From d001b34f816f1f65d0625aebf33e5cfc5ba93e49 Mon Sep 17 00:00:00 2001
From: mergify[bot]
Date: Thu, 16 Jun 2022 23:15:42 +0000
Subject: Define leading '_' as API for creating temporaries (backport #2580)
 (#2581)

* Define leading '_' as API for creating temporaries

Chisel and FIRRTL have long used signals with names beginning with an
underscore as an API to specify that the name does not really matter.
Tools like Verilator follow a similar convention and exclude signals
with underscore names from waveform dumps by default. With the
introduction of compiler-plugin prefixing in Chisel 3.4, the convention
remained but was hard for users to use unless the unnnamed signal
existed outside of any prefix domain. Notably, unnamed signals are most
useful when creating wires inside of utility methods which almost always
results in the signal ending up with a prefix.

With this commit, Chisel explicitly recognizes signals whos val names
start with an underscore and preserve that underscore regardless of any
prefixing. Chisel will also ignore such underscores when generating
prefixes based on the temporary signal, preventing accidental double
underscores in the names of signals that are prefixed by the temporary.

(cherry picked from commit bd94366290886f3489d58f88b9768c7c11fa2cb6)

* Remove unused defaultPrefix argument from _computeName

(cherry picked from commit ec178aa20a830df2c8c756b9e569709a59073554)

# Conflicts:
#	core/src/main/scala/chisel3/Module.scala
#	core/src/main/scala/chisel3/experimental/hierarchy/ModuleClone.scala

* Resolve backport conflicts

* Waive false positive binary compatibility errors

Co-authored-by: Jack Koenig <koenig@sifive.com>---
 plugin/src/main/scala/chisel3/internal/plugin/ChiselComponent.scala | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

(limited to 'plugin/src/main/scala/chisel3/internal')

diff --git a/plugin/src/main/scala/chisel3/internal/plugin/ChiselComponent.scala b/plugin/src/main/scala/chisel3/internal/plugin/ChiselComponent.scala
index f98049e2..b3bbdbe4 100644
--- a/plugin/src/main/scala/chisel3/internal/plugin/ChiselComponent.scala
+++ b/plugin/src/main/scala/chisel3/internal/plugin/ChiselComponent.scala
@@ -181,8 +181,11 @@ class ChiselComponent(val global: Global, arguments: ChiselPluginArguments)
         // If a Data or a Memory, get the name and a prefix
         else if (shouldMatchNamedComp(tpe)) {
           val str = stringFromTermName(name)
+          // Starting with '_' signifies a temporary, we ignore it for prefixing because we don't
+          // want double "__" in names when the user is just specifying a temporary
+          val prefix = if (str.head == '_') str.tail else str
           val newRHS = transform(rhs)
-          val prefixed = q"chisel3.experimental.prefix.apply[$tpt](name=$str)(f=$newRHS)"
+          val prefixed = q"chisel3.experimental.prefix.apply[$tpt](name=$prefix)(f=$newRHS)"
           val named = q"chisel3.internal.plugin.autoNameRecursively($str)($prefixed)"
           treeCopy.ValDef(dd, mods, name, tpt, localTyper.typed(named))
           // If an instance, just get a name but no prefix
-- 
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