[PATCH] D72048: [InstCombine] Preserve nuw on sub of geps (PR44419)

[Nikita Popov via Phabricator via llvm-commits]

nikic updated this revision to Diff 235769.
nikic added a comment.

Only add nuw to final mul.

As the scope of this optimization turned out to be a lot smaller than I originally expected, I've moved this as a fixup operation in OptimizePointerDifference(), instead of directly emitting the nuw flag in EmitGEPOffset(), which is somewhat tricky to do.

The code is slightly odd due to the need to match the zero add, which I wasn't able to simply drop for the reason mentioned above.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D72048/new/

https://reviews.llvm.org/D72048

Files:
  llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
  llvm/lib/Transforms/InstCombine/InstCombineInternal.h
  llvm/test/Transforms/InstCombine/sub-gep.ll


Index: llvm/test/Transforms/InstCombine/sub-gep.ll
===================================================================
--- llvm/test/Transforms/InstCombine/sub-gep.ll
+++ llvm/test/Transforms/InstCombine/sub-gep.ll
@@ -16,7 +16,7 @@
 
 define i64 @test_inbounds_nuw([0 x i32]* %base, i64 %idx) {
 ; CHECK-LABEL: @test_inbounds_nuw(
-; CHECK-NEXT:    [[P2_IDX:%.*]] = shl nsw i64 [[IDX:%.*]], 2
+; CHECK-NEXT:    [[P2_IDX:%.*]] = shl nuw nsw i64 [[IDX:%.*]], 2
 ; CHECK-NEXT:    ret i64 [[P2_IDX]]
 ;
   %p1 = getelementptr inbounds [0 x i32], [0 x i32]* %base, i64 0, i64 0
@@ -84,3 +84,18 @@
   %d = sub nuw i64 %i2, %i1
   ret i64 %d
 }
+
+define i64 @test_inbounds_nuw_multi_index([0 x [2 x i32]]* %base, i64 %idx, i64 %idx2) {
+; CHECK-LABEL: @test_inbounds_nuw_multi_index(
+; CHECK-NEXT:    [[P2_IDX:%.*]] = shl nsw i64 [[IDX:%.*]], 3
+; CHECK-NEXT:    [[P2_IDX1:%.*]] = shl nsw i64 [[IDX2:%.*]], 2
+; CHECK-NEXT:    [[P2_OFFS2:%.*]] = add i64 [[P2_IDX]], [[P2_IDX1]]
+; CHECK-NEXT:    ret i64 [[P2_OFFS2]]
+;
+  %p1 = getelementptr inbounds [0 x [2 x i32]], [0 x [2 x i32]]* %base, i64 0, i64 0, i64 0
+  %p2 = getelementptr inbounds [0 x [2 x i32]], [0 x [2 x i32]]* %base, i64 0, i64 %idx, i64 %idx2
+  %i1 = ptrtoint i32* %p1 to i64
+  %i2 = ptrtoint i32* %p2 to i64
+  %d = sub nuw i64 %i2, %i1
+  ret i64 %d
+}
Index: llvm/lib/Transforms/InstCombine/InstCombineInternal.h
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineInternal.h
+++ llvm/lib/Transforms/InstCombine/InstCombineInternal.h
@@ -369,7 +369,8 @@
   Instruction *visitFNeg(UnaryOperator &I);
   Instruction *visitAdd(BinaryOperator &I);
   Instruction *visitFAdd(BinaryOperator &I);
-  Value *OptimizePointerDifference(Value *LHS, Value *RHS, Type *Ty);
+  Value *OptimizePointerDifference(
+      Value *LHS, Value *RHS, Type *Ty, bool isNUW);
   Instruction *visitSub(BinaryOperator &I);
   Instruction *visitFSub(BinaryOperator &I);
   Instruction *visitMul(BinaryOperator &I);
Index: llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1585,7 +1585,7 @@
 ///  &A[10] - &A[0]: we should compile this to "10".  LHS/RHS are the pointer
 /// operands to the ptrtoint instructions for the LHS/RHS of the subtract.
 Value *InstCombiner::OptimizePointerDifference(Value *LHS, Value *RHS,
-                                               Type *Ty) {
+                                               Type *Ty, bool IsNUW) {
   // If LHS is a gep based on RHS or RHS is a gep based on LHS, we can optimize
   // this.
   bool Swapped = false;
@@ -1653,6 +1653,15 @@
   // Emit the offset of the GEP and an intptr_t.
   Value *Result = EmitGEPOffset(GEP1);
 
+  // If this is a single inbounds GEP and the original sub was nuw,
+  // then the final multiplication is also nuw. We match an extra add zero
+  // here, because that's what EmitGEPOffset() generates.
+  Instruction *I;
+  if (IsNUW && !GEP2 && !Swapped && GEP1->isInBounds() &&
+      match(Result, m_Add(m_Instruction(I), m_Zero())) &&
+      I->getOpcode() == Instruction::Mul)
+    I->setHasNoUnsignedWrap();
+
   // If we had a constant expression GEP on the other side offsetting the
   // pointer, subtract it from the offset we have.
   if (GEP2) {
@@ -1983,13 +1992,15 @@
   Value *LHSOp, *RHSOp;
   if (match(Op0, m_PtrToInt(m_Value(LHSOp))) &&
       match(Op1, m_PtrToInt(m_Value(RHSOp))))
-    if (Value *Res = OptimizePointerDifference(LHSOp, RHSOp, I.getType()))
+    if (Value *Res = OptimizePointerDifference(LHSOp, RHSOp, I.getType(),
+                                               I.hasNoUnsignedWrap()))
       return replaceInstUsesWith(I, Res);
 
   // trunc(p)-trunc(q) -> trunc(p-q)
   if (match(Op0, m_Trunc(m_PtrToInt(m_Value(LHSOp)))) &&
       match(Op1, m_Trunc(m_PtrToInt(m_Value(RHSOp)))))
-    if (Value *Res = OptimizePointerDifference(LHSOp, RHSOp, I.getType()))
+    if (Value *Res = OptimizePointerDifference(LHSOp, RHSOp, I.getType(),
+                                               /* IsNUW */ false))
       return replaceInstUsesWith(I, Res);
 
   // Canonicalize a shifty way to code absolute value to the common pattern.


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