Background

Currently there isn't an easy way to enable or disable portions of assembly based on configuration. This can be a annoyance for asm! that can be worked around with multiple assembly blocks, but it is especially a problem in global_asm! and naked_asm! because small config-based changes mean the entire assembly block needs to be duplicated.

Another workaround is possible by defining multiple macro_rules! of the same name that are enabled/disabled based on the desired config and expand to strings. This option is inconvenient and fragments the assembly.

Proposal

Allow #[cfg(...)] in all assembly macros:

asm!( // or global_asm! or naked_asm! "nop", #[cfg(target_feature = "sse2")] "nop", // ... #[cfg(target_feature = "sse2")] a = const 123, // only used on sse2 );

The configuration applies to a single comma-separated segment.

We may want to also support blocks in order to allow grouping instructions without merging strings, or grouping directives:

asm!( "nop", #[cfg(target_feature = "sse2")] { "nop", "nop", "nop", } // does allowing/requiring a comma here make sense? "nop", #[cfg(target_feature = "sse2")] { a = const 123, a = const 123, } );

This design has some concerns and there may be a better option, see discussion in the thread.

Zulip discussion: #project-inline-asm > `cfg` in assembly blocks