llvm dev - Jun 2009 - [LLVMdev] Alloca and malloc alignment

According to the LLVM manual, for both malloc and alloca:

  "If a constant alignment is specified, the value result of the
allocation is guaranteed to be aligned to at least that boundary. If
not specified, or if zero, the target can choose to align the
allocation on any convenient boundary."

I don't see the rationale for the second sentence, because it means
that alloca and malloc without an alignment are basically useless.
Why?  Because without a specified alignment the allocated memory might
not be properly aligned for the type that was specified.
It would be nice if the type argument to alloca&malloc were used to
determine a minimal acceptable alignment.

I discovered this by using alloca for a 2-element vector of i64, and
sure enough, the memory wasn't properly aligned and thus useless for
storing the vector (this was on i386).

  -- Lennart

Hi Lennart,
>   "If a constant alignment is specified, the value result of the
> allocation is guaranteed to be aligned to at least that boundary. If
> not specified, or if zero, the target can choose to align the
> allocation on any convenient boundary."
> 
> I don't see the rationale for the second sentence, because it means
> that alloca and malloc without an alignment are basically useless.
> Why?  Because without a specified alignment the allocated memory might
> not be properly aligned for the type that was specified.
> It would be nice if the type argument to alloca&malloc were used to
> determine a minimal acceptable alignment.
it is already used for that: if you supply an alignment of zero then it
uses the ABI alignment for the type.
> I discovered this by using alloca for a 2-element vector of i64, and
> sure enough, the memory wasn't properly aligned and thus useless for
> storing the vector (this was on i386).
Then that's a bug, please open a bug report with details.

Ciao,

Duncan.