Re: RFR(M): 8248188: [PATCH] Add HotSpotIntrinsicCandidate and API for Base64 decoding

[Corey Ashford]

Due to the switchover to Git from Mercurial, this patch set got a new 
thread on the mailing lists on 9/24, but it has the same title. as this 
thread.

- Corey
On 9/7/20 3:08 AM, Doerr, Martin wrote:
Hi Corey,

thanks for investigating.

Note that we use xlclang++ on AIX. It may possibly understand the directives as 
gcc on linux.
Gcc 7.3.1 is the minimum for BE linux.
But if you protect your code by #ifdef VM_LITTLE_ENDIAN no compiler except gcc 
>= 7.4.0 should ever look at it.

Best regards,
Martin


-----Original Message-----
From: Corey Ashford <cjash...@linux.ibm.com>
Sent: Dienstag, 1. September 2020 02:17
To: Doerr, Martin <martin.do...@sap.com>
Cc: Michihiro Horie <ho...@jp.ibm.com>; hotspot-compiler-
d...@openjdk.java.net; core-libs-dev <core-libs-dev@openjdk.java.net>;
Kazunori Ogata <oga...@jp.ibm.com>; jos...@br.ibm.com
Subject: Re: RFR(M): 8248188: [PATCH] Add HotSpotIntrinsicCandidate and
API for Base64 decoding

On 8/27/20 8:07 AM, Doerr, Martin wrote:
I will use __attribute__ ((align(16))) instead of __vector, and make
them arrays of 16 unsigned char.
Maybe __vectors works as expected, too, now. Whatever we use, I'd
appreciate to double-check the alignment e.g. by using gdb.
I don't remember what we had tried and why it didn't work as desired.


I just now tried on gcc-7.5.0, declaring a __vector at 1, 2, 3, 8, 9,
and 15 byte offsets in a struct, trying to force a misalignment, but the
compiler realigned all of them on 16-byte boundaries.

If someone decides to make the intrinsic work on AIX (big endian), and
compiles with 7.3.1, I don't know what will happen w.r.t. alignment, so
to be on the safe side, I will make the declarations 16-byte unsigned
char arrays with an align attribute.

Looking a bit deeper, I see that the __vector type comes out of the C
preprocessor as: __attribute__((altivec(vector__))). It's part of the
compiler's basic set of predefined macros, and can be seen using this
command:

%  gcc -dM -E - < /dev/null | grep __vector

#define __vector __attribute__((altivec(vector__)))

Some information here:
https://gcc.gnu.org/onlinedocs/gcc/PowerPC-Type-Attributes.html

I don't know if this is helpful or not, but it might answer part of your
question about the meaning of __vector.

Regards,

- Corey