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

[Chris Lattner]

On Dec 30, 2010, at 8:34 PM, Jakob Stoklund Olesen wrote:

> 
> 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.

Yes, this should work for global register variables.  llvm-gcc does this IIRC.

-Chris

> 
> 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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