[GSoC-19] Expanding fromfp variants on AArch64

[GSoC-19] Expanding fromfp variants on AArch64

[Tejas Joshi]

Hello.
The fromfp/fromfpx variants round to integers with a specified number
of bits, to a specified rounding mode. They come with their own
complications as Joseph had stated in an initial mail and expected to
expand them in AArch64 :

> The fromfp / fromfpx / ufromfp / ufromfpx functions (round to integers
> of a specified number of bits, in a specified rounding mode, with
> specified handling of inexact results) are a case with some other
> complications.  Typically I'd expect them to be expanded inline only (for
> constant arguments or) for constant values of the number of bits and
> rounding mode, if the target machine has an appropriate instruction.  A
> target hook would need adding for a target to specify the FP_INT_* values
> used in libm, since that's an ABI that's defined by libm, not by GCC.
> Then you'd need instruction patterns that might only be supported in
> certain cases.

How can I add a target hook to specify the FP_INT_* values from libm ?
Also as this includes rounding to integers, does it involve any RTL
related complications that we have encountered in FADD ?

Thanks,
Tejas

Re: [GSoC-19] Expanding fromfp variants on AArch64

[Joseph Myers]

On Mon, 19 Aug 2019, Tejas Joshi wrote:

> How can I add a target hook to specify the FP_INT_* values from libm ?

See target.def.

You'll need a GCC-specific enum (GCC_FP_INT_*, say) that GCC uses 
internally, and a hook that maps between that and FP_INT_*.  I'm guessing 
that for the likely uses, maybe the hook should map from FP_INT_* to 
GCC_FP_INT_* (so it gets used on constant arguments to the built-in 
function to say which rounding direction they are in GCC's internal enum).  
It will need to be able to return that a constant doesn't map to a known 
rounding mode (not an error, just means that call can't be expanded inline 
or optimized to a constant).

Then the relevant macro giving the default for glibc systems should be 
defined in config/gnu-user.h (see how it defines e.g. 
TARGET_LIBC_HAS_FUNCTION).

> Also as this includes rounding to integers, does it involve any RTL
> related complications that we have encountered in FADD ?

The new RTL would effectively be variants of the fix_trunc and 
fixuns_trunc patterns, which can use (fix) and (unsigned_fix) RTL; the new 
variants would take an argument in a floating-point mode, returning one in 
an integer mode - but with extra information involved about the number of 
bits, rounding direction, handling of "inexact".

fix_trunc and fixuns_trunc / (fix) and (unsigned_fix) always use 
FP_INT_TOWARDZERO, always use the width of the mode and have unspecified 
"inexact" handling for non-integer in-range values (they correspond to C 
casts) so are not themselves suitable for implementing the new built-in 
functions (but the particular instructions those patterns expand to are 
likely to be suitable for certain arguments to certain of the new built-in 
functions).

-- 
Joseph S. Myers
joseph@codesourcery.com