[Valgrind-developers] [PATCH 1/2] x86: optimize XCHG to MOV for same-register forms

[Jan Beulich]

On 03.07.2026 08:09, Paul Floyd wrote:
> On 2026-07-03 08:00, Jan Beulich wrote:
>> On 03.07.2026 06:55, Paul Floyd wrote:
>>> Would it be possible for gas to only do this transformation if the source and destination registers are different?
>> When the registers are different, this transformation is invalid to do.
> 
> OK so you are optimising a no-op. Does GCC use it as a no-op?

I don't expect so. In fact, my take is that -O... should not be used on
compiler generated code. The compiler should do whatever optimizations
are possible / sensible, and it should not emit code which can (easily)
further be optimized. (Easily because the assembler really only does
very simple and pretty obvious transformations.)

I can see that there may be a desire to optimize asm()-generated code.
That imo shouldn't be done by passing -Wa,-O... but rather by some
other, to be invented machinery. (We could tie it to the #APP / #NO_APP
markers that the compiler emits, for example.)

Jan