[PATCH] Allow encoded 8-bit floating point constants in ARM vmov instructions
dpeixott at codeaurora.org
Wed Dec 18 09:44:27 PST 2013
On 12/18/2013 4:13 AM, Renato Golin wrote:
> On 17 December 2013 21:58, Jim Grosbach <grosbach at apple.com
> <mailto:grosbach at apple.com>> wrote:
> What does the ARM documentation indicate? Either way, you’re right
> that we should clean up the implementation so that if we reject or
> accept the different forms of constants, we do so intentionally and
> not inadvertently.
> ARM docs mention that both VFPv3+ and NEON VMOV.f32 Rn, #imm support
> encoded floating-point constants. Tables A7-15 and A7-18 (ARMv7-AR):
> "The bit pattern represents the floating-point number (–1)S × 2exp ×
> mantissa, where S = UInt(a), exp = UInt(NOT(b):c:d)-3 and mantissa =
I tried with both gcc (4.6) and armasm (5.03) and neither accept an
encoded fp constant (e.g. 0x70) as an operand to vmov.f* instructions.
Interestingly, objdump prints the constant in its encoded form (0x70
instead of 1.0) so it is not possible to take the objdump output and
feed it back to the assembler. I checked with llvm-objdump and it prints
the floating-point value not the encoded value.
It seems our options are:
1. Support encoded fp constants in vmov.f* functions. This option is
what my current patch does.
2. Explicitly reject encoded fp constants in vmov.f* functions. This
option amounts to deleting some code in the floating point parsing
routine and adding an error message.
Which option do you like best?
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