Duh, correct. Over-alignment factor proportional to log2(heap_size / intended_alignment).


Sebastian Wittmeier wrote:


It sums to



50% * 1 + 25% * 2 + 12.5% * 4 + ...

= .5 + .5 + .5 + ...

Worse:



So a 64 KiB SRAM or heap in the SRAM would for a naximum alignment of 32 KiB and actually stored 32-bit values on average have 7x the amount of alignment: 28 bytes instead of 4 bytes.



For a maximum alignment of 128 bytes, it would be 3x or 12 bytes.

-----Ursprüngliche Nachricht-----
Von: Marc Edouard Gauthier via Std-Proposals <std-proposals_at_[hidden]<mailto:std-proposals_at_[hidden]>>
Gesendet: Fr 31.10.2025 18:08
Betreff: Re: [std-proposals] realloc(3)
An: std-proposals_at_[hidden]rg<mailto:std-proposals_at_[hidden]>;
CC: Marc Edouard Gauthier <MarcEdouard.Gauthier_at_[hidden]<mailto:MarcEdouard.Gauthier_at_[hidden]>>;
Bo Persson wrote:
> On 2025-10-31 at 11:41, Marcin Jaczewski via Std-Proposals wrote:
> > Would it be possible to recover this information from the pointer
> > itself? If it had alignment to some specific limit then the allocator
> > is required to return the same on the new pointer.
> > It could sometimes have "false positives" but overall it probably
> > should not have a negative impact on total memory usage.
>
> I guess it would be more than "sometimes". Assuming that your very first
> allocation from a fresh heap happens to be page aligned, what is its "true"
> alignment?

On average, my guess is everything would be twice too much aligned,
either creating a huge amount of fragmentation or taking up twice the
allocation space depending on implementation.

A pointer aligned to bit N has a 50% probability of being aligned to bit N+1
(twice the expected alignment), 25% aligned to N+2 (4 times expected), etc...
Summing this converges on average to twice the overall expected alignments.

Marc
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