Re: [testsuite] require arm_little_endian in two tests

[Richard Earnshaw <rearnsha at arm dot com>]

On 13/10/11 15:56, Joseph S. Myers wrote:
> On Thu, 13 Oct 2011, Richard Earnshaw wrote:
> 
>> 2) Change the compiler to make initializers of vectors assign elements
>> of initializers to consecutive lanes in a vector, rather than the
>> current behaviour of 'casting' an array of elements to a vector.
>>
>> While the second would be my preferred change, I suspect it's too hard
>> to fix, and may well cause code written for other targets to break on
>> big-endian (altivec for example).
> 
> Indeed, vector initializers are part of the target-independent GNU C 
> language and have target-independent semantics that the elements go in 
> memory order, corresponding to the target-independent semantics of lane 
> numbers where they appear in GENERIC, GIMPLE and (non-UNSPEC) RTL and any 
> target-independent built-in functions that use such numbers.  (The issue 
> here being, as you saw, that the lane numbers used in ARM-specific NEON 
> intrinsics are for big-endian not the same as those used in 
> target-independent features of GNU C and target-independent internal 
> representations in GCC - hence various code to translate them between the 
> two conventions when processing intrinsics into non-UNSPEC RTL, and to 
> translate back when generating assembly instructions that encode lane 
> numbers with the ARM conventions, as expounded at greater length at 
> <http://gcc.gnu.org/ml/gcc-patches/2010-06/msg00409.html>.)
> 

This is all rather horrible, and leads to THREE different layouts for a
128-bit vector for big-endian Neon.

GCC format
'VLD1.n' format
'ABI' format

GCC format and 'ABI' format differ in that the 64-bit words of the
128-bit vector are swapped.

All this and they are all expected to share a single machine mode.

Furthermore, the definitions in GCC are broken, in that the types
defined in arm_neon.h (eg int8x16_t) are supposed to be ABI format, not
GCC format.

Eukkkkkk! :-(

R.