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From: "vmakarov at redhat dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/50176] [4.7 Regression] 4.7 generates spill-fill dealing with char->int conversion
Date: Tue, 13 Dec 2011 20:06:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50176

--- Comment #9 from Vladimir Makarov <vmakarov at redhat dot com> 2011-12-13 20:04:04 UTC ---
(In reply to comment #0)
> Created attachment 25088 [details]
> 
> 
>  After expanding 4.7 contains:
> 
> (insn 52 51 53 6 (set (reg:QI 83 [ D.2723 ])
>         (mem:QI (plus:SI (reg/v/f:SI 75 [ inptr1 ])
>                 (reg/v:SI 117 [ col ])) [0 MEM[base: inptr1_19, index: col_90,
> offset: 0B]+0 S1 A8])) test_4_6.c:42 -1
>      (nil))
> 
>  and 4.6 contains
> 
> (insn 52 51 53 6 (parallel [
>             (set (reg/v:SI 86 [ cb ])
>                 (zero_extend:SI (mem:QI (plus:SI (reg/v/f:SI 76 [ inptr1 ])
>                             (reg/v:SI 78 [ col ])) [0 MEM[base: inptr1_19, 
> index: col_22, offset: 0B]+0 S1 A8])))
>             (clobber (reg:CC 17 flags))
>         ]) test_4_6.c:42 -1
>      (nil))
> 
> 

The reason of different outcome in RA is that p83 generated by 4.7 we can use
only q regs vs. general regs for p86 generated by 4.6.  It decreases # of
possible hard regs for p83 in two times and failure to assign p83 a hard
register.  More accurately IRA assigns dx to p83 then reload spills p83 because
it needs a hard register then reload asks IRA to reassign a hard register to
p83 and IRA fails.