pseudo-ucode expansion for LOOP <dest>

gistfile1.txt

	lea	rcx, [rcx-1]	; decrement rcx w/o flag update
	mov	temp0, rax	; save rax that we're about to trash
	lahf			; save original flags
	test	rcx, rcx	; check whether updated rcx is zero
	setz	temp1		; temp1 = 1 if rcx=0, 0 otherwise
	sahf			; restore flags
	mov	rax, temp0	; restore rax
	jecxz	temp1, dest	; jump if temp1 is zero, not rcx (doesn't exist in regular ISA but rcx is renamed anyway so the internal uop can do any source)

NOTE the actual ucode expansion probably doesn't have the MOVs since I would expect the internal LAHF/SAHF uops
to have explicit sources, meaning the actual code is more like

	lea     rcx, [rcx-1]	; decrement rcx w/o flag update
	lahf	temp0		; save original flags to ucode temp
	test	rcx, rcx	; check whether updated rcx is zero
	setz	temp1		; temp1 = 1 if rcx=0, 0 otherwise
	sahf	temp0		; restore flags
	jecxz	temp1, dest	; jump if temp1 is zero, not rcx (doesn't exist in regular ISA but rcx is renamed anyway so the internal uop can do any source)

also note that jecxz is internally expanded into two uops and these would be "spelled out" in the actual ucode

anyway, that's 7 uops, which matches the uop counts reported for LOOP on Skylake.