Re: Unaligned access trade-offs for SFrame FRE layout

On Fri, 12 Sep 2025 10:34:42 -0700
Indu Bhagat <[email protected]> wrote:

> TL;DR: Thinking and experimenting a bit on the possible approaches for 
> avoiding unaligned accesses in the SFrame FRE layout (in SFrame V3), I 
> am not convinced that avoiding unaligned accesses for performance is 
> worth it.  IMO, forsaking compactness for avoiding unaligned accesses is 
> not a good trade off for SFrame.
> 
> Problem Statement
> On architectures such as x86_64, AArch64, and s390x, unaligned memory 
> accesses are handled transparently by the hardware but incur a 
> performance penalty. The objective of this analysis is to evaluate if 
> these unaligned accesses can be eliminated from the SFrame FRE layout 
> and if doing so provides a net performance benefit.

I guess the question is really, is it that big of a performance hit?

I know some others were worried about the performance, but we should look
at measurements too. Is it going to be a big enough issue in the stack
unwinding code to even notice?

> 
> The central challenge is that any alternative must demonstrate a clear
> performance improvement while avoiding significant size overhead. 
> Introducing "bloat" to the format to solve a potential performance issue 
> is a poor trade-off.

Correct. I would like to see performance numbers before we invest too much
time in this.

> 
> Source of unaligned accesses in SFrame FRE
>   - (#1) Access to the SFrame FRE start address (sfre_start_address)
>   - (#2) Access to the SFrame FRE stack offsets,  This is varlen data 
> tailing SFrame FRE top-level members (sfre_start_address and FRE info), 
> usually interpreted as stack offsets)

BTW, we should also look at how often are there unaligned accesses? All the
time? or just a percentage of time? If it is a percentage, what is that
percentage?

> 
> (Note that in the SFrame specification, SFrame Header, and SFrame FDE 
> (function descriptor entry) have aligned accesses.)
> 
> Updated notes on the various approaches and respective evaluation notes 
> on the wiki page:
> https://sourceware.org/binutils/wiki/sframe/sframev3todo#Avoid_unaligned_accesses
> 
> Summary of Approaches and Analysis/Notes
> Unaligned accesses may mean lower performance, but the alternative we 
> pick must at least provide better performance.  It is also important 
> that the chosen approach does not add bloat to the format.  Avoiding 
> unaligned accesses at the expense of bloating up the format is not a 
> good idea IMO.
> 
> Approach 1a: Bucketed members
>   Pros: Negligible bloat.
>   Cons: 1. Writing out the FRE data is somewhat more involved. Affects
>    assemblers, linkers. 2. For the common case though, accessing stack 
> offsets now needs more memory accesses per FRE.  This approach will not 
> bring clear performance benefits; the additional complexity in SFrame 
> readers and writers is not justified then either.

Right. If this causes more cache misses or worse, more page faults, to save
from an unaligned access, I don't think it's worth it.

> 
> Approach 1b: Bucketed members with Index
>   Cons: Significant bloat (~30%).

I personally believe 30% is too much overhead.

> 
> Approach 2: De-duplicated "stack offsets"
>   Pros: Will help reduce the size of SFrame sections.
>   Cons: 1. SFrame FRE layout is designed to be flexible so that it can
>    serve needs of new ABIs:  The varlen data is interpreted as stack 
> offsets on x86_64, and AArch64, but may not be the case for other ABIs. 
> De-duplicating non-structured data is not meaningful. 2. Writing out the 
> FRE data is quite more involved, increasing the complexity in Toolchain.

I don't know enough to comment about the above.

> 
> Approach 3: Good old basic padding
>   Cons: Significant bloat (~22%).  Performance win arguable as well.

I think 22% is also too much.

> 
> IMO, none of these approaches provide viable way to move forward. The 
> proposed methods either fail to deliver the desired clear performance 
> gain or introduce a significant size penalty or complexity, which is an 
> unacceptable trade-off.
> 
> Would like to gather inputs from the interested folks on this. Please 
> take a look and chime in.  Other ideas welcome.

As stated above, I'd like to know how much of a performance benefit this
is. It may not be worth it.

I wasn't one of the people who brought up unaligned accesses. I'd like to
hear from them to get their input.

-- Steve