Re: [PATCH][binutils][Arm][2/3] Add support for float16 (IEEE and Alternative formats) for Arm backend.

["Richard Earnshaw (lists)" <Richard dot Earnshaw at arm dot com>]

On 11/07/2019 16:26, Barnaby Wilks wrote:
> Hello,
>
> This is part of a patch series that implements a directive for assembling 16-bit floating point constants for Arm and AArch64.
>
> This patch implements the float16 directive for both the IEEE 754 format and the Arm alternative format for the
> Arm backend >
> The syntax of the directive is:
> .float16 <0-n decimal numbers>
> e.g.
> .float16 12.0
> .float16 0.23, 433.1, 0.06
>
> The Arm alternative format is almost identical to the IEEE 754 format, except that it doesn't
> encode for NaNs or Infinity (instead an exponent of 0x1F represents a normalized number in the range
> 65536 to 131008).
>
> The alternative format is documented in the reference manual:
> https://static.docs.arm.com/ddi0487/db/DDI0487D_b_armv8_arm.pdf?_ga=2.72318806.49764181.1561632697-999473562.1560847439
>
> Which format is used is controlled by the
> .eabi_attribute Tag_ABI_FP_16bit_format, <...>
> directive, where if
> <...> = 1 then use the IEEE 754 half-precision format else if
> <...> = 2 then use the Arm alternative format
>
> Added testcases to verify the correct encoding for both formats (for both big and little endian targets),
> as well as tests verifying that warnings are thrown if trying to encode NaN or Infinity when using
> the Arm alternative format.
> The encodings have been cross referenced with GCC's encoding for __fp16 types to ensure consistency.
>
> Cross compiled and regtested on arm-none-eabi and arm-none-linux-gnueabihf
>
> I don't have write access, so if it's OK then could someone commit on my behalf?

Hi Barney,

thanks for the patch.

I'm not sure that using the build attributes to control selection of the 
format is the best idea.  Firstly, build attributes are not available 
when not assembling for ELF (we have PE-coff support in the assembler as 
well); secondly, build attributes should reflect what was placed in the 
assembly file, not control what goes into it.  So this is a case of the 
coach before the horse.

I think, if we want to support the Arm alternate FP format in GAS at 
all, we need either a new command line option, or a new directive (or 
possibly both).  Then the code in BFD that does automatic 
build-attribute selection should be updated to reflect the tri-state 
situation: no format specified; IEEE format specified; or ARM Alternate 
format specified.

Note that GAS can currently only reflect show build attributes that 
affect the whole file, so switching during assembly is a little dubious 
as things stand.


R.


> Thanks,
> Barney
>
> gas/ChangeLog:
>
> 2019-07-11  Barnaby Wilks<barnaby.wilks@arm.com>
>
> 	* config/tc-arm.c (md_atof): Set precision for float16 type.
> 	(enum fp_16bit_format): Add enum to represent the 2 float16 encodings.
> 	(arm_is_largest_exponent_ok): Check for whether to encode with the IEEE or alternative
> 	format.
> 	* config/tc-arm.h (TC_LARGEST_EXPONENT_IS_NORMAL): Macro that expands to
> 	arm_is_largest_exponent_ok.
> 	(arm_is_largest_exponent_ok): Add prototype for arm_is_largest_exponent_ok function.
> 	* testsuite/gas/arm/float16-bad.d: New test.
> 	* testsuite/gas/arm/float16-bad.l: New test.
> 	* testsuite/gas/arm/float16-bad.s: New test.
> 	* testsuite/gas/arm/float16-be.d: New test.
> 	* testsuite/gas/arm/float16-be.s: New test.
> 	* testsuite/gas/arm/float16-le.d: New test.
> 	* testsuite/gas/arm/float16-le.s: New test.