What is the difference between the Firrtl option -X and -E ?

[itviewer]

It seems that they did the same thing. For example, both -X verilog and -E verilog can generate Verilog files.

[ekiwi]

We might deprecate -X at some point. -E is the more "modern" option. I think the one important difference is that you can have multiple -E but only one -X if I remember correctly.

[seldridge]

Historically, there were two options: -X/--compiler and --split-verilog. -X was telling the compiler "to what form should I run" and --split-verilog was turning on split verilog output. At this point, the emission was a special step in the compilation.

Eventually the Scala FIRRTL Compiler was refactored to make emission a transform. This opened up interesting possibilities like running multiple emitters. However, this was incompatible with the notion of --split-verilog. (To which emitter does --split-verilog apply?) To make this more clear --split-verilog was removed and -E/--emit-circuit and -e/--emit-modules was added. These add an emitter that will either do circuit emission or one-module-per-file emission at a specific point in the compilation.

The original -X/--compiler argument was never deprecated. For a time, -X/--compiler was still mandatory. This created a weird situation where -X would set the "end point" of the compilation and would give you a default emitter at that point unless you provide a -E/-e which will override the default emitter. Eventually @ekiwi made it so you didn't need to do this and the -X option was then entirely vestigial.

[ekiwi]

I actually use firrtl without the -X option all the time and it generally works for me. The only time it did not do what I expected was when I was trying to get -E chirrtl.

@seldridge You deprecated the CompilerAnnotation last year in chipsalliance/firrtl#1870 and since that is somewhat synonymous with -X I assumed that -X is deprecated as well.

[ekiwi]

Some examples of me using the firrtl compiler without -X:

• https://github.com/ucb-bar/chisel-testers2/blob/c43aef63b74ffabe3e3f42ecaa5ed05e1fc67f65/src/main/scala/chiseltest/simulator/Compiler.scala#L45
• https://github.com/ucb-bar/chisel-testers2/blob/c43aef63b74ffabe3e3f42ecaa5ed05e1fc67f65/src/main/scala/chiseltest/formal/backends/Maltese.scala#L67
• https://github.com/ucb-bar/chisel-testers2/blob/c43aef63b74ffabe3e3f42ecaa5ed05e1fc67f65/src/main/scala/chiseltest/formal/backends/Maltese.scala#L93

[seldridge]

Weird. I didn't recall that that removed the requirement that a CompilerAnnotation exists. 😅 With that, -X is entirely vestigial and we should just remove it.

[seldridge]

I've tried to update the parent response indicating that this was how things used to work. I'm in favor of only advising -E/-e going forward. 👍