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

[H.J. Lu]

On Mon, Jul 13, 2026 at 5:28 PM Mark Wielaard <mark@klomp.org> wrote:
>
> Hi Jan,
>
> On Mon, Jul 13, 2026 at 08:09:23AM +0200, Jan Beulich wrote:
> > On 12.07.2026 17:37, Mark Wielaard wrote:
> > > I am not sure I understand why you want inline assembly excluded. Yes,
> > > inline assembly is "hand-written", but only because there was no exact
> > > way to make the compiler produce a specific sequence or encoding of
> > > some instructions. How about something like:
> > >
> > > "Assembler optimization is intended primarily for hand-written
> > > assembly files, not for use with compiler generated code (or inline
> > > assembly crafted explicitly to produce a specific instruction sequence
> > > or encoding).  If you notice any effects on compiler generated code,
> > > please consider raising a bug against the compiler about a missed
> > > optimization.  Note, however, that this does not extend to there
> > > possibly being effects on code originating from hand-written assembly
> > > code embedded as a whole into a higher-level language compiler."
> >
> > Well. You've seen H.J.'s reply, haven't you? His take is that -O very much
> > is for compiler generated code. As just indicated on another thread, what
> > is needed here first is an agreed upon understanding what we want to
> > optimize (for). Intentions originally weren't made sufficiently clear, and
> > hence views looks to have heavily diverged.
>
> OK, but lets document that then so people don't accidentially use -O
> without knowing that the option doesn't have clear semantics. How
> about simply:
>
> "Assembler optimization is currently an experimental feature that
> might result in changing instruction encoding, replacement of
> instructions by equivalent ones or removal of instructions that could
> be seen as nops. Don't use it when you have code that depends on the
> exact assembler instruction encodings."
>

GNU assembler is used by GCC to generate binaries.  Assembler
optimizations should be 100% safe for all applications, hand-written
codes or compiler generated codes.

-- 
H.J.