[Bug c++/97642] Incorrect replacement of vmovdqu32 with vpblendd can cause fault

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97642

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
                 CC|                            |crazylht at gmail dot com,
                   |                            |hjl.tools at gmail dot com,
                   |                            |jakub at gcc dot gnu.org
   Last reconfirmed|                            |2020-10-30
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
The problem is that in the RTL representation there is nothing that would tell
cse, forward propagation or combiner etc. not to optimize the
(insn 7 6 8 2 (set (reg:QI 89)
        (const_int 31 [0x1f])) "include/avx512vlintrin.h":865:20 77
{*movqi_internal}
     (nil))
(insn 8 7 9 2 (set (reg:V8SI 87)
        (vec_merge:V8SI (mem:V8SI (reg/v/f:DI 86 [ arr ]) [0  S32 A8])
            (reg:V8SI 88)
            (reg:QI 89))) "include/avx512vlintrin.h":865:20 1423
{avx512vl_loadv8si_mask}
     (nil))
into:
(insn 8 7 9 2 (set (reg:V8SI 87)
        (vec_merge:V8SI (mem:V8SI (reg/v/f:DI 86 [ arr ]) [0  S32 A8])
            (reg:V8SI 88 [ tmp ])
            (const_int 31 [0x1f]))) "include/avx512vlintrin.h":865:20 4402
{avx2_pblenddv8si}
     (expr_list:REG_DEAD (reg:QI 89)
        (expr_list:REG_DEAD (reg:V8SI 88 [ tmp ])
            (expr_list:REG_DEAD (reg/v/f:DI 86 [ arr ])
                (nil)))))
Guess we'd need to use some UNSPEC for the masked loads and have patterns for
combine to optimize those that have -1 masks into normal loads, or disable the
blend patterns with MEM operands for avx512f+ (i.e. force those into
registers).
Because the RTL representation really matches more the blend behavior than the
avx512 masking, where exceptions from the masked off elts just don't show up.