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Re: store_bit_field, CONCATs and subregs

> In the testcases, store_bit_field is trying to store a CHImode CONCAT to
> an unaligned address.  It first tries to mode-pun the value to SImode:
>   /* If VALUE is a floating-point mode, access it as an integer of the
>      corresponding size.  This can occur on a machine with 64 bit registers
>      that uses SFmode for float.  This can also occur for unaligned float
>      structure fields.  */
>   orig_value = value;
>   if (GET_MODE_CLASS (GET_MODE (value)) != MODE_INT
>     value = gen_lowpart ((GET_MODE (value) == VOIDmode
> 			  ? word_mode : int_mode_for_mode (GET_MODE (value))),
> 			 value);
> and the problem is with the rtx that gen_lowpart returns.
> Things first go wrong because:
>     simplify_subreg (SImode, (concat:CHImode X Y), CHImode, 0)
> returns (subreg:SI X 0), i.e. a paradoxical SImode subreg of the
> real part.  This is caused by the CONCAT case picking either the
> real or imaginary part without checking whether the requested subreg
> is entirely contained within it.

Same on the SPARC.

> These two hunks stop us generating the invalid subreg, but gen_lowpart
> has nothing to fall back on, so the call quoted above now fails.  The
> right fix for that depends on where we're headed.  Is this something
> that gen_lowpart_general should handle, or should store_bit_field
> not be asking for this lowpart in the first place?
> The third hunk adds a case to gen_lowpart_general.  It creates an integer
> pseudo of the lowpart mode, takes a paradoxical complex subreg of it,
> then moves the CONCAT into that subreg.  This works correctly thanks
> to the new emit_move_insn handling.

And this fixes all the remaining struct-layout-1 failures, both on SPARC 
32-bit and SPARC 64-bit!  Congratulations. :-)

Eric Botcazou

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