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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.