llvm dev - Sep 2015 - Do Frontends need to care about alignment?

Meant to add more text, but somehow the email got sent.

Could you explain why for some of these cases? If I set the right target
and only want to operate with the system C ABI, will LLVM not generate
correct code if I want to pay say structs to a system defined function?

I see clang specifying alignment, but I'm not sure why.

I want to understand beyond alignment and function ABI (which for my case
is also system's C ABI) what makes my LLVM IR platform dependent.

On Mon, Aug 31, 2015 at 4:16 PM, Philip Reames <listmail at
philipreames.com>
wrote:
> Sometimes.
>
> On 08/31/2015 03:48 PM, Rohan Bajaj via llvm-dev wrote:
>
>
>
> _______________________________________________
> LLVM Developers mailing listllvm-dev at
lists.llvm.orghttp://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
>
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LLVM will always generate correct code if you don’t specify alignment, but may
generate inefficient code.  For example, if you are targeting MIPS (or older ARM
ISAs) then the hardware does not handle unaligned loads and stores, and so you
will enter a trap-and-emulate path if you do a load or store with
lower-than-natural alignment.  To avoid this, LLVM will emit a slower sequence
of loads, shifts and masks (or load-right + load-left on MIPS) for all cases
where the load / store does not have a sufficiently high alignment in the IR.

The alignment is used to guarantee the alignment on allocas and globals, though
in most cases this is unnecessary (most targets have a sufficiently high default
alignment that they’ll be fine).  It is also used to provide a contract to the
back end saying ‘either this load/store has this alignment, or it is undefined
behaviour’.  This means that the back end is free to emit instructions that rely
on that alignment (and mid-level optimisers are free to perform transforms that
require that alignment).  For x86, it doesn’t make much difference, as almost
all instructions are alignment-independent.  For MIPS, it can make a big
difference.

David
> On 1 Sep 2015, at 01:14, Rohan Bajaj via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Meant to add more text, but somehow the email got sent.
> 
> Could you explain why for some of these cases? If I set the right target
and only want to operate with the system C ABI, will LLVM not generate correct
code if I want to pay say structs to a system defined function?
> 
> I see clang specifying alignment, but I'm not sure why.
> 
> I want to understand beyond alignment and function ABI (which for my case
is also system's C ABI) what makes my LLVM IR platform dependent.
> 
> On Mon, Aug 31, 2015 at 4:16 PM, Philip Reames <listmail at
philipreames.com> wrote:
> Sometimes.
> 
> On 08/31/2015 03:48 PM, Rohan Bajaj via llvm-dev wrote:
>> 
>> 
>> _______________________________________________
>> LLVM Developers mailing list
>> 
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

The other case is that your ABI might require a particular alignment.  
While some of the details of calling conventions are handled by LLVM 
itself, some have to be specified by the frontend. There's been 
discussion on this divide on the mailing list before.

(p.s. I know you mentioned this in your question.  Just mentioning it 
for completeness.)

On 09/01/2015 02:48 AM, David Chisnall wrote:
> LLVM will always generate correct code if you don’t specify alignment, but
may generate inefficient code.  For example, if you are targeting MIPS (or older
ARM ISAs) then the hardware does not handle unaligned loads and stores, and so
you will enter a trap-and-emulate path if you do a load or store with
lower-than-natural alignment.  To avoid this, LLVM will emit a slower sequence
of loads, shifts and masks (or load-right + load-left on MIPS) for all cases
where the load / store does not have a sufficiently high alignment in the IR.
>
> The alignment is used to guarantee the alignment on allocas and globals,
though in most cases this is unnecessary (most targets have a sufficiently high
default alignment that they’ll be fine).  It is also used to provide a contract
to the back end saying ‘either this load/store has this alignment, or it is
undefined behaviour’.  This means that the back end is free to emit instructions
that rely on that alignment (and mid-level optimisers are free to perform
transforms that require that alignment).  For x86, it doesn’t make much
difference, as almost all instructions are alignment-independent.  For MIPS, it
can make a big difference.
>
> David
>
>> On 1 Sep 2015, at 01:14, Rohan Bajaj via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
>>
>> Meant to add more text, but somehow the email got sent.
>>
>> Could you explain why for some of these cases? If I set the right
target and only want to operate with the system C ABI, will LLVM not generate
correct code if I want to pay say structs to a system defined function?
>>
>> I see clang specifying alignment, but I'm not sure why.
>>
>> I want to understand beyond alignment and function ABI (which for my
case is also system's C ABI) what makes my LLVM IR platform dependent.
>>
>> On Mon, Aug 31, 2015 at 4:16 PM, Philip Reames <listmail at
philipreames.com> wrote:
>> Sometimes.
>>
>> On 08/31/2015 03:48 PM, Rohan Bajaj via llvm-dev wrote:
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>>
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev