Blackfin eh broken with dataflow merge

[Bernd Schmidt]

Exception handling on the Blackfin was broken by the dataflow merge.
Inspecting the differences in the generated code, it appears that when
compiling functions like _Unwind_Resume in unwind-dw2.c, the dse pass
deletes necessary instructions.

Before:
(insn 56 55 57 7 (set (reg/v:SI 56 [ offset ])
        (reg:SI 0 R0)) 14 {*movsi_insn} (expr_list:REG_DEAD (reg:SI 0 R0)
        (nil)))

(insn 57 56 67 7 (set (reg/v/f:SI 55 [ handler ])
        (mem/s/f/c:SI (plus:SI (reg/f:SI 15 FP)
                (const_int -376 [0xfffffe88])) [12 cur_context.ra+0 S4
A32])) 14 {*movsi_insn} (nil))

(insn 67 57 68 7 (set (reg:SI 10 P2 [ offset ])
        (reg/v:SI 56 [ offset ])) 14 {*movsi_insn} (expr_list:REG_DEAD
(reg/v:SI 56 [ offset ])
        (nil)))

(insn 68 67 69 7 (set (mem:SI (plus:SI (reg/f:SI 15 FP)
                (const_int 4 [0x4])) [0 S4 A32])
        (reg/v/f:SI 55 [ handler ])) 14 {*movsi_insn}
(expr_list:REG_DEAD (reg/v/f:SI 55 [ handler ])
        (nil)))

(jump_insn 69 68 0 7 (set (pc)
        (unspec_volatile [
                (reg:SI 10 P2)
            ] 0)) 126 {eh_return_internal} (expr_list:REG_DEAD (reg:SI
10 P2)
        (expr_list:REG_EQUAL (unspec_volatile [
                    (reg:SI 10 P2)
                ] 0)
            (nil))))

After:
(insn 56 55 67 7 (set (reg/v:SI 56 [ offset ])
        (reg:SI 0 R0)) 14 {*movsi_insn} (expr_list:REG_DEAD (reg:SI 0 R0)
        (nil)))

(insn 67 56 69 7 (set (reg:SI 10 P2 [ offset ])
        (reg/v:SI 56 [ offset ])) 14 {*movsi_insn} (expr_list:REG_DEAD
(reg/v:SI 56 [ offset ])
        (nil)))

(jump_insn 69 67 0 7 (set (pc)
        (unspec_volatile [
                (reg:SI 10 P2)
            ] 0)) 126 {eh_return_internal} (expr_list:REG_DEAD (reg:SI
10 P2)
        (expr_list:REG_EQUAL (unspec_volatile [
                    (reg:SI 10 P2)
                ] 0)
            (nil))))

[FP + 4] is part of the register save area.  Insn 68, which stores a
value into it, gets deleted.

There is a reference to current_function_uses_eh_return in dse.c, but
it's not clear to me what it's trying to achieve.  Anyway, turning the
store into an unspec_volatile solves the problem.  I've committed the
following.


Bernd
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