llvm dev - Oct 2012 - [LLVMdev] [llvm-commits] [cfe-commits] [PATCH] [llvm+clang] memset for non-8-bit bytes

> I'm a bit confused by this concept. 
For the term byte, I use the "archaic" definition in the C (and C++)
standard (section 3.6):

  addressable unit of data storage large enough to hold any member of the basic
character
  set of the execution environment

/Patrik Hägglund 

-----Original Message-----
From: Jakob Stoklund Olesen [mailto:stoklund at 2pi.dk] 
Sent: den 19 oktober 2012 18:28
To: Patrik Hägglund H
Cc: Chris Lattner; llvmdev at cs.uiuc.edu
Subject: Re: [llvm-commits] [cfe-commits] [PATCH] [llvm+clang] memset for
non-8-bit bytes


On Oct 19, 2012, at 2:24 AM, Patrik Hägglund H <patrik.h.hagglund at
ericsson.com> wrote:
>> non-8-bit byte 
I'm a bit confused by this concept. I'm aware of the archaic meaning of
the word byte, but it has meant 8 bits for the last 30 years. There's even
an ISO/IEC standard.

I know of architectures like Texas' C55x DSPs that address 16 bits at a
time, but even their data sheets state:

. 256K Bytes Zero-Wait State On-Chip RAM, Composed of:
  . -  64K Bytes of Dual-Access RAM (DARAM), 8 Blocks of 4K x 16-Bit
  . -  192K Bytes of Single-Access RAM (SARAM), 24 Blocks of 4K x 16-Bit

Perhaps you could begin by defining more accurately what you're talking
about?

/jakob

On Oct 19, 2012, at 11:43 AM, Patrik Hägglund H <patrik.h.hagglund at
ericsson.com> wrote:
>> I'm a bit confused by this concept. 
> 
> For the term byte, I use the "archaic" definition in the C (and
C++) standard (section 3.6):
> 
>  addressable unit of data storage large enough to hold any member of the
basic character
>  set of the execution environment
That definition isn't really relevant to LLVM, though.  You can define char
to be (say) 16 bits, and your frontend (clang?) just needs to set CHAR_BIT
properly, and generate code with i16 whenever you wrote char.

I suspect what you want to talk about, and the part that is relevant to LLVM as
opposed to clang, is supporting architectures where the minimum addressable unit
is not 8 bits in size.

--Owen

On Fri, Oct 19, 2012 at 12:45 PM, Owen Anderson <resistor at mac.com>
wrote:
>
> On Oct 19, 2012, at 11:43 AM, Patrik Hägglund H <patrik.h.hagglund at
ericsson.com> wrote:
>
>>> I'm a bit confused by this concept.
>>
>> For the term byte, I use the "archaic" definition in the C
(and C++) standard (section 3.6):
>>
>>  addressable unit of data storage large enough to hold any member of
the basic character
>>  set of the execution environment
>
> That definition isn't really relevant to LLVM, though.  You can define
char to be (say) 16 bits, and your frontend (clang?) just needs to set CHAR_BIT
properly, and generate code with i16 whenever you wrote char.
That's not true; SimplifyLibCalls, for example, would perform all
sorts of bad optimizations if CHAR_BIT is not 8.
> I suspect what you want to talk about, and the part that is relevant to
LLVM as opposed to clang, is supporting architectures where the minimum
addressable unit is not 8 bits in size.
There's also this.

-Eli

> You can define char to be (say) 16 bits, and your
> frontend (clang?) just needs to set CHAR_BIT properly, and
> generate code with i16 whenever you wrote char.
Sorry, but this is naive. As a starting point (many more changes are needed), I
suggest you take a look at this patch (not provided by me) refered in my
originial email:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20120702/146050.html

/Patrik Hägglund

-----Original Message-----
From: Owen Anderson [mailto:resistor at mac.com] 
Sent: den 19 oktober 2012 21:45
To: Patrik Hägglund H
Cc: Jakob Stoklund Olesen; llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] [llvm-commits] [cfe-commits] [PATCH] [llvm+clang] memset
for non-8-bit bytes


On Oct 19, 2012, at 11:43 AM, Patrik Hägglund H <patrik.h.hagglund at
ericsson.com> wrote:
>> I'm a bit confused by this concept. 
> 
> For the term byte, I use the "archaic" definition in the C (and
C++) standard (section 3.6):
> 
>  addressable unit of data storage large enough to hold any member of the
basic character
>  set of the execution environment
That definition isn't really relevant to LLVM, though.  You can define char
to be (say) 16 bits, and your frontend (clang?) just needs to set CHAR_BIT
properly, and generate code with i16 whenever you wrote char.

I suspect what you want to talk about, and the part that is relevant to LLVM as
opposed to clang, is supporting architectures where the minimum addressable unit
is not 8 bits in size.

--Owen

> That definition isn't really relevant to LLVM, though.
> [...] the part that is relevant to LLVM as opposed to clang, is
> supporting architectures where the minimum addressable unit is
> not 8 bits in size.
The C standard don't make any distinction between front-end and back-end
parts of the implementation. Therefore, this is not excluded.

Having a different byte size in the C implementation than supported by the ISA
is mostly of theoretical value, and not what I intended here.

/Patrik Hägglund

-----Original Message-----
From: Owen Anderson [mailto:resistor at mac.com] 
Sent: den 19 oktober 2012 21:45
To: Patrik Hägglund H
Cc: Jakob Stoklund Olesen; llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] [llvm-commits] [cfe-commits] [PATCH] [llvm+clang] memset
for non-8-bit bytes


On Oct 19, 2012, at 11:43 AM, Patrik Hägglund H <patrik.h.hagglund at
ericsson.com> wrote:
>> I'm a bit confused by this concept. 
> 
> For the term byte, I use the "archaic" definition in the C (and
C++) standard (section 3.6):
> 
>  addressable unit of data storage large enough to hold any member of the
basic character
>  set of the execution environment
That definition isn't really relevant to LLVM, though.  You can define char
to be (say) 16 bits, and your frontend (clang?) just needs to set CHAR_BIT
properly, and generate code with i16 whenever you wrote char.

I suspect what you want to talk about, and the part that is relevant to LLVM as
opposed to clang, is supporting architectures where the minimum addressable unit
is not 8 bits in size.

--Owen