Hi!
On Mon, Feb 27, 2023 at 08:11:09PM +0100, Jakub Jelinek wrote:
> (insn 52 48 53 2 (set (reg:CC 66 cc)
> (compare:CC (reg:SI 130)
> (const_int 0 [0]))) "pr108803.c":12:25 437 {cmpsi}
> (expr_list:REG_DEAD (reg:SI 130)
> (expr_list:REG_EQUAL (compare:CC (const_int -64 [0xffffffffffffffc0])
> (const_int 0 [0]))
> (nil))))
> (insn 53 52 57 2 (set (reg:DI 152 [ _6+8 ])
> (if_then_else:DI (ge (reg:CC 66 cc)
> (const_int 0 [0]))
> (reg:DI 132)
> (const_int 0 [0]))) "pr108803.c":12:25 490 {*cmovdi_insn}
> (expr_list:REG_DEAD (reg:DI 132)
> (nil)))
> (insn 57 53 59 2 (set (reg:DI 151 [ _6 ])
> (if_then_else:DI (ge (reg:CC 66 cc)
> (const_int 0 [0]))
> (const_int 0 [0])
> (reg:DI 126))) "pr108803.c":12:25 490 {*cmovdi_insn}
> (expr_list:REG_DEAD (reg:CC 66 cc)
> (nil)))
> ...
> (insn 71 68 72 2 (set (reg:CC 66 cc)
> (compare:CC (reg:SI 137)
> (const_int 0 [0]))) "pr108803.c":12:42 437 {cmpsi}
> (expr_list:REG_DEAD (reg:SI 137)
> (expr_list:REG_EQUAL (compare:CC (const_int -64 [0xffffffffffffffc0])
> (const_int 0 [0]))
> (nil))))
> (insn 72 71 76 2 (set (reg:DI 153 [ _8 ])
> (if_then_else:DI (ge (reg:CC 66 cc)
> (const_int 0 [0]))
> (reg:DI 139)
> (reg:DI 153 [ _8 ]))) "pr108803.c":12:42 490 {*cmovdi_insn}
> (expr_list:REG_DEAD (reg:DI 139)
> (nil)))
> (insn 76 72 77 2 (set (reg:DI 154 [ _8+8 ])
> (if_then_else:DI (ge (reg:CC 66 cc)
> (const_int 0 [0]))
> (reg:DI 138)
> (reg:DI 127))) "pr108803.c":12:42 490 {*cmovdi_insn}
> (expr_list:REG_DEAD (reg:DI 138)
> (expr_list:REG_DEAD (reg:DI 127)
> (expr_list:REG_DEAD (reg:CC 66 cc)
> (nil)))))
> (insn 77 76 78 2 (set (reg:DI 159 [ b ])
> (ior:DI (reg:DI 151 [ _6 ])
> (reg:DI 126))) "pr108803.c":12:12 537 {iordi3}
> (expr_list:REG_DEAD (reg:DI 126)
> (expr_list:REG_DEAD (reg:DI 151 [ _6 ])
> (nil))))
> (insn 78 77 80 2 (set (reg:DI 160 [ b+8 ])
> (reg:DI 152 [ _6+8 ])) "pr108803.c":12:12 65 {*movdi_aarch64}
> (expr_list:REG_DEAD (reg:DI 152 [ _6+8 ])
> (nil)))
Both CC's are used twice, in if_then_else all times, a situation that
does not happen frequently at all, and that combine is apparently not
prepared for at all. It is the same (hard!) register in all cases as
well.
> but as you can see, because cc reg has been REG_DEAD before on insn 57
> rather than on insn 53, nothing really removed REG_DEAD note from there
> and just adds it on insn 78 (note, besides this REG_DEAD issue the
> IL is otherwise still sane, the previous cc setter 71 and its previous
> uses 72 and 76 in between the move have been optimized away already in
> an earlier successful combination).
> And things go wild with the next successful combination:
Yup.
Segher
