Re: [PATCH 5/6] mips: Implement vec_perm_const.

[Richard Henderson <rth at redhat dot com>]

On 12/11/2011 04:50 AM, Richard Sandiford wrote:
> [Mingjie, please could you help with the Loongson question near the end?]

Actually, can you tell me how to test these abi combinations?  I keep trying to use mips-sim or mips64-sim and get linker errors complaining of abi combinations.

>      Little-endian:
> 
>         The semantics of the RTL pattern are:
> 
> 	{ 0L, 0U } = { X[I3], X[I4 + 2] }, where X = { 1L, 1U, 2L, 2U }
> 
> 	so: 0L = { 1L, 1U }[I3] (= <bop><bUL>)
> 	    0U = { 2L, 2U }[I4] (= <aop><aUL>)
> 
> 	    <aop> = 2, <aUL> = I4 ? U : L
> 	    <bop> = 1, <bUL> = I3 ? U : L
> 
> 	    [LL] !I4 && !I3   [UL] I4 && !I3
> 	    [LU] !I4 && I3    [UU] I4 && I3
> 
>      Big-endian:
> 
>         The semantics of the RTL pattern are:
> 
> 	{ 0U, 0L } = { X[I3], X[I4 + 2] }, where X = { 1U, 1L, 2U, 2L }
> 
> 	so: 0U = { 1U, 1L }[I3] (= <aop><aUL>)
> 	    0L = { 2U, 2L }[I4] (= <bop><bUL>)
> 
> 	    <aop> = 1, <aUL> = I3 ? L : U
> 	    <bop> = 2, <bUL> = I4 ? L : U
> 
> 	    [UU] !I3 && !I4   [UL] !I3 && I4
> 	    [LU] I3 && !I4    [LL] I3 && I4.  */
> 
> which suggests that the PUL and PLU entries for big-endian should be
> the other way around.  Does that sound right, or have I misunderstood?

Yes, that sounds right.

> ...for little-endian, we need to pass the "U" and "L" components of the
> mnemonic in the reverse order: the MIPS instruction specifies the upper
> part first, whereas the rtl pattern specifies the lower part first.
> And for little-endian, U refers to memory element 1 and L to memory
> element 0.  So I think this should be:

... Except that the actual output of the LE insn actually swaps the operands too.  So I think these expanders should not *also* swap the operands.  I've tidied these up a bit since then.

>> +static bool
>> +mips_expand_vpc_ps (struct expand_vec_perm_d *d)

I've eliminated this function since then.

>> +  /* Convert the selector into the packed 8-bit form for pshufh.  */
>> +  for (i = mask = 0; i < 4; i++)
>> +    mask |= (d->perm[i] & 3) << (i * 2);
> 
> I think this is endian-dependent.  For little-endian, the bottom two bits
> of the mask determine element 0; for big-endian, the top two bits of the
> mask do. 

Recall that loongson can only run in little-endian.  I added comments about that in the md file, but it would do no harm to add another here.

> (There's a machine in the farm, but bootstrapping on it is rather slow.)

Yeah, I started checking out the tree there yesterday and it never completed.

> I think a lot of the endianness stuff in the patch is dependent on byte
> endianness rather than word endianness.  Since we only support two out
> of the four combinations, it seems better not to worry which and simply
> use TARGET_{BIG,LITTLE}_ENDIAN instead of {WORDS,BYTES}_{BIG,LITTLE}_ENDIAN.

Sure.

This is my current patch, which doesn't have the pul/plu insns swapped, as suggested above.  I did change the loongson.h interface as H-P suggested.


r~

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