-NEXT:      Non vectorizable stores to invariant address were not found in loop.
+; CHECK-NEXT:      SCEV assumptions:
+; CHECK-EMPTY:
+; CHECK-NEXT:      Expressions re-written:
+;
 entry:
   %invariant.gep = getelementptr i32, ptr %A, i64 1
   br label %for.body