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
