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

[Madhur Amilkanthwar via llvm-commits]

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@@ -3330,6 +3333,235 @@ void GVNPass::assignValNumForDeadCode() {
   }
 }
 
+// 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
+//
+// *** 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,
+    Value *IndexVal, Value *OffsetVal) {
+
+  assert(IndexVal && "IndexVal is null");
+  AAResults *AA = VN.getAliasAnalysis();
+  assert(AA && "AA is null");
+
+  IRBuilder<> Builder(Preheader->getTerminator());
+  Value *InitialMinIndex =
+      dyn_cast<PHINode>(IndexVal)->getIncomingValueForBlock(Preheader);
+
+  // Insert PHI node at the top of this block.
+  // This PHI node will be used to memoize the current minimum value so far.
+  PHINode *KnownMinPhi = PHINode::Create(LoadType, 2, "known_min", BB->begin());
+
+  // Hoist the load and build the necessary operations.
+  // 1. hoist_0 = sext i32 1 to i64
+  Value *HoistedSExt =
+      Builder.CreateSExt(InitialMinIndex, Builder.getInt64Ty(), "hoist_sext");
+
+  // 2. hoist_gep1 = getelementptr float, ptr BasePtr, i64 HoistedSExt
+  Value *HoistedGEP1 =
+      Builder.CreateGEP(LoadType, BasePtr, HoistedSExt, "hoist_gep1");
+
+  // 3. hoist_gep2 = getelementptr i8, ptr HoistedGEP1, i64 OffsetVal
+  Value *HoistedGEP2 = Builder.CreateGEP(Builder.getInt8Ty(), HoistedGEP1,
+                                         OffsetVal, "hoist_gep2");
+
+  MemoryLocation NewLoc = MemoryLocation(
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madhur13490 wrote:

Done

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