[llvm] [InstCombine] Fold pointer adding in integer to arithmetic add (PR #91596)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Monad (YanWQ-monad)

<details>
<summary>Changes</summary>

Fold
``` llvm
define i32 @<!-- -->src(i32 %x, i32 %y) {
  %base = inttoptr i32 %x to ptr
  %ptr = getelementptr inbounds i8, ptr %base, i32 %y
  %r = ptrtoint ptr %ptr to i32
  ret i32 %r
}
```
where both `%base` and `%ptr` have only one use, to
``` llvm
define i32 @<!-- -->tgt(i32 %x, i32 %y) {
  %r = add nuw i32 %x, %y
  ret i32 %r
}
```

### Motivation

It seems unnecessary to convert `int` to `ptr` just to get its offset. In most cases, they generates the same assembly, but sometimes it may miss some optimizations since the analysis of `GEP` is not as perfect as that of arithmetic operation. One example is

https://github.com/dtcxzyw/llvm-opt-benchmark/blob/e3c822bf41df3a88ca38eba884a52b0cc7e70bf2/bench/protobuf/optimized/generated_message_reflection.cc.ll#L39860-L39873

``` llvm
  %conv.i188 = zext i32 %145 to i64
  %add.i189 = add i64 %conv.i188, %125
  %146 = load i16, ptr %num_aux_entries10.i, align 2
  %conv2.i191 = zext i16 %146 to i64
  %mul.i192 = shl nuw nsw i64 %conv2.i191, 3
  %add3.i193 = add i64 %add.i189, %mul.i192
  %147 = inttoptr i64 %add3.i193 to ptr
  %sub.ptr.lhs.cast.i195 = ptrtoint ptr %144 to i64
  %sub.ptr.rhs.cast.i196 = ptrtoint ptr %143 to i64
  %sub.ptr.sub.i197 = sub i64 %sub.ptr.lhs.cast.i195, %sub.ptr.rhs.cast.i196
  %add.ptr = getelementptr inbounds i8, ptr %147, i64 %sub.ptr.sub.i197
  %sub.ptr.lhs.cast = ptrtoint ptr %add.ptr to i64
  %sub.ptr.sub = sub i64 %sub.ptr.lhs.cast, %125
```

where `%conv.i188` first adds `%125` and then subtracts `%125` (the result is `%sub.ptr.sub`), which can be optimized.

But this fold involves `ptr`, which Alive2 can't verify, plus that in most cases, they generate the same assembly, I am not sure whether it is profitable.

---
Full diff: https://github.com/llvm/llvm-project/pull/91596.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp (+10) 
- (modified) llvm/test/Transforms/InstCombine/cast_ptr.ll (+54) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 11e31877de38..18ad8706f884 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -2073,6 +2073,16 @@ Instruction *InstCombinerImpl::visitPtrToInt(PtrToIntInst &CI) {
     return InsertElementInst::Create(Vec, NewCast, Index);
   }
 
+  // (ptrtoint (gep (inttoptr Base), Offset)) -> Base + Offset
+  Value *Base, *Offset;
+  if (match(SrcOp, m_OneUse(m_PtrAdd(m_OneUse(m_IntToPtr(m_Value(Base))),
+                                     m_Value(Offset)))) &&
+      Base->getType() == Ty && Offset->getType() == Ty) {
+    auto *NewOp = BinaryOperator::CreateAdd(Base, Offset);
+    NewOp->setHasNoUnsignedWrap(true);
+    return NewOp;
+  }
+
   return commonCastTransforms(CI);
 }
 
diff --git a/llvm/test/Transforms/InstCombine/cast_ptr.ll b/llvm/test/Transforms/InstCombine/cast_ptr.ll
index 5c6c012064e0..0e191748226d 100644
--- a/llvm/test/Transforms/InstCombine/cast_ptr.ll
+++ b/llvm/test/Transforms/InstCombine/cast_ptr.ll
@@ -244,3 +244,57 @@ define <2 x i32> @insertelt_extra_use2(<2 x i32> %x, ptr %p) {
   %r = ptrtoint <2 x ptr> %i to <2 x i32>
   ret <2 x i32> %r
 }
+
+define i32 @ptr_add_in_int(i32 %x, i32 %y) {
+; CHECK-LABEL: @ptr_add_in_int(
+; CHECK-NEXT:    [[R:%.*]] = add nuw i32 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i32 [[R]]
+;
+  %ptr = inttoptr i32 %x to ptr
+  %p2 = getelementptr inbounds i8, ptr %ptr, i32 %y
+  %r = ptrtoint ptr %p2 to i32
+  ret i32 %r
+}
+
+define i32 @ptr_add_in_int_const(i32 %x) {
+; CHECK-LABEL: @ptr_add_in_int_const(
+; CHECK-NEXT:    [[R:%.*]] = add nuw i32 [[X:%.*]], 4096
+; CHECK-NEXT:    ret i32 [[R]]
+;
+  %ptr = inttoptr i32 %x to ptr
+  %p2 = getelementptr inbounds i8, ptr %ptr, i32 4096
+  %r = ptrtoint ptr %p2 to i32
+  ret i32 %r
+}
+
+declare void @use_ptr(ptr)
+
+define i32 @ptr_add_in_int_extra_use1(i32 %x) {
+; CHECK-LABEL: @ptr_add_in_int_extra_use1(
+; CHECK-NEXT:    [[PTR:%.*]] = inttoptr i32 [[X:%.*]] to ptr
+; CHECK-NEXT:    call void @use_ptr(ptr [[PTR]])
+; CHECK-NEXT:    [[P2:%.*]] = getelementptr inbounds i8, ptr [[PTR]], i32 4096
+; CHECK-NEXT:    [[R:%.*]] = ptrtoint ptr [[P2]] to i32
+; CHECK-NEXT:    ret i32 [[R]]
+;
+  %ptr = inttoptr i32 %x to ptr
+  call void @use_ptr(ptr %ptr)
+  %p2 = getelementptr inbounds i8, ptr %ptr, i32 4096
+  %r = ptrtoint ptr %p2 to i32
+  ret i32 %r
+}
+
+define i32 @ptr_add_in_int_extra_use2(i32 %x) {
+; CHECK-LABEL: @ptr_add_in_int_extra_use2(
+; CHECK-NEXT:    [[PTR:%.*]] = inttoptr i32 [[X:%.*]] to ptr
+; CHECK-NEXT:    [[P2:%.*]] = getelementptr inbounds i8, ptr [[PTR]], i32 4096
+; CHECK-NEXT:    call void @use_ptr(ptr nonnull [[P2]])
+; CHECK-NEXT:    [[R:%.*]] = ptrtoint ptr [[P2]] to i32
+; CHECK-NEXT:    ret i32 [[R]]
+;
+  %ptr = inttoptr i32 %x to ptr
+  %p2 = getelementptr inbounds i8, ptr %ptr, i32 4096
+  call void @use_ptr(ptr %p2)
+  %r = ptrtoint ptr %p2 to i32
+  ret i32 %r
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/91596