[gcc r14-4883] i386: Avoid paradoxical subreg dests in vector zero_extend

[gcc r14-4883] i386: Avoid paradoxical subreg dests in vector zero_extend

[Richard Sandiford]

https://gcc.gnu.org/g:58de8e933d19516d1a9de66a617378493ffa29a5

commit r14-4883-g58de8e933d19516d1a9de66a617378493ffa29a5
Author: Richard Sandiford <richard.sandiford@arm.com>
Date:   Tue Oct 24 11:26:45 2023 +0100

    i386: Avoid paradoxical subreg dests in vector zero_extend
    
    For the V2HI -> V2SI zero extension in:
    
      typedef unsigned short v2hi __attribute__((vector_size(4)));
      typedef unsigned int v2si __attribute__((vector_size(8)));
      v2si f (v2hi x) { return (v2si) {x[0], x[1]}; }
    
    ix86_expand_sse_extend would generate:
    
       (set (reg:V2HI 102)
            (const_vector:V2HI [(const_int 0 [0])
                                (const_int 0 [0])]))
       (set (subreg:V8HI (reg:V2HI 101) 0)
            (vec_select:V8HI
              (vec_concat:V16HI (subreg:V8HI (reg/v:V2HI 99 [ x ]) 0)
                                (subreg:V8HI (reg:V2HI 102) 0))
              (parallel [(const_int 0 [0])
                         (const_int 8 [0x8])
                         (const_int 1 [0x1])
                         (const_int 9 [0x9])
                         (const_int 2 [0x2])
                         (const_int 10 [0xa])
                         (const_int 3 [0x3])
                         (const_int 11 [0xb])])))
      (set (reg:V2SI 100)
           (subreg:V2SI (reg:V2HI 101) 0))
        (expr_list:REG_EQUAL (zero_extend:V2SI (reg/v:V2HI 99 [ x ])))
    
    But using (subreg:V8HI (reg:V2HI 101) 0) as the destination of
    the vec_select means that only the low 4 bytes of the destination
    are stored.  Only the lower half of reg 100 is well-defined.
    
    Things tend to happen to work if the register allocator ties reg 101
    to reg 100.  But it caused problems with the upcoming late-combine pass
    because we propagated the set of reg 100 into its uses.
    
    gcc/
            * config/i386/i386-expand.cc (ix86_split_mmx_punpck): Allow the
            destination to be wider than the sources.  Take the mode from the
            first source.
            (ix86_expand_sse_extend): Pass the destination directly to
            ix86_split_mmx_punpck, rather than using a fresh register that
            is half the size.

Diff:
---
 gcc/config/i386/i386-expand.cc | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/gcc/config/i386/i386-expand.cc b/gcc/config/i386/i386-expand.cc
index 1eae9d7c78c2..2361ff77af3f 100644
--- a/gcc/config/i386/i386-expand.cc
+++ b/gcc/config/i386/i386-expand.cc
@@ -1110,7 +1110,9 @@ ix86_split_mmx_pack (rtx operands[], enum rtx_code code)
   ix86_move_vector_high_sse_to_mmx (op0);
 }
 
-/* Split MMX punpcklXX/punpckhXX with SSE punpcklXX.  */
+/* Split MMX punpcklXX/punpckhXX with SSE punpcklXX.  This is also used
+   for a full unpack of OPERANDS[1] and OPERANDS[2] into a wider
+   OPERANDS[0].  */
 
 void
 ix86_split_mmx_punpck (rtx operands[], bool high_p)
@@ -1118,7 +1120,7 @@ ix86_split_mmx_punpck (rtx operands[], bool high_p)
   rtx op0 = operands[0];
   rtx op1 = operands[1];
   rtx op2 = operands[2];
-  machine_mode mode = GET_MODE (op0);
+  machine_mode mode = GET_MODE (op1);
   rtx mask;
   /* The corresponding SSE mode.  */
   machine_mode sse_mode, double_sse_mode;
@@ -5660,7 +5662,7 @@ ix86_expand_sse_extend (rtx dest, rtx src, bool unsigned_p)
       gcc_unreachable ();
     }
 
-  ops[0] = gen_reg_rtx (imode);
+  ops[0] = dest;
 
   ops[1] = force_reg (imode, src);
 
@@ -5671,7 +5673,6 @@ ix86_expand_sse_extend (rtx dest, rtx src, bool unsigned_p)
 				  ops[1], pc_rtx, pc_rtx);
 
   ix86_split_mmx_punpck (ops, false);
-  emit_move_insn (dest, lowpart_subreg (GET_MODE (dest), ops[0], imode));
 }
 
 /* Unpack SRC into the next wider integer vector type.  UNSIGNED_P is