[cfe-commits] r122643 - in /cfe/trunk: lib/CodeGen/CGDecl.cpp lib/CodeGen/CGStmt.cpp test/CodeGen/asm-variable.c

[Jakob Stoklund Olesen]

On Dec 30, 2010, at 4:08 PM, Benjamin Kramer wrote:

> 
> On 30.12.2010, at 23:59, Rafael Espindola wrote:
> 
>> Author: rafael
>> Date: Thu Dec 30 16:59:32 2010
>> New Revision: 122643
>> 
>> URL: http://llvm.org/viewvc/llvm-project?rev=122643&view=rev
>> Log:
>> Add support for declaring register contraints in variables. They are only used
>> in asm statements:
>> 
>> register int foo asm("rdi");
>> 
>> asm("..." : ... "r" (foo) ...
>> 
>> We also only accept these variables if the constraint in the asm statement is "r".
>> 
>> This fixes most of PR3933.
> 
> I'm not very happy with this fix. Using such a pinned variable with inline asm looks like an edge case.

It is all that is required for local variables, see http://gcc.gnu.org/onlinedocs/gcc/Local-Reg-Vars.html

> Most code (e.g. ruby 1.9) uses this as a (premature) optimization so it shouldn't hurt there but on the other hand
> the linux kernel uses this extension to access the stack pointer like a variable.
> 
> register unsigned long current_stack_pointer asm("esp");
> foo = current_stack_pointer;

Presumably this is a global variable, otherwise it could legally be compiled to:

  foo = undef

I think we can support global variables like this one by replacing all reads and writes with empty inline assembly statements.

That will work for reserved registers like %esp. As for using allocatable registers for global variables, I really hope we can avoid that.

/jakob

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