9 Declarations [dcl.dcl]

9.10 The asm declaration [dcl.asm]

1
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An asm declaration has the form
The asm declaration is conditionally-supported; its meaning is 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]