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From: "steven at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/23451] [3.4/4.0/4.1/4.2 regression] Redundant reloading from stack frame
Date: Tue, 10 Jan 2006 20:57:00 -0000	[thread overview]
Message-ID: <20060110205739.18426.qmail@sourceware.org> (raw)
In-Reply-To: <bug-23451-5724@http.gcc.gnu.org/bugzilla/>



------- Comment #4 from steven at gcc dot gnu dot org  2006-01-10 20:57 -------
On the trunk, we have the following situation in the .csa RTL dump (on AMD64
-m32 -march=i686):

;; Start of basic block 5, registers live:
 4 [si] 5 [di] 6 [bp] 7 [sp] 20 [frame]
(code_label:HI 38 37 39 5 2 "" [1 uses])

(note:HI 39 38 41 5 [bb 5] NOTE_INSN_BASIC_BLOCK)

(insn:HI 41 39 42 5 (set (reg:CCZ 17 flags)
        (compare:CCZ (mem/f/c:SI (plus:SI (reg/f:SI 6 bp)
                    (const_int -20 [0xffffffffffffffec])) [7 val+0 S4 A8])
            (const_int 0 [0x0]))) 3 {*cmpsi_ccno_1} (nil)
    (nil))

(jump_insn:HI 42 41 44 5 (set (pc)
        (if_then_else (ne (reg:CCZ 17 flags)
                (const_int 0 [0x0]))
            (label_ref 63)
            (pc))) 511 {*jcc_1} (insn_list:REG_DEP_TRUE 41 (nil))
    (expr_list:REG_DEAD (reg:CCZ 17 flags)
        (expr_list:REG_BR_PROB (const_int 8100 [0x1fa4])
            (nil))))
;; End of basic block 5, registers live:
 4 [si] 5 [di] 6 [bp] 7 [sp] 20 [frame]

(...)

;; Start of basic block 7, registers live: 4 [si] 5 [di] 6 [bp] 7 [sp] 20
[frame]
(code_label:HI 63 119 64 7 5 "" [1 uses])

(note:HI 64 63 125 7 [bb 7] NOTE_INSN_BASIC_BLOCK)

(insn 125 64 66 7 (set (reg:SI 0 ax)
        (mem/f/c:SI (plus:SI (reg/f:SI 6 bp)
                (const_int -20 [0xffffffffffffffec])) [7 val+0 S4 A8])) 40
{*movsi_1} (nil)
    (nil))

(insn:HI 66 125 67 7 (set (mem:SI (reg/f:SI 7 sp) [0 S4 A32])
        (reg:SI 0 ax)) 40 {*movsi_1} (nil)
    (expr_list:REG_DEAD (reg:SI 0 ax)
        (nil)))

(call_insn/u:HI 67 66 68 7 (set (reg:SI 0 ax)
        (call (mem:QI (symbol_ref:SI ("strlen") [flags 0x41]
                       <function_decl 0x2aaaaae6fb00 strlen>) [0 S1 A8])
            (const_int 4 [0x4]))) 731 {*call_value_0} (nil)
    (expr_list:REG_EH_REGION (const_int 0 [0x0])
        (nil))
    (expr_list:REG_DEP_TRUE (use (mem:BLK (scratch) [0 A8]))
        (nil)))
(...)


Then we peephole2 the MEM for the compare to a set, see the .peephole2 dump:

;; Start of basic block 5, registers live:
  4 [si] 5 [di] 6 [bp] 7 [sp] 20 [frame]
(code_label:HI 38 37 39 5 2 "" [1 uses])

(note:HI 39 38 142 5 [bb 5] NOTE_INSN_BASIC_BLOCK)

(insn 142 39 143 5 (set (reg:SI 0 ax)
        (mem/f/c:SI (plus:SI (reg/f:SI 6 bp)
                (const_int -20 [0xffffffffffffffec])) [7 val+0 S4 A8])) -1
(nil)
    (nil))

(insn 143 142 42 5 (set (reg:CCZ 17 flags)
        (compare:CCZ (reg:SI 0 ax)
            (const_int 0 [0x0]))) -1 (nil)
    (expr_list:REG_DEAD (reg:SI 0 ax)
        (nil)))

(jump_insn:HI 42 143 44 5 (set (pc)
        (if_then_else (ne (reg:CCZ 17 flags)
                (const_int 0 [0x0]))
            (label_ref 63)
            (pc))) 511 {*jcc_1} (insn_list:REG_DEP_TRUE 41 (nil))
    (expr_list:REG_DEAD (reg:CCZ 17 flags)
        (expr_list:REG_BR_PROB (const_int 8100 [0x1fa4])
            (nil))))
;; End of basic block 5, registers live:
 4 [si] 5 [di] 6 [bp] 7 [sp] 20 [frame]

(...)

;; Start of basic block 7, registers live: 4 [si] 5 [di] 6 [bp] 7 [sp] 20
[frame]
(code_label:HI 63 119 64 7 5 "" [1 uses])

(note:HI 64 63 125 7 [bb 7] NOTE_INSN_BASIC_BLOCK)

(insn 125 64 66 7 (set (reg:SI 0 ax)
        (mem/f/c:SI (plus:SI (reg/f:SI 6 bp)
                (const_int -20 [0xffffffffffffffec])) [7 val+0 S4 A8])) 40
{*movsi_1} (nil)
    (nil))

(insn:HI 66 125 67 7 (set (mem:SI (reg/f:SI 7 sp) [0 S4 A32])
        (reg:SI 0 ax)) 40 {*movsi_1} (nil)
    (expr_list:REG_DEAD (reg:SI 0 ax)
        (nil)))

(call_insn/u:HI 67 66 68 7 (set (reg:SI 0 ax)
        (call (mem:QI (symbol_ref:SI ("strlen") [flags 0x41]
                       <function_decl 0x2aaaaae6fb00 strlen>) [0 S1 A8])
            (const_int 4 [0x4]))) 731 {*call_value_0} (nil)
    (expr_list:REG_EH_REGION (const_int 0 [0x0])
        (nil))
    (expr_list:REG_DEP_TRUE (use (mem:BLK (scratch) [0 A8]))
        (nil)))


Essentially the same thing happens in GCC 3.3: Up to peephole2, the first MEM
is burried in the compare, and split out by some peephole. 

I don't think this is related to register allocation, really.  Someone should
try to see what the flow2 dump looks like for GCC 3.2, which is apparently the
last release that did not have this problem.  In any case, it will be difficult
to find out when this problem was introduced.  But it doesn't look like a
really severe problem to me anyway, really...


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23451



  parent reply	other threads:[~2006-01-10 20:57 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <bug-23451-5724@http.gcc.gnu.org/bugzilla/>
2005-10-31  5:05 ` [Bug target/23451] [3.4/4.0/4.1 " mmitchel at gcc dot gnu dot org
2006-01-10 20:36 ` [Bug target/23451] [3.4/4.0/4.1/4.2 " steven at gcc dot gnu dot org
2006-01-10 20:57 ` steven at gcc dot gnu dot org [this message]
2006-01-10 21:27 ` steven at gcc dot gnu dot org
2006-01-10 21:33 ` steven at gcc dot gnu dot org
2006-01-10 22:44 ` herbert at gondor dot apana dot org dot au
2006-01-10 22:58 ` steven at gcc dot gnu dot org
2006-01-10 23:00 ` steven at gcc dot gnu dot org
2006-01-10 23:00 ` steven at gcc dot gnu dot org
2006-01-10 23:01 ` steven at gcc dot gnu dot org
2006-01-10 23:02 ` steven at gcc dot gnu dot org
2006-01-10 23:04 ` steven at gcc dot gnu dot org
2006-01-10 23:25 ` herbert at gondor dot apana dot org dot au
2006-01-10 23:36 ` steven at gcc dot gnu dot org
2006-01-10 23:41 ` herbert at gondor dot apana dot org dot au
2006-01-10 23:44 ` [Bug target/23451] Redundant reloading from stack frame on i386 steven at gcc dot gnu dot org
2006-01-10 23:45 ` herbert at gondor dot apana dot org dot au
2006-01-29 18:09 ` pinskia at gcc dot gnu dot org
2009-04-22 23:06 ` pinskia at gcc dot gnu dot org

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