Question about constraints and reload

[Andrew Pinski]

On Sep 26, 2004, at 5:30 PM, Geoffrey Keating wrote:

> Steven Bosscher <stevenb@suse.de> writes:
>> is there any reason to rely on reload to turn a constant into a MEM?
>
> Yes, there is.
>
> Consider rs6000, where there's no instruction to load a constant into
> a FP register, but it's pretty easy to get a constant into a GPR.
> Suppose reload is seeing a pattern like
>
> (mov:SF (reg:SF 1234) (const_double:SF 0))
>
> if it decides that register 1234 should be allocated to a GPR (for
> instance, because it's going to be a parameter to a varargs function),
> this can be coded as a 'li' instruction, an immediate load.  If it
> decides to allocate it to a FPR (for instance, because it's about to
> be compared with another FP value), it must be coded as a 'lfs'
> instruction, a floating-point load, of a value in the constant pool.
> If the zero's just being stored out to somewhere else, either choice
> is acceptable and which should be picked would depend on what
> registers are available.

But this breaks some testcases, see PR 17606 for an example where this
causes some problems.

-- Pinski