[llvm-dev] Vectorization with fast-math on irregular ISA sub-sets

Stephen Canon via llvm-dev llvm-dev at lists.llvm.org
Mon Feb 15 06:22:23 PST 2016

> On Feb 15, 2016, at 3:34 AM, James Molloy via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> Hi,
>> James, is that a correct assessment?
> Yes, it is also my belief that the only way ARMv7 NEON differs from IEEE754 is lack of denormal support.

- ARMv7 NEON ignores the rounding mode set in bits 23:22 of FPSCR and always uses round to nearest.
- ARMv7 NEON ignores the trap enable bits (15:8) in FPSCR and always uses default exception handling.

As with denormal support, the issue at hand is not so much that these differ from IEEE 754 as it is that they differ from the behavior of the scalar (VFP) arithmetic.

- Steve

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