[SimpleGVN] Improve casting (#2620)

* [SimpleGVN] Improve casting

* add test

Author: wsmoses

enzyme/Enzyme/SimpleGVN.cpp

@@ -115,6 +115,10 @@ Value *extractValue(IRBuilder<> &Builder, Value *StoredVal, Type *LoadType,
     return StoredVal;
   }
 
+  if (RelativeOffset == 0 && isa<PointerType>(LoadType) &&
+      isa<PointerType>(StoreType)) {
+    return Builder.CreatePointerCast(StoredVal, LoadType);
+  }
   // Handle extraction with offset or type mismatch
   // First, bitcast to an integer type if needed
   if (!StoreType->isIntegerTy()) {

enzyme/test/Enzyme/SimpleGVN/pointer_conversion.ll

@@ -0,0 +1,18 @@
+; RUN: if [ %llvmver -lt 16 ]; then %opt < %s %loadEnzyme -simple-gvn -S | FileCheck %s; fi
+; RUN: %opt < %s %newLoadEnzyme -passes="simple-gvn" -S | FileCheck %s
+
+define i8* @test_byte_extraction(i8** noalias nocapture %ptr, float* %x) {
+entry:
+  %c = bitcast i8** %ptr to float**
+  store float* %x, float** %c
+  %val = load i8*, i8** %ptr
+  ret i8* %val
+}
+
+; CHECK: define i8* @test_byte_extraction(i8** noalias nocapture %ptr, float* %x) {
+; CHECK-NEXT: entry:
+; CHECK-NEXT:   %c = bitcast i8** %ptr to float**
+; CHECK-NEXT:   store float* %x, float** %c, align 8
+; CHECK-NEXT:   %0 = bitcast float* %x to i8*
+; CHECK-NEXT:   ret i8* %0
+; CHECK-NEXT: }