[llvm-branch-commits] [llvm] [SDAG] Introduce inbounds flag for pointer arithmetic (PR #131862)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-selectiondag

Author: Fabian Ritter (ritter-x2a)

<details>
<summary>Changes</summary>

This patch introduces an inbounds SDNodeFlag, to show that a pointer
addition SDNode implements an inbounds getelementptr operation (i.e.,
the pointer operand is in bounds wrt. the allocated object it is based
on, and the arithmetic does not change that). The flag is set in the DAG
construction when lowering inbounds GEPs.

Inbounds information is useful in the ISel when selecting memory
instructions that perform address computations whose intermediate steps
must be in the same memory region as the final result. A follow-up patch
will start using it for AMDGPU's flat memory instructions, where the
immediate offset must not affect the memory aperture of the address.

A similar patch for gMIR and GlobalISel will follow.

For SWDEV-516125.

---
Full diff: https://github.com/llvm/llvm-project/pull/131862.diff


4 Files Affected:

- (modified) llvm/include/llvm/CodeGen/SelectionDAGNodes.h (+7-2) 
- (modified) llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp (+3) 
- (modified) llvm/lib/CodeGen/SelectionDAG/SelectionDAGDumper.cpp (+3) 
- (modified) llvm/test/CodeGen/X86/merge-store-partially-alias-loads.ll (+1-1) 


``````````diff
diff --git a/llvm/include/llvm/CodeGen/SelectionDAGNodes.h b/llvm/include/llvm/CodeGen/SelectionDAGNodes.h
index 2283f99202e2f..13ac65f5d731c 100644
--- a/llvm/include/llvm/CodeGen/SelectionDAGNodes.h
+++ b/llvm/include/llvm/CodeGen/SelectionDAGNodes.h
@@ -415,12 +415,15 @@ struct SDNodeFlags {
     Unpredictable = 1 << 13,
     // Compare instructions which may carry the samesign flag.
     SameSign = 1 << 14,
+    // Pointer arithmetic instructions that remain in bounds, e.g., implementing
+    // an inbounds GEP.
+    InBounds = 1 << 15,
 
     // NOTE: Please update LargestValue in LLVM_DECLARE_ENUM_AS_BITMASK below
     // the class definition when adding new flags.
 
     PoisonGeneratingFlags = NoUnsignedWrap | NoSignedWrap | Exact | Disjoint |
-                            NonNeg | NoNaNs | NoInfs | SameSign,
+                            NonNeg | NoNaNs | NoInfs | SameSign | InBounds,
     FastMathFlags = NoNaNs | NoInfs | NoSignedZeros | AllowReciprocal |
                     AllowContract | ApproximateFuncs | AllowReassociation,
   };
@@ -455,6 +458,7 @@ struct SDNodeFlags {
   void setAllowReassociation(bool b) { setFlag<AllowReassociation>(b); }
   void setNoFPExcept(bool b) { setFlag<NoFPExcept>(b); }
   void setUnpredictable(bool b) { setFlag<Unpredictable>(b); }
+  void setInBounds(bool b) { setFlag<InBounds>(b); }
 
   // These are accessors for each flag.
   bool hasNoUnsignedWrap() const { return Flags & NoUnsignedWrap; }
@@ -472,6 +476,7 @@ struct SDNodeFlags {
   bool hasAllowReassociation() const { return Flags & AllowReassociation; }
   bool hasNoFPExcept() const { return Flags & NoFPExcept; }
   bool hasUnpredictable() const { return Flags & Unpredictable; }
+  bool hasInBounds() const { return Flags & InBounds; }
 
   bool operator==(const SDNodeFlags &Other) const {
     return Flags == Other.Flags;
@@ -481,7 +486,7 @@ struct SDNodeFlags {
 };
 
 LLVM_DECLARE_ENUM_AS_BITMASK(decltype(SDNodeFlags::None),
-                             SDNodeFlags::SameSign);
+                             SDNodeFlags::InBounds);
 
 inline SDNodeFlags operator|(SDNodeFlags LHS, SDNodeFlags RHS) {
   LHS |= RHS;
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 14bb1d943d2d6..f2e5cb64d5b52 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -4293,6 +4293,7 @@ void SelectionDAGBuilder::visitGetElementPtr(const User &I) {
         if (NW.hasNoUnsignedWrap() ||
             (int64_t(Offset) >= 0 && NW.hasNoUnsignedSignedWrap()))
           Flags |= SDNodeFlags::NoUnsignedWrap;
+        Flags.setInBounds(NW.isInBounds());
 
         N = DAG.getNode(ISD::ADD, dl, N.getValueType(), N,
                         DAG.getConstant(Offset, dl, N.getValueType()), Flags);
@@ -4336,6 +4337,7 @@ void SelectionDAGBuilder::visitGetElementPtr(const User &I) {
         if (NW.hasNoUnsignedWrap() ||
             (Offs.isNonNegative() && NW.hasNoUnsignedSignedWrap()))
           Flags.setNoUnsignedWrap(true);
+        Flags.setInBounds(NW.isInBounds());
 
         OffsVal = DAG.getSExtOrTrunc(OffsVal, dl, N.getValueType());
 
@@ -4398,6 +4400,7 @@ void SelectionDAGBuilder::visitGetElementPtr(const User &I) {
       // pointer index type (add nuw).
       SDNodeFlags AddFlags;
       AddFlags.setNoUnsignedWrap(NW.hasNoUnsignedWrap());
+      AddFlags.setInBounds(NW.isInBounds());
 
       N = DAG.getNode(ISD::ADD, dl, N.getValueType(), N, IdxN, AddFlags);
     }
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGDumper.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGDumper.cpp
index 64ecff8d71f98..6360deb1ed528 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGDumper.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGDumper.cpp
@@ -663,6 +663,9 @@ void SDNode::print_details(raw_ostream &OS, const SelectionDAG *G) const {
   if (getFlags().hasSameSign())
     OS << " samesign";
 
+  if (getFlags().hasInBounds())
+    OS << " inbounds";
+
   if (getFlags().hasNonNeg())
     OS << " nneg";
 
diff --git a/llvm/test/CodeGen/X86/merge-store-partially-alias-loads.ll b/llvm/test/CodeGen/X86/merge-store-partially-alias-loads.ll
index c1fdd71c04948..6e16f946f8ac0 100644
--- a/llvm/test/CodeGen/X86/merge-store-partially-alias-loads.ll
+++ b/llvm/test/CodeGen/X86/merge-store-partially-alias-loads.ll
@@ -16,7 +16,7 @@
 ; DBGDAG-LABEL: Optimized legalized selection DAG: %bb.0 'merge_store_partial_overlap_load:'
 ; DBGDAG: [[ENTRYTOKEN:t[0-9]+]]: ch,glue = EntryToken
 ; DBGDAG-DAG: [[BASEPTR:t[0-9]+]]: i64,ch = CopyFromReg [[ENTRYTOKEN]],
-; DBGDAG-DAG: [[ADDPTR:t[0-9]+]]: i64 = add {{(nuw )?}}[[BASEPTR]], Constant:i64<2>
+; DBGDAG-DAG: [[ADDPTR:t[0-9]+]]: i64 = add {{(nuw )?}}{{(inbounds )?}}[[BASEPTR]], Constant:i64<2>
 
 ; DBGDAG-DAG: [[LD2:t[0-9]+]]: i16,ch = load<(load (s16) from %ir.tmp81, align 1)> [[ENTRYTOKEN]], [[BASEPTR]], undef:i64
 ; DBGDAG-DAG: [[LD1:t[0-9]+]]: i8,ch = load<(load (s8) from %ir.tmp12)> [[ENTRYTOKEN]], [[ADDPTR]], undef:i64

``````````

</details>


https://github.com/llvm/llvm-project/pull/131862