[llvm] [LAA] Document reasoning in multiple places in isDependent (NFC). (PR #89381)

[Florian Hahn via llvm-commits]

https://github.com/fhahn created https://github.com/llvm/llvm-project/pull/89381

As suggested in https://github.com/llvm/llvm-project/pull/88039, add extra documentation for reasoning in isDependent.

>From 2e9a43a783290a9ea50dec7a778badb3b8d5030f Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Fri, 19 Apr 2024 13:47:42 +0100
Subject: [PATCH] [LAA] Document reasoning in multiple places in isDependent
 (NFC).

As suggested in https://github.com/llvm/llvm-project/pull/88039, add
extra documentation for reasoning in isDependent.
---
 llvm/lib/Analysis/LoopAccessAnalysis.cpp | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/Analysis/LoopAccessAnalysis.cpp b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
index 3bfc9700a14559..b1ba8e7c0f6014 100644
--- a/llvm/lib/Analysis/LoopAccessAnalysis.cpp
+++ b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
@@ -2034,6 +2034,10 @@ MemoryDepChecker::Dependence::DepType MemoryDepChecker::isDependent(
 
   ScalarEvolution &SE = *PSE.getSE();
   auto &DL = InnermostLoop->getHeader()->getModule()->getDataLayout();
+  // If the distance between the acecsses is larger than their absolute stride
+  // multiplied by the backedge taken count, the accesses are independet, i.e.
+  // they are far enough appart that accesses won't access the same location
+  // across all loop ierations.
   if (!isa<SCEVCouldNotCompute>(Dist) && HasSameSize &&
       isSafeDependenceDistance(DL, SE, *(PSE.getBackedgeTakenCount()), *Dist,
                                Stride, TypeByteSize))
@@ -2049,7 +2053,8 @@ MemoryDepChecker::Dependence::DepType MemoryDepChecker::isDependent(
   const APInt &Val = C->getAPInt();
   int64_t Distance = Val.getSExtValue();
 
-  // Attempt to prove strided accesses independent.
+  // If the distance between accesses and their strides are known constants,
+  // check whether the accesses interlace each other.
   if (std::abs(Distance) > 0 && Stride > 1 && HasSameSize &&
       areStridedAccessesIndependent(std::abs(Distance), Stride, TypeByteSize)) {
     LLVM_DEBUG(dbgs() << "LAA: Strided accesses are independent\n");
@@ -2059,9 +2064,13 @@ MemoryDepChecker::Dependence::DepType MemoryDepChecker::isDependent(
   // Negative distances are not plausible dependencies.
   if (Val.isNegative()) {
     bool IsTrueDataDependence = (AIsWrite && !BIsWrite);
-    // There is no need to update MaxSafeVectorWidthInBits after call to
-    // couldPreventStoreLoadForward, even if it changed MinDepDistBytes,
-    // since a forward dependency will allow vectorization using any width.
+    // Check if the first access writes to a location that is read in a later
+    // iteration, where the distance between them is not a multiple of a vector
+    // factor and relatively small.
+    //
+    // NOTE: There is no need to update MaxSafeVectorWidthInBits after call to
+    // couldPreventStoreLoadForward, even if it changed MinDepDistBytes, since a
+    // forward dependency will allow vectorization using any width.
     if (IsTrueDataDependence && EnableForwardingConflictDetection &&
         (!HasSameSize || couldPreventStoreLoadForward(Val.abs().getZExtValue(),
                                                       TypeByteSize))) {