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);
