https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114902
--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Segher Boessenkool from comment #6)
> (In reply to Andrew Pinski from comment #2)
> > Looks like the issue is during combine.
> >
> > We go from CCGC with a sign_extend to a zero_extend with CCZ. that can't be
> > right.
>
> Why is that not correct? zero_extend is preferred over sign_extend, and both
> are equivalent when only checking for zero.
For Equality they are equivalent yes. But when doing `a >=s 0` a sign
extend/extract will cause different results from a zero extend/extract.
> Is there something wrong in target code here, perhaps?
For arm, x86 and mips?
For testcase in comment #4 on x86_64:
Before combine we start with:
```
(insn 16 15 17 2 (parallel [
(set (reg:SI 106 [ t_4 ])
(and:SI (reg:SI 105 [ tt1_3 ])
(const_int 1 [0x1])))
(clobber (reg:CC 17 flags))
]) "/app/example.cpp":6:9 617 {*andsi_1}
(expr_list:REG_DEAD (reg:SI 105 [ tt1_3 ])
(expr_list:REG_UNUSED (reg:CC 17 flags)
(nil))))
(insn 17 16 20 2 (parallel [
(set (reg:SI 107 [ e_5 ])
(neg:SI (reg:SI 106 [ t_4 ])))
(clobber (reg:CC 17 flags))
]) "/app/example.cpp":7:9 804 {*negsi_1}
(expr_list:REG_DEAD (reg:SI 106 [ t_4 ])
(expr_list:REG_UNUSED (reg:CC 17 flags)
(nil))))
(insn 20 17 21 2 (set (reg:CCGC 17 flags)
(compare:CCGC (reg:SI 107 [ e_5 ])
(const_int -1 [0xffffffffffffffff]))) "/app/example.cpp":8:16 11
{*cmpsi_1}
(expr_list:REG_DEAD (reg:SI 107 [ e_5 ])
(nil)))
(insn 21 20 22 2 (set (reg:QI 109)
(ge:QI (reg:CCGC 17 flags)
(const_int 0 [0]))) "/app/example.cpp":8:16 1125 {*setcc_qi}
(expr_list:REG_DEAD (reg:CCGC 17 flags)
(nil)))
(insn 22 21 23 2 (set (reg:SI 108 [ _1 ])
(zero_extend:SI (reg:QI 109))) "/app/example.cpp":8:16 169
{*zero_extendqisi2}
(expr_list:REG_DEAD (reg:QI 109)
(nil)))
(insn 23 22 24 2 (set (reg:CCZ 17 flags)
(compare:CCZ (reg:SI 108 [ _1 ])
(const_int 0 [0]))) "/app/example.cpp":9:8 7 {*cmpsi_ccno_1}
(expr_list:REG_DEAD (reg:SI 108 [ _1 ])
(nil)))
(jump_insn 24 23 30 2 (set (pc)
(if_then_else (eq (reg:CCZ 17 flags)
(const_int 0 [0]))
(label_ref 30)
(pc))) "/app/example.cpp":9:8 1130 {*jcc}
(expr_list:REG_DEAD (reg:CCZ 17 flags)
(int_list:REG_BR_PROB 7 (nil)))
-> 30)
```
We first combine 16->17 into:
```
(parallel [
(set (reg:SI 107 [ e_5 ])
(sign_extract:SI (reg:SI 105 [ tt1_3 ])
(const_int 1 [0x1])
(const_int 0 [0])))
(clobber (reg:CC 17 flags))
])
```
which is correct and good
And then when combining 17 -> 20 combine does:
Trying 17 -> 20:
17: {r107:SI=sign_extract(r105:SI,0x1,0);clobber flags:CC;}
REG_DEAD r105:SI
REG_UNUSED flags:CC
20: flags:CCGC=cmp(r107:SI,0xffffffffffffffff)
REG_DEAD r107:SI
Successfully matched this instruction:
(set (reg:CCZ 17 flags)
(compare:CCZ (zero_extract:SI (reg:SI 105 [ tt1_3 ])
(const_int 1 [0x1])
(const_int 0 [0]))
(const_int 0 [0])))
Successfully matched this instruction:
(set (reg:QI 109)
(ne:QI (reg:CCZ 17 flags)
(const_int 0 [0])))
Which is also replacing insn 21 incorrectly.
We go from `-(a&1) >= -1` (which is always true) to `(a&1) != 0`.
Maybe we go to `(a&1) <= 1` (still always true) and we mess up somehow to `(a &
1) != 0`
