Performance regression

[David Edelsohn]

>>>>> Dale Johannesen writes:

Dale> The obvious difference is the lack
Dale> of the REG_EQUAL note.  Is this right?

	I have concurrently tracked the bug back to the same REG_EQUAL
note, although I do not know whether the NOTE is wrong or GCC is handling
the NOTE incorrectly or missing another NOTE.

	In the buggy case, cse_insn analyzes the next unrolled load:

(insn 195 194 196 0 30038e00 (set (reg:SI 156)
        (zero_extend:SI (reg/v:HI 171))) -1 (nil)
    (expr_list:REG_EQUAL (zero_extend:SI (mem:HI (plus:SI (reg:SI 133)
                    (symbol_ref:SI ("bndpsd"))) [4 S2 A16]))
        (nil)))

and examines the REG_EQUAL note to find a SRC_EQV.  It finds the first
load, so it substitutes (reg:SI 156):

(insn 195 194 196 0 30038e00 (set (reg:SI 156)
        (reg:SI 156)) 291 {*movsi_internal1} (nil)
    (expr_list:REG_EQUAL (zero_extend:SI (mem:HI (plus:SI (reg:SI 133)
                    (symbol_ref:SI ("bndpsd"))) [4 S2 A16]))
        (nil)))

ignoring the intervening SET of (reg:HI 171).

	The three possibilities are:

1) Load of (reg:SI 156) from (reg:HI 171) should not have a REG_EQUAL
note.

2) Store to (reg:HI 171) also *should* have a REG_EQUAL note.

3) cse_insn should not validate the change due to the intervening store to
(reg:HI 171).

	The semantics of REG_EQUAL are that the register value is
equivalent to the NOTE value at runtime at that instant, but not
necessarily elsewhere in the function, so I think the NOTE is okay, but
the substitution is not checking validity correctly.  I do not know if
adding a REG_EQUAL to the store should be necessary.

David