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[Bug target/29401] [4.0/4.1/4.2 Regression] missed-optimization (in unneeded code elimination)



------- Comment #4 from hubicka at ucw dot cz  2006-10-15 22:20 -------
Subject: Re:  [4.0/4.1/4.2 Regression] missed-optimization (in unneeded code
elimination)

> (insn:TI 38 37 26 2 (parallel [
>             (set (reg:SI 1 dx [+4 ])
>                 (ashiftrt:SI (reg:SI 1 dx [+4 ])
>                     (const_int 15 [0xf])))
>             (clobber (reg:CC 17 flags))
>         ]) 443 {*ashrsi3_1} (nil)
>     (expr_list:REG_UNUSED (reg:CC 17 flags)
>         (expr_list:REG_UNUSED (reg:SI 1 dx [+4 ])
>             (nil))))
> 
> note the problematic partially dead DI ax:dx which flow does not handle,
> so the redundant instruction does not get deleted.  A peephole might be
> able to fix this until new dataflow maybe handles this case(?).

It seems to me that the instruction is completely dead from post-reload
dataflow point of view (ie return value is just eax and both sets are
correctly marked as unused).  Perhaps we just somehow managed to drop
post-read DCE?

Honza


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29401


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