Re: Unaligned access trade-offs for SFrame FRE layout

On 9/16/25 9:32 AM, Fangrui Song wrote:
> On Tue, Sep 16, 2025 at 9:03 AM Indu Bhagat <[email protected]> wrote:
>>
>> On 9/15/25 11:05 PM, Fangrui Song wrote:
>>> On Mon, Sep 15, 2025 at 9:12 AM Steven Rostedt <[email protected]> wrote:
>>>>
>>>> On Sun, 14 Sep 2025 22:42:46 -0700
>>>> Indu Bhagat <[email protected]> wrote:
>>>>
>>>>> In such cases, the routines reading the SFrame data under consideration
>>>>> here (SFrame FRE start addr, and SFrame FRE stack offsets) from memory
>>>>> will need to use a memcpy to copy out the data to an aligned location.
>>>>>
>>>>> In GNU Binutils libsframe (used by ld), we do the above. Such a "SFrame
>>>>> FRE decoding" routine could be provided in a arch-specific manner in
>>>>> SFrame stack tracers.
>>>>
>>>> I'm perfectly fine with making it a requirement for the reader of the
>>>> SFrame section having to use memcpy into an aligned structure for reading
>>>> if the architecture requires it. Let only the architectures that have
>>>> issues with unaligned access take the performance hit.
>>>>
>>>> -- Steve
>>>
>>> I agree. Unaligned access has nearly zero performance impact on modern
>>> architectures, provided the access doesn't span additional cache
>>> lines.
>>> The padding required for alignment would increase the size, likely
>>> creating more overhead than any alignment benefit would justify.
>>>
>>> (
>>>   From a linker and binary utilities perspective, I'd even suggest
>>> adopting a universal little-endian format regardless of the target
>>> system's native endianness.
>>> This would eliminate the need for endianness templates in the C++ code
>>> and simplify toolchain implementation across platforms.
>>>
>>
>> (Perhaps I am missing something) Wouldnt a toolchain implementation need
>> endianness handling anyway to support cross toolchains?
>>
>>> On the big-endian z/Architecture, this is efficient: the LOAD REVERSED
>>> instructions are used by the bswap versions in the following program,
>>> not even requiring extra instructions.
>>> #define WIDTH(x) \
>>> typedef __UINT##x##_TYPE__ [[gnu::aligned(1)]] uint##x; \
>>> uint##x load_inc##x(uint##x *p) { return *p+1; } \
>>> uint##x load_bswap_inc##x(uint##x *p) { return __builtin_bswap##x(*p)+1; }; \
>>> uint##x load_eq##x(uint##x *p) { return *p==3; } \
>>> uint##x load_bswap_eq##x(uint##x *p) { return __builtin_bswap##x(*p)==3; }; \
>>>
>>> WIDTH(16);
>>> WIDTH(32);
>>> WIDTH(64);
>>> )
>>
>> For AArch64 which SFrame supports too, this is not true. AArch64 has
>> both LE and BE.
> 
> While runtime consumers typically handle a single endianness, other
> tools like linkers and binary utilities must support both. They have
> to support cross compilation, producing a big-endian executable from a
> little-endian host.
> 

Right.  Sorry, I am still missing the link between "complexity of 
endianness templates in the C++ code" vs what you say in the next 
paragraph: endian aware read/write is anyway necessary.

> A universal little-endian approach simplifies code. Instead of using a
> function like read32le(config, p), where config->endian specifies the
> object file's endianness, or read32(p) with an internal endianness
> check, the code can simply use read32le(p).
> 
> The read32le(p) function is either a standard read or a byte-swapped
> read. This byte-swapping is fast on aarch64be (thanks to REV16 and
> REV32 instructions) and s390x (byte-swap load).

The rev* instruction is in the data dependency chain.  This means that 
using little-endian for AArch64 BE then defers the task of endian swap 
on to the stack tracers. E.g., aarch64 (added insn in dependency chain):

load_inc16:
         ldrh    w0, [x0]
         add     w0, w0, 1
         ret
load_bswap_inc16:
         ldrh    w0, [x0]
         rev16   w0, w0
         add     w0, w0, 1
         ret

s390x (same height of dependency chain):

load_inc16:
         lh      %r2,0(%r2)
         ahi     %r2,1
         llghr   %r2,%r2
         br      %r14
load_bswap_inc16:
         lrvh    %r2,0(%r2)
         ahi     %r2,1
         llghr   %r2,%r2
         br      %r14