gcc4.6.0:combining operate+test

[cirrus75 <cirrus75 at uol dot com dot br>]

 Hi All,

 I have been looking at a case in x86 architecture where gcc could generate better code for:

if(a+=25)
 d=c;

 
 Insns for operation and test are:


(insn 5 2 6 2 (set (reg:SI 62 [ a ])
        (mem/c/i:SI (symbol_ref:DI ("a")  <var_decl 0x2aaaae259500 a>) [2 a+0 S4 A32])) test_and.c:9 64 {*movsi_internal}
     (nil))

(insn 6 5 7 2 (parallel [
            (set (reg:SI 60 [ a.1 ])
                (plus:SI (reg:SI 62 [ a ])
                    (const_int 25 [0x19])))
            (clobber (reg:CC 17 flags))
        ]) test_and.c:9 252 {*addsi_1}
     (expr_list:REG_DEAD (reg:SI 62 [ a ])
        (expr_list:REG_UNUSED (reg:CC 17 flags)
            (expr_list:REG_EQUAL (plus:SI (mem/c/i:SI (symbol_ref:DI ("a")  <var_decl 0x2aaaae259500 a>) [2 a+0 S4 A32])
                    (const_int 25 [0x19]))
                (nil)))))

(insn 7 6 8 2 (set (mem/c/i:SI (symbol_ref:DI ("a")  <var_decl 0x2aaaae259500 a>) [2 a+0 S4 A32])
        (reg:SI 60 [ a.1 ])) test_and.c:9 64 {*movsi_internal}
     (nil))

(insn 8 7 9 2 (set (reg:CCZ 17 flags)
        (compare:CCZ (reg:SI 60 [ a.1 ])
            (const_int 0 [0]))) test_and.c:9 2 {*cmpsi_ccno_1}
     (nil))


  I noticed combine.c is not able to combine insns 6 and 8. This is because create_log_links function only creates (as far as I could understand) links between the reg setter and the first reg user, but not the other reg users. Thus, combine.c do try to combine 6 and 7, but without success.

  Why does not create_log_links create links between the reg setter and all the reg users ?

  I compiled it on powerpc and got the same results (3 instructions: operate, store, test), so this behavior affects not only x86 architectures. It seems something good to optimize.
  
best regards,
Alex Rocha Prado