[PATCH] Don't bypass blocks with multiple latch edges (PR middle-end/54838)

[Eric Botcazou]

> We then end up with
> Redirecting fallthru edge 3->4 to 6
> JUMP-BYPASS: Proved reg 59 in jump_insn 15 equals constant (const_int 1
> [0x1]) Bypass edge from 3->4 to 6
> Redirecting fallthru edge 9->4 to 5
> JUMP-BYPASS: Proved reg 59 in jump_insn 15 equals constant (const_int 3
> [0x3]) Bypass edge from 9->4 to 5
> i.e., it is assumed that in one reg there "are" two constants, that can't
> be right, right?!

No, I don't think that's the problem.  The above messages are admittedly a bit 
terse, they should say:

JUMP-BYPASS: Proved reg 59 in jump_insn 15 equals constant (const_int 3 [0x3])
             when BB 4 is entered from BB 9.  Redirect edge 9->4 to 5.

so you can have different constants for BB 3 and BB 9.  The patch to tweak the 
dump messages along these lines is pre-approved.

The ICE in merge_latch_edges means that the loop structure and the CFG aren't 
in sync anymore.  Does the cprop pass modify the former without declaring it?

-- 
Eric Botcazou