[LLVMdev] Alloca Requirements

[Lennart Augustsson]

And you really want your allocas in the entry block so they are
implemented by just stack pointer manipulation rather than calling
alloca().  The latter is slower, and there's also a bug that makes
calling alloca() not getting the alignment right (if it's > 8).

On Tue, Oct 13, 2009 at 9:44 AM, Duncan Sands <baldrick at free.fr> wrote:
> Hi,
>
>> On Mon, Oct 12, 2009 at 1:07 PM, David Greene <dag at cray.com> wrote:
>>> Are there any implicit assumptions about where alloca instructions
>>> can appear.
>>
>> Static allocas should appear as a continuous chunk in the entry block,
>> otherwise other passes might make bad assumptions.
>
> an alloca can appear anywhere, but when they are outside the entry block
> then some optimizations may not occur.  The important distinction is
> between alloca's that are appear in a loop and those that are not in a
> loop.  Rather than detect loops, optimizers tend to just check whether
> alloca's are in the entry block or not (the entry block is never part
> of a loop).
>
> Ciao,
>
> Duncan.
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