[llvm] [MCA] Do not allocate space for DependenceEdge by default in DependencyGraphNode (NFC) (PR #125080)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-tools-llvm-mca

Author: Anton Sidorenko (asi-sc)

<details>
<summary>Changes</summary>

For each instruction from the input assembly sequence, DependencyGraph has a dedicated node (DGNode). Outgoing edges (data, resource and memory dependencies) are tracked as SmallVector<..., 8> for each DGNode in the graph. However, it's unlikely that a usual input instruction will have approximately eight dependent instructions. Below is my statistics for several RISC-V input sequences:

```
Number of  | Number of nodes with
edges      | this # of edges
---------------------------------
         0 | 8239447
         1 | 464252
         2 | 6164
         3 | 6783
         4 | 939
         5 | 500
         6 | 545
         7 | 116
         8 | 2
         9 | 1
        10 | 1
```

Approximately the same distribution is produced by llvm-mca lit tests for X86, AArch and RISC-V (even modified ones with extra dependencies added).
On a rather big input asm sequences, the use of SmallVector<..., 8> dramatically increases memory consumption without any need for it. In my case, replacing it with SmallVector<...,0> reduces memory usage by ~28% or ~1700% of input file size (2.2GB in absolute values).

There is no change in execution time, I verified it on mca lit-tests and on my big test (execution time is ~30s in both cases).

This change was made with the same intention as #<!-- -->124904 and optimizes I believe quite an unusual scenario. However, if there is no negative impact on other known scenarios, I'd like to have the change in llvm-project.

---
Full diff: https://github.com/llvm/llvm-project/pull/125080.diff


1 Files Affected:

- (modified) llvm/tools/llvm-mca/Views/BottleneckAnalysis.h (+1-1) 


``````````diff
diff --git a/llvm/tools/llvm-mca/Views/BottleneckAnalysis.h b/llvm/tools/llvm-mca/Views/BottleneckAnalysis.h
index 529090cf543fc4..2621efb0413ae2 100644
--- a/llvm/tools/llvm-mca/Views/BottleneckAnalysis.h
+++ b/llvm/tools/llvm-mca/Views/BottleneckAnalysis.h
@@ -228,7 +228,7 @@ class DependencyGraph {
     unsigned Depth;
 
     DependencyEdge CriticalPredecessor;
-    SmallVector<DependencyEdge, 8> OutgoingEdges;
+    SmallVector<DependencyEdge, 0> OutgoingEdges;
   };
   SmallVector<DGNode, 16> Nodes;
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/125080