[all-commits] [llvm/llvm-project] 9cf8ee: [MCA] Do not allocate space for DependenceEdge by ...

[Anton Sidorenko via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 9cf8ee91451788b08a0bcda924bfbb3754c454da
      https://github.com/llvm/llvm-project/commit/9cf8ee91451788b08a0bcda924bfbb3754c454da
  Author: Anton Sidorenko <anton.sidorenko at syntacore.com>
  Date:   2025-01-31 (Fri, 31 Jan 2025)

  Changed paths:
    M llvm/tools/llvm-mca/Views/BottleneckAnalysis.h

  Log Message:
  -----------
  [MCA] Do not allocate space for DependenceEdge by default in DependencyGraphNode (NFC) (#125080)

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.



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