[LLVMdev] Odd PPC inline asm constraint

[Peter Bergner]

On Fri, 2012-04-27 at 14:54 -0500, Hal Finkel wrote:
> There is a comment in the file which reads:
> 
> /* The weird 'i#*X' constraints on the following suppress a gcc
>    warning when __excepts is not a constant.  Otherwise, they mean the
>    same as just plain 'i'.  */
[sinp]
>  ("mtfsb0 %s0"  : : "i#*X"(__builtin_ffs (__excepts)));
[snip]
> Does anyone know what that "weird" asm constraint actually means?


The "i" and "X" constraints are documented here:

  http://gcc.gnu.org/onlinedocs/gcc-4.7.0/gcc/Simple-Constraints.html

`i'
    An immediate integer operand (one with constant value) is allowed.
    This includes symbolic constants whose values will be known only
    at assembly time or later.

`X'
    Any operand whatsoever is allowed.


The # and * constraint modifiers are documented here:

  http://gcc.gnu.org/onlinedocs/gcc-4.7.0/gcc/Modifiers.html

`#'
    Says that all following characters, up to the next comma, are to
    be ignored as a constraint. They are significant only for choosing
    register preferences.

`*'
    Says that the following character should be ignored when choosing
    register preferences. `*' has no effect on the meaning of the constraint
    as a constraint, and no effect on reloading. 

For more info about PowerPC specific constraints, you'll want to look here:

  http://gcc.gnu.org/onlinedocs/gcc-4.7.0/gcc/Machine-Constraints.html


I'll note that the "s" in the %s0 means to only print the low 5 bits of
operand 0.  I think that may only be documented in the src:

  gcc/config/rs6000/rs6000.c:print_operand()


Peter