[Bug target/107172] [13 Regression] wrong code with "-O1 -ftree-vrp" on x86_64-linux-gnu since r13-1268-g8c99e307b20c502e

[segher at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107172

--- Comment #29 from Segher Boessenkool <segher at gcc dot gnu.org> ---
(In reply to Hongtao.liu from comment #23)
> looking at i386.c put_condition_code used by *setcc_qi, it looks like (EQ
> (reg:CCCmode FLAG_REG) (const_int 0)) means get carry flag.
> Not (LTU: (REG:CCCmode FLAGS_REG) (const_int 0)).
> Now I got more confused.

(eq (reg:CCC 17) (const_int 0))  means that the comparison that did set reg 17
returned "equal".

CCCmode is just like CCmode, but *only* the carry flag is valid.