[cfe-dev] Specifying registers for local variables

[Simon Atanasyan]

Hi,

Does LLVM/Clang support specifying registers for local variables?

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