9 Declarations [dcl.dcl]

9.10 The asm declaration [dcl.asm]

An asm declaration has the form
The asm declaration is conditionally-supported; any restrictions on the balanced-token-seq and its meaning are implementation-defined.
The optional attribute-specifier-seq in an asm-declaration appertains to the asm declaration.
[Note 1: 
Typically it is used to pass information through the implementation to an assembler.
— end note]