Re: ICE after folding svld1rq to vec_perm_expr duing forwprop

[Richard Biener <richard.guenther@gmail.com>]

On Tue, Jul 12, 2022 at 9:12 PM Prathamesh Kulkarni via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> Hi Richard,
> For the following test:
>
> svint32_t f2(int a, int b, int c, int d)
> {
>   int32x4_t v = (int32x4_t) {a, b, c, d};
>   return svld1rq_s32 (svptrue_b8 (), &v[0]);
> }
>
> The compiler emits following ICE with -O3 -mcpu=generic+sve:
> foo.c: In function ‘f2’:
> foo.c:4:11: error: non-trivial conversion in ‘view_convert_expr’
>     4 | svint32_t f2(int a, int b, int c, int d)
>       |           ^~
> svint32_t
> __Int32x4_t
> _7 = VIEW_CONVERT_EXPR<__Int32x4_t>(_8);
> during GIMPLE pass: forwprop
> dump file: foo.c.109t.forwprop2
> foo.c:4:11: internal compiler error: verify_gimple failed
> 0xfda04a verify_gimple_in_cfg(function*, bool)
>         ../../gcc/gcc/tree-cfg.cc:5568
> 0xe9371f execute_function_todo
>         ../../gcc/gcc/passes.cc:2091
> 0xe93ccb execute_todo
>         ../../gcc/gcc/passes.cc:2145
>
> This happens because, after folding svld1rq_s32 to vec_perm_expr, we have:
>   int32x4_t v;
>   __Int32x4_t _1;
>   svint32_t _9;
>   vector(4) int _11;
>
>   <bb 2> :
>   _1 = {a_3(D), b_4(D), c_5(D), d_6(D)};
>   v_12 = _1;
>   _11 = v_12;
>   _9 = VEC_PERM_EXPR <_11, _11, { 0, 1, 2, 3, ... }>;
>   return _9;
>
> During forwprop, simplify_permutation simplifies vec_perm_expr to
> view_convert_expr,
> and the end result becomes:
>   svint32_t _7;
>   __Int32x4_t _8;
>
> ;;   basic block 2, loop depth 0
> ;;    pred:       ENTRY
>   _8 = {a_2(D), b_3(D), c_4(D), d_5(D)};
>   _7 = VIEW_CONVERT_EXPR<__Int32x4_t>(_8);
>   return _7;
> ;;    succ:       EXIT
>
> which causes the error duing verify_gimple since VIEW_CONVERT_EXPR
> has incompatible types (svint32_t, int32x4_t).
>
> The attached patch disables simplification of VEC_PERM_EXPR
> in simplify_permutation, if lhs and rhs have non compatible types,
> which resolves ICE, but am not sure if it's the correct approach ?

It for sure papers over the issue.  I think the error happens earlier,
the V_C_E should have been built with the type of the VEC_PERM_EXPR
which is the type of the LHS.  But then you probably run into the
different sizes ICE (VLA vs constant size).  I think for this case you
want a BIT_FIELD_REF instead of a VIEW_CONVERT_EXPR,
selecting the "low" part of the VLA vector.

>
> Alternatively, should we allow assignments from fixed-width to SVE
> vector, so the above
> VIEW_CONVERT_EXPR would result in dup ?
>
> Thanks,
> Prathamesh