[llvm] [SDAG] Preserve InBounds in DAGCombines (PR #165424)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-amdgpu

Author: Fabian Ritter (ritter-x2a)

<details>
<summary>Changes</summary>

This PR preserves the InBounds flag (#<!-- -->162477) where possible in PTRADD-related
DAGCombines. We can't preserve them in all the cases that we could in the
analogous GISel change (#<!-- -->152495) because SDAG usually represents pointers as
integers, which means that pointer provenance is not preserved between PTRADD
operations (see the discussion at PR #<!-- -->162477 for more details). This PR marks
the places in the DAGCombiner where this is relevant explicitly.

For SWDEV-516125.

---
Full diff: https://github.com/llvm/llvm-project/pull/165424.diff


1 Files Affected:

- (modified) llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (+19-2) 


``````````diff
diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index cf221bba1e3a3..6aa8a4a4a202a 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -2715,6 +2715,12 @@ SDValue DAGCombiner::visitPTRADD(SDNode *N) {
           (N->getFlags() & N0->getFlags()) & SDNodeFlags::NoUnsignedWrap;
       SDValue Add = DAG.getNode(ISD::ADD, DL, IntVT, {Y, Z}, Flags);
       AddToWorklist(Add.getNode());
+      // We can't set InBounds even if both original ptradds were InBounds and
+      // NUW: SDAG usually represents pointers as integers, therefore, the
+      // matched pattern behaves as if it had implicit casts:
+      //   (ptradd inbounds (inttoptr (ptrtoint (ptradd inbounds x, y))), z)
+      // The outer inbounds ptradd might therefore rely on a provenance that x
+      // does not have.
       return DAG.getMemBasePlusOffset(X, Add, DL, Flags);
     }
   }
@@ -2740,6 +2746,12 @@ SDValue DAGCombiner::visitPTRADD(SDNode *N) {
         // that.
         SDNodeFlags Flags =
             (N->getFlags() & N0->getFlags()) & SDNodeFlags::NoUnsignedWrap;
+        // We can't set InBounds even if both original ptradds were InBounds and
+        // NUW: SDAG usually represents pointers as integers, therefore, the
+        // matched pattern behaves as if it had implicit casts:
+        //   (ptradd inbounds (inttoptr (ptrtoint (ptradd inbounds GA, v))), c)
+        // The outer inbounds ptradd might therefore rely on a provenance that
+        // GA does not have.
         SDValue Inner = DAG.getMemBasePlusOffset(GAValue, N1, DL, Flags);
         AddToWorklist(Inner.getNode());
         return DAG.getMemBasePlusOffset(Inner, N0.getOperand(1), DL, Flags);
@@ -2763,8 +2775,13 @@ SDValue DAGCombiner::visitPTRADD(SDNode *N) {
     bool ZIsConstant = DAG.isConstantIntBuildVectorOrConstantInt(Z);
 
     // If both additions in the original were NUW, reassociation preserves that.
-    SDNodeFlags ReassocFlags =
-        (N->getFlags() & N1->getFlags()) & SDNodeFlags::NoUnsignedWrap;
+    SDNodeFlags CommonFlags = N->getFlags() & N1->getFlags();
+    SDNodeFlags ReassocFlags = CommonFlags & SDNodeFlags::NoUnsignedWrap;
+    if (CommonFlags.hasNoUnsignedWrap()) {
+      // If both operations are NUW and the PTRADD is inbounds, the offests are
+      // both non-negative, so the reassociated PTRADDs are also inbounds.
+      ReassocFlags |= N->getFlags() & SDNodeFlags::InBounds;
+    }
 
     if (ZIsConstant != YIsConstant) {
       if (YIsConstant)

``````````

</details>


https://github.com/llvm/llvm-project/pull/165424