[cfe-dev] Specifying registers for local variables

[Rafael Espíndola]

On 18 December 2014 at 10:12, Simon Atanasyan <simon at atanasyan.com> wrote:
> Hi,
>
> Does LLVM/Clang support specifying registers for local variables?

In some way, yes. We implement what is documented by gcc:

    Local register variables in specific registers do not reserve the
     registers, except at the point where they are used as input or
     output operands in an 'asm' statement and the 'asm' statement
     itself is not deleted.

That is, it is just a way of writing a asm constraint.

> The following code is accepted by the Clang. It parses the `asm`
> attribute in the `Parser::ParseAsmAttributesAfterDeclarator()` routine
> and checks the declaration in the `Sema::ActOnVariableDeclarator()`.
> But it looks like the asm label expression is not used anywhere
> further. Is this language extension is unsupported by the Clang?
>
> % cat check.c
> int foo() {
>   register int v asm ("edx") = 3;
>   v = v + 1;
>   return v;
> }
>
> The assembler output demonstrates the problem:
>
> % gcc -S check.c && cat check.s
> ...
>         movl    $3, %edx        ; use register
>         movl    %edx, %eax
>         addl    $1, %eax
>         movl    %eax, %edx
>         movl    %edx, %eax
> ...
>
> % clang -S check.c && cat check.s
> ...
>         movl    $3, -4(%rbp)    ; use stack
>         movl    -4(%rbp), %eax
>         addl    $1, %eax
>         movl    %eax, -4(%rbp)
>         movl    -4(%rbp), %eax
> ...
>
> --
> Simon
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