Re: [PATCH] x86: Disable XCHG to MOV optimization

On 15.07.2026 10:17, H.J. Lu wrote:
> GNU assembler is used by GCC to generate binaries.  GCC may not
> always generate the optimal encoding.   That is why I added -O to
> assembler in the first place.  There is no point in adding it if it isn't safe.
> We can't break applications because of some assembler optimizations.

So this kind of thing will break with any optimization changing encoding
size:

	test	eax, eax
	jnz	$+9
	test	rcx, 0x21

(Intel syntax for all examples, as that's what I'm more used to.)

This clearly breaks as well:

	test	eax, eax
	jz	1f+2
1:	test	bx, 0x21

As does this:

	test	ecx, ecx
	jz	$+4
	mov	rcx, 0xc9634890

While all of these may look contrived, I've seen code (not written by
myself) doing similar things. A construct branching into the middle of
an insn was (transiently) even considered to address one of the many
speculation issues we've seen over the last 8+ years.

Jan