llvm dev - Sep 2014 - [LLVMdev] Runtime alignment

For the size of a type to be calculated at runtime, there is a known trick with
gep. Is there any similar trick for determining the alignment of a type at
runtime for target-independent IR?
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On 24 September 2014 15:09, Mark Boyall <wolfeinstein at gmail.com> wrote:
> For the size of a type to be calculated at runtime, there is a known trick
> with gep. Is there any similar trick for determining the alignment of a
> type at runtime for target-independent IR?
>
There's ConstantExpr::getAlignOf to go with getSizeOf and getOffsetOf. The
implementation has this comment:

  // alignof is implemented as: (i64) gep ({i1,Ty}*)null, 0, 1
  // Note that a non-inbounds gep is used, as null isn't within any object.

Nick
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Thanks, that's sweet. I've been thinking about using this in various
contexts and I've come to the conclusion that it's not that useful
because
the alloca alignment cannot be a ConstantExpr, so there's no way to defer
alignment for trying to create IR that's more target-independent. I've
been
thinking about suggesting that some IR-level constructs that currently
require literal constants, like array sizes and alloca alignment, should be
changed to be ConstantExprs.

On 25 September 2014 00:33, Nick Lewycky <nlewycky at google.com> wrote:
> On 24 September 2014 15:09, Mark Boyall <wolfeinstein at gmail.com>
wrote:
>
>> For the size of a type to be calculated at runtime, there is a known
>> trick with gep. Is there any similar trick for determining the
alignment of
>> a type at runtime for target-independent IR?
>>
>
> There's ConstantExpr::getAlignOf to go with getSizeOf and getOffsetOf.
The
> implementation has this comment:
>
>   // alignof is implemented as: (i64) gep ({i1,Ty}*)null, 0, 1
>   // Note that a non-inbounds gep is used, as null isn't within any
object.
>
> Nick
>
>
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