[PATCH] Add a new macro to mask a float

[Joseph Myers]

On Wed, 29 Jun 2016, Adhemerval Zanella wrote:

> My understanding of this optimization is to just make the the FP to GPR move,
> bitwise operation and GRP to FP move again to a more simple bitwise operation
> on FP register itself.  It is indeed equivalent to integer masking and I 
> believe the 'normalized' here means to make the float mask to represented
> as internal double required in VSX operations.

What do you mean by "internal double"?  Is this purely some fixed 
rearrangement of bits, so that e.g. subnormal float values still get 
represented as subnormals rather than like normal doubles?

Say the number is the least subnormal float - 0x1p-149f, integer 
representation 1 - and that it's masked with 0xfffff000, as in the various 
MASK_FLOAT calls.  Can you confirm that the instruction sequence in the 
patch produces 0.0f, as the integer masking does, when executed on a 
POWER8 processor?  And that if instead the value is 0x1p-137f, it's 
returned unchanged?

If equivalent to integer masking for all inputs including subnormals and 
infinities and NaNs, then my previous point applies that this should be a 
compiler optimization instead of a glibc patch.

-- 
Joseph S. Myers
joseph@codesourcery.com