[PATCH] D44833: [InstCombine] peek through bitcasted vector/array pointer GEP operand

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
spatel added reviewers: efriedma, majnemer, hfinkel.
Herald added a subscriber: mcrosier.

The bitcast may be interfering with other combines or vectorization as shown in PR16739:
https://bugs.llvm.org/show_bug.cgi?id=16739

I haven't looked at GEPs very much, so I'm not sure if I've missed any safety checks here. I'm also not sure if this warning (still) applies:

http://llvm.org/docs/GetElementPtr.html#can-gep-index-into-vector-elements
"This hasn’t always been forcefully disallowed, though it’s not recommended. It leads to awkward special cases in the optimizers, and fundamental inconsistency in the IR. In the future, it will probably be outright disallowed."

...but it's 5 years since the bug was filed, and the IR looks the same.


https://reviews.llvm.org/D44833

Files:
  lib/Transforms/InstCombine/InstructionCombining.cpp
  test/Transforms/InstCombine/gep-vector.ll


Index: test/Transforms/InstCombine/gep-vector.ll
===================================================================
--- test/Transforms/InstCombine/gep-vector.ll
+++ test/Transforms/InstCombine/gep-vector.ll
@@ -27,3 +27,23 @@
   ret <2 x i8*> %1
 }
 
+define i32* @bitcast_vec_to_array_gep(<7 x i32>* %x, i64 %y, i64 %z) {
+; CHECK-LABEL: @bitcast_vec_to_array_gep(
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr <7 x i32>, <7 x i32>* [[X:%.*]], i64 [[Y:%.*]], i64 [[Z:%.*]]
+; CHECK-NEXT:    ret i32* [[GEP]]
+;
+  %arr_ptr = bitcast <7 x i32>* %x to [7 x i32]*
+  %gep = getelementptr [7 x i32], [7 x i32]* %arr_ptr, i64 %y, i64 %z
+  ret i32* %gep
+}
+
+define i32* @bitcast_array_to_vec_gep([3 x i32]* %x, i64 %y, i64 %z) {
+; CHECK-LABEL: @bitcast_array_to_vec_gep(
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds [3 x i32], [3 x i32]* [[X:%.*]], i64 [[Y:%.*]], i64 [[Z:%.*]]
+; CHECK-NEXT:    ret i32* [[GEP]]
+;
+  %vec_ptr = bitcast [3 x i32]* %x to <3 x i32>*
+  %gep = getelementptr inbounds <3 x i32>, <3 x i32>* %vec_ptr, i64 %y, i64 %z
+  ret i32* %gep
+}
+
Index: lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- lib/Transforms/InstCombine/InstructionCombining.cpp
+++ lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1963,14 +1963,34 @@
       PtrOp = BC;
   }
 
-  /// See if we can simplify:
-  ///   X = bitcast A* to B*
-  ///   Y = gep X, <...constant indices...>
-  /// into a gep of the original struct.  This is important for SROA and alias
-  /// analysis of unions.  If "A" is also a bitcast, wait for A/X to be merged.
   if (auto *BCI = dyn_cast<BitCastInst>(PtrOp)) {
     Value *SrcOp = BCI->getOperand(0);
     PointerType *SrcType = cast<PointerType>(BCI->getSrcTy());
+    Type *SrcEltType = SrcType->getElementType();
+
+    // GEP directly using the source operand if this GEP is accessing an element
+    // of a bitcasted pointer to vector or array of the same dimensions:
+    // gep (bitcast <c x ty>* X to [c x ty]*), Y, Z --> gep X, Y, Z
+    // gep (bitcast [c x ty]* X to <c x ty>*), Y, Z --> gep X, Y, Z
+    auto areMatchingArrayAndVecTypes = [](Type *ArrTy, Type *VecTy) {
+      return ArrTy->getArrayElementType() == VecTy->getVectorElementType() &&
+             ArrTy->getArrayNumElements() == VecTy->getVectorNumElements();
+    };
+    if (GEP.getNumOperands() == 3 &&
+        ((GEPEltType->isArrayTy() && SrcEltType->isVectorTy() &&
+          areMatchingArrayAndVecTypes(GEPEltType, SrcEltType)) ||
+         (GEPEltType->isVectorTy() && SrcEltType->isArrayTy() &&
+          areMatchingArrayAndVecTypes(SrcEltType, GEPEltType)))) {
+      GEP.setOperand(0, SrcOp);
+      GEP.setSourceElementType(SrcEltType);
+      return &GEP;
+    }
+
+    // See if we can simplify:
+    //   X = bitcast A* to B*
+    //   Y = gep X, <...constant indices...>
+    // into a gep of the original struct. This is important for SROA and alias
+    // analysis of unions. If "A" is also a bitcast, wait for A/X to be merged.
     unsigned OffsetBits = DL.getIndexTypeSizeInBits(GEPType);
     APInt Offset(OffsetBits, 0);
     if (!isa<BitCastInst>(SrcOp) && GEP.accumulateConstantOffset(DL, Offset)) {


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