[PATCH] D98588: [InstCombine] Restrict a GEP transform to avoid changing provenance

[Simonas Kazlauskas via Phabricator via llvm-commits]

nagisa updated this revision to Diff 330471.
nagisa added a comment.

Rebase onto precommited test cases, adjust style


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D98588/new/

https://reviews.llvm.org/D98588

Files:
  llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
  llvm/test/Transforms/InstCombine/getelementptr.ll


Index: llvm/test/Transforms/InstCombine/getelementptr.ll
===================================================================
--- llvm/test/Transforms/InstCombine/getelementptr.ll
+++ llvm/test/Transforms/InstCombine/getelementptr.ll
@@ -1087,7 +1087,11 @@
 
 define %struct.C* @test45(%struct.C* %c1, %struct.C** %c2) {
 ; CHECK-LABEL: @test45(
-; CHECK-NEXT:    [[GEP:%.*]] = bitcast %struct.C** [[C2:%.*]] to %struct.C*
+; CHECK-NEXT:    [[PTRTOINT1:%.*]] = ptrtoint %struct.C* [[C1:%.*]] to i64
+; CHECK-NEXT:    [[PTRTOINT2:%.*]] = ptrtoint %struct.C** [[C2:%.*]] to i64
+; CHECK-NEXT:    [[SUB:%.*]] = sub i64 [[PTRTOINT2]], [[PTRTOINT1]]
+; CHECK-NEXT:    [[SHR:%.*]] = sdiv i64 [[SUB]], 7
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds [[STRUCT_C:%.*]], %struct.C* [[C1]], i64 [[SHR]]
 ; CHECK-NEXT:    ret %struct.C* [[GEP]]
 ;
   %ptrtoint1 = ptrtoint %struct.C* %c1 to i64
Index: llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -2173,14 +2173,15 @@
           Matched = true;
       }
 
-      if (Matched) {
-        // Canonicalize (gep i8* X, (ptrtoint Y)-(ptrtoint X))
-        // to (bitcast Y)
-        Value *Y;
-        if (match(V, m_Sub(m_PtrToInt(m_Value(Y)),
-                           m_PtrToInt(m_Specific(GEP.getOperand(0))))))
-          return CastInst::CreatePointerBitCastOrAddrSpaceCast(Y, GEPType);
-      }
+      // Canonicalize (gep i8* X, (ptrtoint Y)-(ptrtoint X)) to (bitcast Y), but
+      // only if both point to the same underlying object (otherwise provenance
+      // is not necessarily retained).
+      Value *Y;
+      Value *X = GEP.getOperand(0);
+      if (Matched &&
+          match(V, m_Sub(m_PtrToInt(m_Value(Y)), m_PtrToInt(m_Specific(X)))) &&
+          getUnderlyingObject(X) == getUnderlyingObject(Y))
+        return CastInst::CreatePointerBitCastOrAddrSpaceCast(Y, GEPType);
     }
   }
 


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