Reduce CDE lookups for IOBinders

[tymcauley]

I recently noticed a performance degradation when emitting FIRRTL for a private SoC project, and tracked the issue down to this bit of IOBinders code. If you add a println statement to the IOBinder class, like this:

diff --git a/generators/chipyard/src/main/scala/iobinders/IOBinders.scala b/generators/chipyard/src/main/scala/iobinders/IOBinders.scala
index 45f72641..f67e7bf3 100644
--- a/generators/chipyard/src/main/scala/iobinders/IOBinders.scala
+++ b/generators/chipyard/src/main/scala/iobinders/IOBinders.scala
@@ -116,7 +116,10 @@ object GetSystemParameters {
 }

 class IOBinder[T](composer: Seq[IOBinderFunction] => Seq[IOBinderFunction])(implicit tag: ClassTag[T]) extends Config((site, here, up) => {
-  case IOBinders => up(IOBinders, site) + (tag.runtimeClass.toString -> composer(up(IOBinders, site)(tag.runtimeClass.toString)))
+  case IOBinders => {
+    println(s"Checking IOBinder ${tag.runtimeClass.toString}")
+    up(IOBinders, site) + (tag.runtimeClass.toString -> composer(up(IOBinders, site)(tag.runtimeClass.toString)))
+  }
 })

 class ConcreteIOBinder[T](composes: Boolean, fn: T => IOBinderTuple)(implicit tag: ClassTag[T]) extends IOBinder[T](

... and then build RocketConfig with:

make -C sims/vcs firrtl

Then you will see 524287 instances of that println message, with many duplicate checks of the same IOBinder classes. Seems that the extra CDE lookup call to up is causing a lot of issues. If you just make a single call to up, as in this PR, you'll only see 19 instances of that println message.

This performance hiccup isn't that noticeable for Rocket, but for my project, it's the difference between the make firrtl step taking 1 minute vs 8 minutes. I'm not adding that many extra IOBinders (maybe about 10), but there seems to be some greater-than-linear blowup in the CDE lookups with the two calls to up.

Related PRs / Issues:

Type of change:

• Bug fix
• New feature
• Other enhancement

Impact:

• RTL change
• Software change (RISC-V software)
• Build system change
• Other

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• Did you set main as the base branch?
• Is this PR's title suitable for inclusion in the changelog and have you added a changelog:<topic> label?
• Did you state the type-of-change/impact?
• Did you delete any extraneous prints/debugging code?
• Did you mark the PR with a changelog: label?
• (If applicable) Did you update the conda .conda-lock.yml file if you updated the conda requirements file?
• (If applicable) Did you add documentation for the feature?
• (If applicable) Did you add a test demonstrating the PR?

• (If applicable) Did you mark the PR as Please Backport?

[tymcauley]

Thinking about this a bit more, the speedup makes sense. Each call to up(IOBinders) is effectively a recursive call, and two recursive calls for each execution of the function leads to 2^n complexity.

[jerryz123]

Thanks for investigating this!