[flang-commits] [flang] [flang] Stop tracking memory source after a load in a more explicit manner. (PR #126156)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-flang-fir-hlfir

Author: Renaud Kauffmann (Renaud-K)

<details>
<summary>Changes</summary>

Typically, we do not track memory sources after a load because of the dynamic nature of the load and the fact that the alias analysis is a simple static analysis.

However the code is written in a way that makes it seem like we are continuing to track memory but in reality we are only doing so when we know that the tracked memory is a leaf and therefore that there will only be one more iteration through the switch statement. In other words, we are iterating one more time, to gather data about a box, anticipating that this will be the last time. This is a hack that  helped avoid cut-and-paste from other case statements but gives the wrong impression about the intention of the code and makes it confusing. 

To make it clear that there is no more tracking, we gather all the necessary data from the memref of the load, in the case statement for the load, and exit the loop. I am also limiting this data gathering for the case when we load a box reference while we were actually following data, as tests have shows, is the only case when we need it for. Other cases will be handled conservatively, but this can change in the future, on a case-by-case basis.




---
Full diff: https://github.com/llvm/llvm-project/pull/126156.diff


2 Files Affected:

- (modified) flang/lib/Optimizer/Analysis/AliasAnalysis.cpp (+37-11) 
- (modified) flang/test/Analysis/AliasAnalysis/alias-analysis-2.fir (+4-2) 


``````````diff
diff --git a/flang/lib/Optimizer/Analysis/AliasAnalysis.cpp b/flang/lib/Optimizer/Analysis/AliasAnalysis.cpp
index 01f3a0326db216e..6f65ca18c3074bf 100644
--- a/flang/lib/Optimizer/Analysis/AliasAnalysis.cpp
+++ b/flang/lib/Optimizer/Analysis/AliasAnalysis.cpp
@@ -51,7 +51,7 @@ static bool hasGlobalOpTargetAttr(mlir::Value v, fir::AddrOfOp op) {
       v, fir::GlobalOp::getTargetAttrName(globalOpName));
 }
 
-mlir::Value getOriginalDef(mlir::Value v) {
+static mlir::Value getOriginalDef(mlir::Value v) {
   mlir::Operation *defOp;
   bool breakFromLoop = false;
   while (!breakFromLoop && (defOp = v.getDefiningOp())) {
@@ -578,16 +578,7 @@ AliasAnalysis::Source AliasAnalysis::getSource(mlir::Value v,
             breakFromLoop = true;
         })
         .Case<fir::LoadOp>([&](auto op) {
-          // If the load is from a leaf source, return the leaf. Do not track
-          // through indirections otherwise.
-          // TODO: Add support to fir.alloca and fir.allocmem
-          auto def = getOriginalDef(op.getMemref());
-          if (isDummyArgument(def) ||
-              def.template getDefiningOp<fir::AddrOfOp>()) {
-            v = def;
-            defOp = v.getDefiningOp();
-            return;
-          }
+          
           // If load is inside target and it points to mapped item,
           // continue tracking.
           Operation *loadMemrefOp = op.getMemref().getDefiningOp();
@@ -600,6 +591,41 @@ AliasAnalysis::Source AliasAnalysis::getSource(mlir::Value v,
             defOp = v.getDefiningOp();
             return;
           }
+
+          // If we are loading a box reference, but following the data,
+          // we gather the attributes of the box to populate the source
+          // and stop tracking.
+          if (auto boxTy = mlir::dyn_cast<fir::BaseBoxType>(ty);
+              boxTy && followingData) {
+
+            if (mlir::isa<fir::PointerType>(boxTy.getEleTy())) {
+              attributes.set(Attribute::Pointer);
+            }
+
+            auto def = getOriginalDef(op.getMemref());
+            if (auto addrOfOp = def.template getDefiningOp<fir::AddrOfOp>()) {
+              global = addrOfOp.getSymbol();
+
+              if (hasGlobalOpTargetAttr(def, addrOfOp))
+                attributes.set(Attribute::Target);
+
+              type = SourceKind::Global;
+            }
+
+            // TODO: Add support to fir.alloca and fir.allocmem
+            // if (auto allocOp = def.template getDefiningOp<fir::AllocaOp>()) {
+            //   ...
+            // }
+
+            if (isDummyArgument(def)) {
+              defOp = nullptr;
+              v = def;
+            }
+            
+            breakFromLoop = true;
+            return;
+          }
+
           // No further tracking for addresses loaded from memory for now.
           type = SourceKind::Indirect;
           breakFromLoop = true;
diff --git a/flang/test/Analysis/AliasAnalysis/alias-analysis-2.fir b/flang/test/Analysis/AliasAnalysis/alias-analysis-2.fir
index ca97c5900281d64..3fbf29ab2eb2926 100644
--- a/flang/test/Analysis/AliasAnalysis/alias-analysis-2.fir
+++ b/flang/test/Analysis/AliasAnalysis/alias-analysis-2.fir
@@ -49,11 +49,13 @@
 
 // TODO: Can the address in a pointer alias the address of a pointer, even when the
 // pointer has no box. Should this be NoAlias?
-// T3: CHECK-DAG: p1.addr#0 <-> p1.tgt#0: MayAlias
+// T3: 
+// CHECK-DAG: p1.addr#0 <-> p1.tgt#0: MayAlias
 
 // The addresses stored in two different pointers can alias, even if one has no
 // box.  In this program, they happen to be the same address.
-// T4: CHECK-DAG: p1.tgt#0 <-> boxp1.addr#0: MayAlias
+// T4: 
+// CHECK-DAG: p1.tgt#0 <-> boxp1.addr#0: MayAlias
 
 func.func @_QFPtest(%arg0: !fir.ref<f32> {fir.bindc_name = "v1", fir.target}, %arg1: !fir.ref<f32> {fir.bindc_name = "v2", fir.target}, %arg2: !fir.ref<!fir.box<!fir.ptr<f32>>> ) attributes {test.ptr = "func"} {
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/126156