[Bug rtl-optimization/110206] [14 Regression] wrong code with -Os -march=cascadelake since r14-1246

[ubizjak at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110206

--- Comment #9 from Uroš Bizjak <ubizjak at gmail dot com> ---
Some more digging through the code:

In cprop.cc/try_replace_reg, we try to simplify the source of the set given our
substitution:

Breakpoint 1, try_replace_reg (from=0x7fffe9f0b7f8, to=0x7fffe9f099e0,
insn=0x7fffea01b6c0) at ../../git/gcc/gcc/cprop.cc:789
789           src = simplify_replace_rtx (SET_SRC (set), from, to);

(gdb) list
784       if (!success && set && reg_mentioned_p (from, SET_SRC (set)))
785         {
786           /* If above failed and this is a single set, try to simplify the
source
787              of the set given our substitution.  We could perhaps try this
for
788              multiple SETs, but it probably won't buy us anything.  */
789           src = simplify_replace_rtx (SET_SRC (set), from, to);

(gdb) p debug_rtx (set)
(set (reg:V8HI 100)
    (zero_extend:V8HI (vec_select:V8QI (subreg:V16QI (reg:V4QI 98) 0)
            (parallel [
                    (const_int 0 [0])
                    (const_int 1 [0x1])
                    (const_int 2 [0x2])
                    (const_int 3 [0x3])
                    (const_int 4 [0x4])
                    (const_int 5 [0x5])
                    (const_int 6 [0x6])
                    (const_int 7 [0x7])
                ]))))

(gdb) p debug_rtx (from)
(reg:V4QI 98)

(gdb) p debug_rtx (to)
(const_vector:V4QI [
        (const_int -52 [0xffffffffffffffcc]) repeated x4
    ])

and simplify_replace_rtx simplifies the above to:

(gdb) p debug_rtx (src)
(const_vector:V8HI [
        (const_int 204 [0xcc]) repeated x8
    ])

which is obviously wrong, we have V4QImode input register holding V4QImode
constant.

Tracing through simplify-rtx.cc brings us to a recursive
simplify_replace_fn_rtx, which gets us to:

Breakpoint 1, simplify_replace_fn_rtx (x=0x7fffe9f0b888,
old_rtx=0x7fffe9f0b7f8, fn=0x0, data=0x7fffe9f099e0) at
../../git/gcc/gcc/simplify-rtx.cc:474
474               op0 = simplify_gen_subreg (GET_MODE (x), op0,

(gdb) list
469           if (code == SUBREG)
470             {
471               op0 = simplify_replace_fn_rtx (SUBREG_REG (x), old_rtx, fn,
data);
472               if (op0 == SUBREG_REG (x))
473                 return x;
474               op0 = simplify_gen_subreg (GET_MODE (x), op0,
475                                          GET_MODE (SUBREG_REG (x)),
476                                          SUBREG_BYTE (x));
477               return op0 ? op0 : x;
478             }

(gdb) p debug_rtx (op0)
(const_vector:V4QI [
        (const_int -52 [0xffffffffffffffcc]) repeated x4
    ])
(gdb) p debug_rtx (x)
(subreg:V16QI (reg:V4QI 98) 0)

and simplify_gen_subreg with the above arguments returns:

(gdb) p debug_rtx (op0)
(const_vector:V16QI [
        (const_int -52 [0xffffffffffffffcc]) repeated x16
    ])

No way! It is not possible to get V16QImode vector from V4QImode vector, even
when all elements are duplicates.

Tracing even deeper to simplify_context::simplify_subreg, we found the
following:

Breakpoint 1, simplify_context::simplify_subreg (this=0x7fffffffd528,
outermode=E_V16QImode, op=0x7fffe9f099e0, innermode=E_V4QImode, byte=...)
    at ../../git/gcc/gcc/simplify-rtx.cc:7561
7561            return gen_vec_duplicate (outermode, elt);

(gdb) list
7556          rtx elt;
7557
7558          if (VECTOR_MODE_P (outermode)
7559              && GET_MODE_INNER (outermode) == GET_MODE_INNER (innermode)
7560              && vec_duplicate_p (op, &elt))
7561            return gen_vec_duplicate (outermode, elt);
7562
7563          if (outermode == GET_MODE_INNER (innermode)
7564              && vec_duplicate_p (op, &elt))
7565            return elt;

(gdb) p outermode
$1 = E_V16QImode
(gdb) p debug_rtx (elt)
(const_int -52 [0xffffffffffffffcc])

(gdb) fin
Run till exit from #0  simplify_context::simplify_subreg (this=0x7fffffffd528,
outermode=E_V16QImode, op=0x7fffe9f099e0, innermode=E_V4QImode, byte=...)
    at ../../git/gcc/gcc/simplify-rtx.cc:7561
0x0000000000eb24d3 in simplify_subreg (byte=..., innermode=E_V4QImode,
op=<optimized out>, outermode=<optimized out>) at ../../git/gcc/gcc/rtl.h:3513
3513      return simplify_context ().simplify_subreg (outermode, op, innermode,
byte);
Value returned is $4 = (rtx_def *) 0x7fffe9f09c10

(gdb) p debug_rtx ($4)
(const_vector:V16QI [
        (const_int -52 [0xffffffffffffffcc]) repeated x16
    ])

Nope. This transformation is valid only for non-paradoxical registers.

Patch is then obvious:

diff --git a/gcc/simplify-rtx.cc b/gcc/simplify-rtx.cc
index d7315d82aa3..87ca25086dc 100644
--- a/gcc/simplify-rtx.cc
+++ b/gcc/simplify-rtx.cc
@@ -7557,6 +7557,7 @@ simplify_context::simplify_subreg (machine_mode
outermode, rtx op,

       if (VECTOR_MODE_P (outermode)
          && GET_MODE_INNER (outermode) == GET_MODE_INNER (innermode)
+         && !paradoxical_subreg_p (outermode, innermode)
          && vec_duplicate_p (op, &elt))
        return gen_vec_duplicate (outermode, elt);