1896833 - Add printf opcode for debugging

Status: NEW

(Core :: JavaScript Engine: JIT, task, P5)

Description

[Yulia Startsev [:yulia] OOO until July 2026]

Sometimes masm.printf isn't enough for debugging what is going on in JITs. I introduced the printf opcode for my own purposed, to record critical loop variables (variables that control the behavior of the loop), but it may be more broadly useful.

Comment 1

[Dan Minor [:dminor]]

Fwiw, I've thought about adding something like this too, I've ended up repurposing JSOp::Debugger for similar things in the past.

Comment 2

[Yulia Startsev [:yulia] OOO until July 2026]

Attachment: Bug 1896833 - Add printf to codegen; r=jandem

This introduces LIR and MIR opcodes, as well as specialization for different MIRTypes with regards
to what gets printed. I added a Printf2 method, because it can be quite frustrating trying to figure
out what part of a string printed from a register is the actual string. I didn't handle dependent
strings at all, those can be added later.