[flang] Set low probability for array repacking code.

This allows LLVM to place the most probably cold blocks
that do the repacking out of the line of the potentially hot code.

Author: vzakhari

flang/include/flang/Optimizer/Dialect/FIROps.td

@@ -2381,6 +2381,16 @@ def fir_IfOp
     static constexpr llvm::StringRef getWeightsAttrAssemblyName() {
       return "weights";
     }
+
+    /// Sets WeightedRegionBranchOpInterface weights to indicate
+    /// that either THEN or ELSE branch is unlikely.
+    /// By default, THEN branch is set to be unlikely.
+    void setUnlikelyIfWeights(bool unlikelyElse = false) {
+      if (unlikelyElse)
+        setWeights({1, 0});
+      else
+        setWeights({0, 1});
+    }
   }];
 }
 

flang/lib/Optimizer/CodeGen/LowerRepackArrays.cpp

@@ -250,6 +250,8 @@ PackArrayConversion::genRepackedBox(fir::FirOpBuilder &builder,
 
   fir::IfOp ifOp =
       builder.create<fir::IfOp>(loc, boxType, doPack, /*withElseRegion=*/true);
+  // Assume that the repacking is unlikely.
+  ifOp.setUnlikelyIfWeights();
 
   // Return original box.
   builder.setInsertionPointToStart(&ifOp.getElseRegion().front());
@@ -322,20 +324,24 @@ UnpackArrayConversion::matchAndRewrite(fir::UnpackArrayOp op,
 
     auto isNotSame = builder.genPtrCompare(loc, mlir::arith::CmpIPredicate::ne,
                                            tempAddr, originalAddr);
-    builder.genIfThen(loc, isNotSame).genThen([&]() {});
-    // Copy from temporary to the original.
-    if (!op.getNoCopy())
-      fir::runtime::genShallowCopy(builder, loc, originalBox, tempBox,
-                                   /*resultIsAllocated=*/true);
-
-    // Deallocate, if it was allocated in heap.
-    // Note that the stack attribute does not always mean
-    // that the allocation was actually done in stack memory.
-    // There are currently cases where we delegate the allocation
-    // to the runtime that uses heap memory, even when the stack
-    // attribute is set on fir.pack_array.
-    if (!op.getStack() || !canAllocateTempOnStack(originalBox))
-      builder.create<fir::FreeMemOp>(loc, tempAddr);
+    builder.genIfThen(loc, isNotSame)
+        .genThen([&]() {
+          // Copy from temporary to the original.
+          if (!op.getNoCopy())
+            fir::runtime::genShallowCopy(builder, loc, originalBox, tempBox,
+                                         /*resultIsAllocated=*/true);
+
+          // Deallocate, if it was allocated in heap.
+          // Note that the stack attribute does not always mean
+          // that the allocation was actually done in stack memory.
+          // There are currently cases where we delegate the allocation
+          // to the runtime that uses heap memory, even when the stack
+          // attribute is set on fir.pack_array.
+          if (!op.getStack() || !canAllocateTempOnStack(originalBox))
+            builder.create<fir::FreeMemOp>(loc, tempAddr);
+        })
+        .getIfOp()
+        .setUnlikelyIfWeights();
   });
   rewriter.eraseOp(op);
   return mlir::success();