How can I set or clear overflow flag in x86 assembly?

Question

I want to write a simple code (or algorithm) to set/clear overflow flag. For setting OF, I know that I can use signed values. But how can I clear that?

Answer 1

Provided:

• you have a register whose content you don't care about,
• you must to preserve CF-Flag

Best solution to clear OF (sar):

Say register is al. (setc only with byte registers r/8)

; clear OF-Flag, preserve CF
setc al
sar al, 1

Note: this is good because it has no partial flag updates, which may cause stalls. (sar xx, 1 writes all flags, not leaving any unmodified, unlike inc/dec) c.f. Intel Optimization Guide, 3.5.2.6: Partial Flag Register Stalls, but note that modern Intel CPUs don't have partial-flag stalls or flag-merging at all: instructions that read FLAGS just read either or both of CF or the SPAZO group as 2 separate inputs. (That's why cmovbe is still 2 uops on Broadwell and later: it needs CF and ZF. https://uops.info/)

Source:Intel Documentation SAR p.1234

General solution (inc/dec):

Say register is al. (works with r/8, r/16, r/32, r/64)

; set OF-Flag, preserve CF
mov al, 0x7F
inc al

; clear OF-Flag, preserve CF
mov al, 0x0
inc al

Source:Intel Documentation INC p.551

Alternatively (adox):

Different apporach, if you can assume:

• an adx enabled processor (You check cpu flags with grep adx /proc/cpuinfo)

Say register is eax. (need r64/r32)

; clear OF-Flag, preserve CF
mov eax, 0x0
adox eax, eax

; set OF-Flag, preserve CF
mov eax, 0xFFFFFFFF
adox eax, eax 

Note: Don't try to replace mov with xor (or similar) since that will clear CF

Source:Intel Documentation ADOX p.150