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Date: Sun, 06 Mar 2005 22:14:00 -0000
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From: "steven at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/19721] [meta-bug] optimizations that CSE still catches
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------- Additional Comments From steven at gcc dot gnu dot org  2005-03-06 22:14 -------
Just to give people an idea of how close we are to optimizing well enough that 
the calls to fold_rtx in CSE are almost all no-ops, here are some numbers 
taken over all cc1-i files on amd64: 
 
Number of times fold_rtx is called: 13882333 
Number of times it returns something other than the incoming rtx x: 70001 
Number of times fold_rtx is called by other functions than itself: 9323647 
Number of times it returns something other than x: 8526 
 
A few rtxes that fold_rtx handles: 
 
Loads from constant pool: 
Trying to fold rtx: 
(float_extend:DF (mem/u/i:SF (symbol_ref/u:DI ("*.LC0") [flags 0x2]) [2 S4 
A32])) 
  Trying to fold rtx: 
  (mem/u/i:SF (symbol_ref/u:DI ("*.LC0") [flags 0x2]) [2 S4 A32]) 
    Trying to fold rtx: 
    (symbol_ref/u:DI ("*.LC0") [flags 0x2]) 
    Returning X unchanged. 
  Returning new rtx: 
  (const_double:SF 1.0e+0 [0x0.8p+1]) 
Returning new rtx: 
(const_double:DF 1.0e+0 [0x0.8p+1]) 
 
Folded jumps: 
Trying to fold rtx: 
(if_then_else (eq (reg:CCZ 17 flags) 
        (const_int 0 [0x0])) 
    (label_ref 73) 
    (pc)) 
  Trying to fold rtx: 
  (pc) 
  Returning X unchanged. 
  Trying to fold rtx: 
  (eq (reg:CCZ 17 flags) 
    (const_int 0 [0x0])) 
    Trying to fold rtx: 
    (reg:SI 66 [ D.10402 ]) 
    Returning X unchanged. 
    Trying to fold rtx: 
    (const_int 4 [0x4]) 
    Returning X unchanged. 
  Returning new rtx: 
  (const_int 1 [0x1]) 
Returning new rtx: 
(label_ref 73) 
 
Apparently an equivalent expression with lower cost: 
Trying to fold rtx: 
(plus:QI (subreg:QI (reg:SI 251) 0) 
    (subreg:QI (reg:SI 251) 0)) 
  Trying to fold rtx: 
  (subreg:QI (reg:SI 251) 0) 
    Trying to fold rtx: 
    (reg:SI 251) 
    Returning X unchanged. 
  Returning X unchanged. 
  Trying to fold rtx: 
  (subreg:QI (reg:SI 251) 0) 
    Trying to fold rtx: 
    (reg:SI 251) 
    Returning X unchanged. 
  Returning X unchanged. 
Returning new rtx: 
(ashift:QI (subreg:QI (reg:SI 251) 0) 
    (const_int 1 [0x1])) 
 
Likewise: 
Trying to fold rtx: 
(mult:DI (reg:DI 63 [ <variable>.comb_vect.length ]) 
    (const_int 4 [0x4])) 
Returning new rtx: 
(ashift:DI (reg:DI 63 [ <variable>.comb_vect.length ]) 
    (const_int 2 [0x2])) 
 
It'd be interesting to find out how many of these things combine and later CSE 
passes would catch (or miss), and the tree-cleanup-branch compares.  I will 
look at the latter first. 
 

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19721