[Bug target/63949] Aarch64 instruction combiner does not optimize subsi_sxth function as expected (gcc.target/aarch64/extend.c fails)

["vekumar at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63949

--- Comment #6 from vekumar at gcc dot gnu.org ---
In the function make_compound_operation, there a check 

      /* See if we have operations between an ASHIFTRT and an ASHIFT.
         If so, try to merge the shifts into a SIGN_EXTEND.  We could
         also do this for some cases of SIGN_EXTRACT, but it doesn't
         seem worth the effort; the case checked for occurs on Alpha.          
*/

if (!OBJECT_P (lhs)
          && ! (GET_CODE (lhs) == SUBREG
                && (OBJECT_P (SUBREG_REG (lhs))))
          && CONST_INT_P (rhs)
          && INTVAL (rhs) < HOST_BITS_PER_WIDE_INT
          && INTVAL (rhs) < mode_width
          && (new_rtx = extract_left_shift (lhs, INTVAL (rhs))) != 0)
        new_rtx = make_extraction (mode, make_compound_operation (new_rtx,
next_code),
                               0, NULL_RTX, mode_width - INTVAL (rhs),
                               code == LSHIFTRT, 0, in_code == COMPARE);

      break;



Our input RTL actually matches this case. 

For (int)i << 1  we are getting incomming RTX as 

(ashiftrt:SI (ashift:SI (reg:SI 1 x1 [ i ])
        (const_int 16 [0x10]))
    (const_int 15 [0xf]))


LHS is ashift:SI (reg:SI 1 x1 [ i ])
        (const_int 16 [0x10]) 

RHS is ashiftrt with a value of 15.

So bacially we get (i<<16)>>15, we can merge these shifts to sign_extends.

With extract_left_shift we get 

(ashift:SI (reg:SI 1 x1 [ i ])
    (const_int 1 [0x1]))

or x1<<1

When we do make_extraction with this shift pattern we get 

    (ashift:SI (sign_extend:SI (reg:HI 1 x1 [ i ]))
        (const_int 1 [0x1])))


But instead this we are the shift RTX, we are actually passing MULT RTX to
make_extraction via another make_compound_operation.

p make_compound_operation(new_rtx,MEM)
$3 = (rtx_def *) 0x7ffff77fd420
(gdb) pr
(mult:SI (reg:SI 1 x1 [ i ])
    (const_int 2 [0x2]))

Which results in 

 (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)

When I changed the original check to

--- a/gcc/combine.c
+++ b/gcc/combine.c
@@ -7896,7 +7896,7 @@ make_compound_operation (rtx x, enum rtx_code in_code)
          && INTVAL (rhs) < HOST_BITS_PER_WIDE_INT
          && INTVAL (rhs) < mode_width
          && (new_rtx = extract_left_shift (lhs, INTVAL (rhs))) != 0)
-       new_rtx = make_extraction (mode, make_compound_operation (new_rtx,
next_
+       new_rtx = make_extraction (mode, new_rtx,
                               0, NULL_RTX, mode_width - INTVAL (rhs),
                               code == LSHIFTRT, 0, in_code == COMPARE)

Combiner was able to match the pattern successfully.

Trying 8 -> 13:
Successfully matched this instruction:
(set (reg/i:SI 0 x0)
    (minus:SI (reg:SI 0 x0 [ a ])
        (ashift:SI (sign_extend:SI (reg:HI 1 x1 [ i ]))
            (const_int 1 [0x1]))))
(minus:SI (reg:SI 0 x0 [ a ])
    (ashift:SI (sign_extend:SI (reg:HI 1 x1 [ i ]))

Any comments about this change?
        (const_int 1 [0x1])))