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

[Madhur Amilkanthwar via llvm-commits]

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@@ -3330,6 +3334,128 @@ void GVNPass::assignValNumForDeadCode() {
   }
 }
 
+bool GVNPass::transformMinFindingSelectPattern(
+    Loop *L, Type *LoadType, BasicBlock *Preheader, BasicBlock *BB, Value *LHS,
+    Value *RHS, CmpInst *Comparison, SelectInst *Select, Value *BasePtr,
+    Value *IndexVal, Value *OffsetVal) {
+  // Hoist the chain of operations for the second load to preheader.
+  // %min.idx.ext = sext i32 %min.idx to i64
+  // %ptr.float.min = getelementptr float, ptr %0, i64 %min.idx.ext
+  // %ptr.second.load = getelementptr i8, ptr %ptr.float.min, i64 -4
+  // %val.current.min = load float, ptr %ptr.second.load, align 4
+  IRBuilder<> Builder(Preheader->getTerminator());
+
+  PHINode *Phi = dyn_cast<PHINode>(IndexVal);
+  if (!Phi) {
+    LLVM_DEBUG(dbgs() << "GVN: IndexVal is not a PHI node\n");
+    return false;
+  }
+
+  Value *InitialMinIndex = Phi->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 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");
+
+  // 4. hoisted_load = load float, ptr HoistedGEP2
+  LoadInst *NewLoad = Builder.CreateLoad(LoadType, HoistedGEP2, "hoisted_load");
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madhur13490 wrote:

Done. Added the loop for checking clobbering.

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