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Re: Combine changes ASHIFT into mult for non-MEM rtx


On 02/04/15 20:39, Bin.Cheng wrote:
> Hi,
> In function make_compound_operation, the code/comment says:
> 
>     case ASHIFT:
>       /* Convert shifts by constants into multiplications if inside
>      an address.  */
>       if (in_code == MEM && CONST_INT_P (XEXP (x, 1))
>       && INTVAL (XEXP (x, 1)) < HOST_BITS_PER_WIDE_INT
>       && INTVAL (XEXP (x, 1)) >= 0
>       && SCALAR_INT_MODE_P (mode))
>     {
> 
> 
> Right now, it changes ASHIFT in any SET into mult because of below code:
> 
>   /* Select the code to be used in recursive calls.  Once we are inside an
>      address, we stay there.  If we have a comparison, set to COMPARE,
>      but once inside, go back to our default of SET.  */
> 
>   next_code = (code == MEM ? MEM
>            : ((code == PLUS || code == MINUS)
>           && SCALAR_INT_MODE_P (mode)) ? MEM         // <--------bogus?
>            : ((code == COMPARE || COMPARISON_P (x))
>           && XEXP (x, 1) == const0_rtx) ? COMPARE
>            : in_code == COMPARE ? SET : in_code);
> 
> This seems an overlook to me.  The effect is all targets have to
> support the generated expression in the corresponding pattern.  Some
> times the generated expression is just too stupid and missed.  For
> example below insn is tried by combine:
> (set (reg:SI 79 [ D.2709 ])
>     (plus:SI (subreg:SI (sign_extract:DI (mult:DI (reg:DI 1 x1 [ i ])
>                     (const_int 2 [0x2]))
>                 (const_int 17 [0x11])
>                 (const_int 0 [0])) 0)
>         (reg:SI 0 x0 [ a ])))
> 
> 
> It actually equals to
> (set (reg/i:SI 0 x0)
>     (plus:SI (ashift:SI (sign_extend:SI (reg:HI 1 x1 [ i ]))
>             (const_int 1 [0x1]))
>         (reg:SI 0 x0 [ a ])))
> 
> equals to below instruction on AARCH64:
> add    w0, w0, w1, sxth 1
> 
> 
> Because of the existing comment, also because it will make backend
> easier (I suppose), is it reasonable to fix this behavior in
> combine.c?  Another question is, if we are going to make the change,
> how many targets might be affected?
> 

I think https://gcc.gnu.org/ml/gcc-patches/2015-01/msg01020.html is
related to this.


Thanks,
kugan


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