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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/115426] ICE: in execute_todo, at
 passes.cc:2138
Date: Tue, 11 Jun 2024 11:07:31 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D115426

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
I think this is a gimplification failure.  'r' is neither TREE_ADDRESSABLE
nor DECL_NOT_GIMPLE_REG and the =3DX constraint results in both allow_reg
and allow_mem but we gimplify it as is_gimple_lvalue which should,
as the base is a gimple register, emit a component extract to pre_p and
a complex build to post_p.

gimplify_compound_lval correctly sees this and forces a register argument
to the __imag operation but I'm not sure that's enough for lvalues.
IIRC a simple

 __imag x =3D 1;

also doesn't have DECL_NOT_GIMPLE_REG on 'x', and gimplify_compound_lval
behaves the same.  Still we eventually gimplify to

  _1 =3D REALPART_EXPR <x>;
  x =3D COMPLEX_EXPR <_1, 1.0e+0>;
  D.2772 =3D x;

which is done via gimplify_modify_expr_complex_part.  That suggests
it's gimplify_asm_expr that would need to do this very same thing as we
seem to rely on this for correctness.

With "=3Dr" we correctly gimplify to

  __asm__("" : "=3Dr" D.2772);
  _1 =3D REALPART_EXPR <r>;
  r =3D COMPLEX_EXPR <_1, D.2772>;
  D.2773 =3D r;=