[llvm] [GVN] Support rnflow pattern matching and transform (PR #162259)

[Madhur Amilkanthwar via llvm-commits]

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@@ -3339,6 +3336,302 @@ void GVNPass::assignValNumForDeadCode() {
   }
 }
 
+/// Return true if the load can be hoisted to the loop preheader (no clobber
+/// in the loop) using MemorySSA's clobbering access.
+static bool canHoistLoadWithMSSA(Loop *L, Instruction *LoadInst,
+                                 MemorySSAUpdater *MSSAU) {
+  MemoryAccess *MA = MSSAU->getMemorySSA()->getMemoryAccess(LoadInst);
+  assert(MA && "MemoryAccess expected when MemorySSA is available");
+  MemoryAccess *Clobber =
+      MSSAU->getMemorySSA()->getSkipSelfWalker()->getClobberingMemoryAccess(MA);
+  if (!Clobber || MSSAU->getMemorySSA()->isLiveOnEntryDef(Clobber))
+    return true;
+  if (!L->contains(Clobber->getBlock()))
+    return true;
+  LLVM_DEBUG(dbgs() << "GVN: Cannot hoist - clobbered in loop by " << *Clobber
+                    << "\n");
+  return false;
+}
+
+/// Return true if the load can be hoisted to the loop preheader (no clobber
+/// in the loop) using MemoryDependenceResults.
+static bool canHoistLoadWithMD(Loop *L, LoadInst *Load,
+                               MemoryDependenceResults *MD) {
+  MemDepResult Dep = MD->getDependency(Load);
+  if (Dep.isLocal() && (Dep.isDef() || Dep.isClobber())) {
+    Instruction *DepInst = Dep.getInst();
+    if (DepInst && L->contains(DepInst->getParent())) {
+      LLVM_DEBUG(dbgs() << "GVN: Cannot hoist - clobbered in loop by "
+                        << *DepInst << "\n");
+      return false;
+    }
+  } else if (Dep.isNonLocal()) {
+    SmallVector<NonLocalDepResult, 64> Deps;
+    MD->getNonLocalPointerDependency(Load, Deps);
+    for (const auto &NLDep : Deps) {
+      if (L->contains(NLDep.getBB()) &&
+          (NLDep.getResult().isDef() || NLDep.getResult().isClobber())) {
+        LLVM_DEBUG(dbgs() << "GVN: Cannot hoist - clobbered in loop (block "
+                          << NLDep.getBB()->getName() << ")\n");
+        return false;
+      }
+    }
+  } else if (Dep.isUnknown()) {
+    LLVM_DEBUG(dbgs() << "GVN: Cannot hoist - unknown memory dependence\n");
+    return false;
+  }
+  return true;
+}
+
+/// Hoist the chain of operations for the second load to preheader.
+/// In this transformation, we hoist the redundant load to the preheader,
+/// caching the first value of the iteration. This value is used to compare with
+/// the current value of the iteration and update the minimum value.
+/// The comparison is done in the loop body using the new select instruction.
+///
+/// *** Before transformation ***
+///
+///  preheader:
+///    ...
+///  loop:
+///    ...
+///    ...
+///    %val.first = load <TYPE>, ptr %ptr.first.load, align 4
+///    %min.idx.ext = sext i32 %min.idx to i64
+///    %ptr.<TYPE>.min = getelementptr <TYPE>, ptr %0, i64 %min.idx.ext
+///    %ptr.second.load = getelementptr i8, ptr %ptr.<TYPE>.min, i64 -4
+///    %val.current.min = load <TYPE>, ptr %ptr.second.load, align 4
+///    ...
+///    ...
+///    br i1 %cond, label %loop, label %exit
+///
+///    We capture <TYPE> as a part of pattern matching and then later
+///    use it in the transformation.
+///
+/// *** After transformation ***
+///
+///  preheader:
+///    %min.idx.ext = sext i32 %min.idx.ext to i64
+///    %hoist_gep1 = getelementptr <TYPE>, ptr %0, i64 %min.idx.ext
+///    %hoist_gep2 = getelementptr i8, ptr %hoist_gep1, i64 -4
+///    %hoisted_load = load <TYPE>, ptr %hoist_gep2, align 4
+///    br label %loop
+///
+///  loop:
+///    %val.first = load <TYPE>, ptr %ptr.first.load, align 4
+///    ...
+///    (new) %val.current.min = select i1 %cond, <TYPE> %hoisted_load, <TYPE>
+///    %val.current.min
+///    ...
+///    ...
+///    br i1 %cond, label %loop, label %exit
+bool GVNPass::transformMinFindingSelectPattern(
+    Loop *L, Type *LoadType, BasicBlock *Preheader, BasicBlock *BB, Value *LHS,
+    Value *LoadVal, CmpInst *Comparison, SelectInst *Select, Value *BasePtr,
+    PHINode *IndexValPhi, Value *OffsetVal) {
+
+  assert(BasePtr && "BasePtr is null");
+  assert(OffsetVal && "OffsetVal is null");
+  assert(IndexValPhi && "IndexValPhi is null");
+  AAResults *AA = VN.getAliasAnalysis();
----------------
madhur13490 wrote:

Done.

https://github.com/llvm/llvm-project/pull/162259