Generate commented assembly with --emit=asm

[jrmuizel]

clang -S produces nicely commented .s files. It would be nice if rustc did this too.

e.g. simd shuffles are commented
.loc directives have the corresponding line number comment
.loc 1 4 25 is_stmt 1 ## test.c:4:25
debug information is commented
.byte 14 ## DW_FORM_strp
.byte 17 ## DW_AT_low_pc
.byte 1 ## DW_FORM_addr
.byte 18 ## DW_AT_high_pc
.byte 1 ## DW_FORM_addr

[eddyb]

There might be a -C or -Z flag already for this. AFAIK what clang emits comes from LLVM.

[Mark-Simulacrum]

This flag is already added, should be usable (issue can be closed?):
-Z asm-comments -- generate comments into the assembly (may change behavior)

[jrmuizel]

Any reason not to make this the default?

[Mark-Simulacrum]

Likely the fact that it may change the behavior (I don't actually know what that means but this seems like a valid assumption).

[jrmuizel]

it looks like -Z asm-comments doesn't do this. MCOptions.AsmVerbose must be set to true for the assembler to emit them.

[Mark-Simulacrum]

So it looks like -Zasm-comments isn't actually used anywhere based on what I can find in the source... or, rather, its uses are in methods that are themselves never called.

[jrmuizel]

Yeah, to get comments I believe I needed to set a verbose flag in some llvm context. Clang does this by default and it would be nice if rustc did too.

[gnzlbg]

IIUC without this neither the playground nor godbolt (compiler-explorer) can map assembly to lines of Rust source code.

[hanna-kruppe]

Godbolt uses debug info AFAIK? Our debug info isn't always great but in general it's working, isn't it?

[gnzlbg]

It seems like they just don't do it much. Godbolt uses .loc directives in the assembly file, which rustc seems to generate correctly, sorry for the noise.