Re: Unaligned access trade-offs for SFrame FRE layout

[Jens Remus <jremus@linux.ibm.com>]

On 9/16/2025 5:58 PM, Steven Rostedt wrote:
> On Mon, 15 Sep 2025 23:05:09 -0700
> Fangrui Song <maskray@sourceware.org> wrote:
> 
>> 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.
>>
>> 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);
> 
> I would like to hear the comments from Jens on this, as he's adapting
> SFrames for the s390 which I believe is big-endian.

This would allow for endianness bugs for the native case only on s390
(and other big-endian architectures).

Load Reversed takes longer than a normal Load, as it obviously needs to
reverse the register contents.

Regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@de.ibm.com

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