llvm dev - May 2017 - How exactly is datatype alignment determined?

On Mon, 22 May 2017, Dr. ERDI Gergo wrote:
> Actually, tracking down the sequence of function calls, it turns out that
'8'
> is ultimately coming from the following call in DataLayout::getAlignment:
>
> getAlignmentInfo(AGGREGATE_ALIGN, 0, abi_or_pref, Ty);
>
> this seems to return 8 with the following datalayout string:
>
> e-S8:p:16:8-i8:8-i16:8-i32:8-i64:8-f32:8-f64:8-n8-a:8
>
> I think my problem is that 'a:8' probably doesn't mean what I
think it should
> mean. What is the difference between 'a:8' and 'a:0'?
OK I just now tried with 'a:0' for identical result. I am now quite sure
I
am fully confused about what these alignment settings mean.

Does 'a:8' mean single byte alignment (i.e. *no* alignment)? If yes,
does
that mean getAlignment for a struct type should return 1? In fact, when 
getAlignment returns 8, does that mean 8 bits (no alignment) or 8 bytes 
(64 bits) alignment? Or does that mean the lowest 8 bits of the address 
needs to be 0, i.e. 256-byte alignment?

Would http://llvm.org/docs/LangRef.html#langref-datalayout be help?

- chenwj

2017-05-22 23:26 GMT+08:00 Dr. ERDI Gergo via llvm-dev <
llvm-dev at lists.llvm.org>:
> On Mon, 22 May 2017, Dr. ERDI Gergo wrote:
>
> Actually, tracking down the sequence of function calls, it turns out that
>> '8' is ultimately coming from the following call in
>> DataLayout::getAlignment:
>>
>> getAlignmentInfo(AGGREGATE_ALIGN, 0, abi_or_pref, Ty);
>>
>> this seems to return 8 with the following datalayout string:
>>
>> e-S8:p:16:8-i8:8-i16:8-i32:8-i64:8-f32:8-f64:8-n8-a:8
>>
>> I think my problem is that 'a:8' probably doesn't mean what
I think it
>> should mean. What is the difference between 'a:8' and
'a:0'?
>>
>
> OK I just now tried with 'a:0' for identical result. I am now quite
sure I
> am fully confused about what these alignment settings mean.
>
> Does 'a:8' mean single byte alignment (i.e. *no* alignment)? If
yes, does
> that mean getAlignment for a struct type should return 1? In fact, when
> getAlignment returns 8, does that mean 8 bits (no alignment) or 8 bytes (64
> bits) alignment? Or does that mean the lowest 8 bits of the address needs
> to be 0, i.e. 256-byte alignment?
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>


-- 
Wei-Ren Chen (陳韋任)
Homepage: https://people.cs.nctu.edu.tw/~chenwj
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The 8 in the data layout string should have been converted to a byte value
by this code before it was passed to setAlignment. As far as I cant ell
getAlignment should return the byte alignment that was passed to
setAlignment, not the bit alignment from the string.
      // ABI alignment.
      if (Rest.empty())
        report_fatal_error(
            "Missing alignment specification in datalayout string");
      Split = split(Rest, ':');
      unsigned ABIAlign = inBytes(getInt(Tok));


~Craig

On Mon, May 22, 2017 at 10:12 AM, 陳韋任 via llvm-dev <llvm-dev at
lists.llvm.org>
wrote:
> Would http://llvm.org/docs/LangRef.html#langref-datalayout be help?
>
> - chenwj
>
> 2017-05-22 23:26 GMT+08:00 Dr. ERDI Gergo via llvm-dev <
> llvm-dev at lists.llvm.org>:
>
>> On Mon, 22 May 2017, Dr. ERDI Gergo wrote:
>>
>> Actually, tracking down the sequence of function calls, it turns out
that
>>> '8' is ultimately coming from the following call in
>>> DataLayout::getAlignment:
>>>
>>> getAlignmentInfo(AGGREGATE_ALIGN, 0, abi_or_pref, Ty);
>>>
>>> this seems to return 8 with the following datalayout string:
>>>
>>> e-S8:p:16:8-i8:8-i16:8-i32:8-i64:8-f32:8-f64:8-n8-a:8
>>>
>>> I think my problem is that 'a:8' probably doesn't mean
what I think it
>>> should mean. What is the difference between 'a:8' and
'a:0'?
>>>
>>
>> OK I just now tried with 'a:0' for identical result. I am now
quite sure
>> I am fully confused about what these alignment settings mean.
>>
>> Does 'a:8' mean single byte alignment (i.e. *no* alignment)? If
yes, does
>> that mean getAlignment for a struct type should return 1? In fact, when
>> getAlignment returns 8, does that mean 8 bits (no alignment) or 8 bytes
(64
>> bits) alignment? Or does that mean the lowest 8 bits of the address
needs
>> to be 0, i.e. 256-byte alignment?
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
>
>
> --
> Wei-Ren Chen (陳韋任)
> Homepage: https://people.cs.nctu.edu.tw/~chenwj
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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