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Re: Fix PR rtl-optimization/33822

> I think rtl.texi already describes the SUBREG_BYTE calculation in enough
> detail, but I noticed that rtl.def was years out of date.  I think that
> shows we should have detailed documentation in one place only (rtl.texi),
> so I simplified the rtl.def version.

Interesting left-overs of an old era. :-)

> I've made the comments for update_reg_offset, gen_rtx_REG_offset
> and gen_reg_rtx_offset say "with OFFSET added to the REG_OFFSET"
> rather than "offsetted by OFFSET".  I think this, in combination
> with the reg_attrs comment, makes the type of offset more explicit.


> 	* rtl.def (SUBREG): Update comments.
> 	* rtl.h (reg_attrs): Be explicit about the type of offset used.
> 	(set_reg_attrs_from_mem): Rename to...
> 	(set_reg_attrs_from_value): ...this.
> 	(adjust_reg_mode, byte_lowpart_offset): Declare.
> 	* emit-rtl.c (byte_lowpart_offset): New function.
> 	(update_reg_offset): Remove special offset handling for big-endian
> 	targets.
> 	(gen_rtx_REG_offset, gen_reg_rtx_offset): Explicitly say that the
> 	offset parameter is added to REG_OFFSET.
> 	(adjust_reg_mode): New function.
> 	(set_reg_attrs_for_mem): Rename to...
> 	(set_reg_attrs_for_value): ...this and generalize to all values.
> 	If the register is a lowpart of the value, adjust the offset
> 	accordingly.
> 	(set_reg_attrs_for_parm): Update after the above renaming.
> 	(set_reg_attrs_for_decl_rtl): New function, split out from
> 	set_decl_incoming_rtl.  Set the offset of plain REGs to the
> 	offset of the REG's mode from the decl's.  Assert that all
> 	subregs are lowparts and handle their inner registers in the
> 	same way as plain REGs.
> 	(set_decl_rtl, set_incoming_decl_rtl): Use reg_attrs_for_decl_rtl.
> 	(subreg_lowpart_offset): Explicitly say that the returned offset
> 	is a SUBREG_BYTE.
> 	* combine.c (do_SUBST_MODE, try_combine, undo_all): Use adjust_reg_mode
> 	instead of PUT_MODE.
> 	* final.c (alter_subreg): Fix/update argument to gen_rtx_REG_offset.
> 	* config/ia64/ia64.c (ia64_expand_load_address): Likewise.
> 	* regclass.c (reg_scan_mark_refs): Use set_reg_attrs_from_value.
> 	* reload.c (find_reloads_subreg_address): Call set_mem_offset
> 	when offseting a MEM.
> 	* var-tracking.c (offset_valid_for_tracked_p): Delete.
> 	(mode_for_reg_attrs): Replace with...
> 	(track_loc_p): ...this new function.  Return the mode and offset
> 	to the caller, checking that the latter is valid.  If the rtx is
> 	a paradoxical lowpart of the decl, use the decl's mode instead.
> 	Do the same when storing to a register that contains the entire decl.
> 	(var_lowpart): Use byte_lowpart_offset rather than
> 	subreg_lowpart_offset when adjusting the offset attribute.
> 	(count_uses, add_uses, add_stores): Use track_reg_p instead of
> 	offset_valid_for_tracked_p and mode_for_reg_attrs.  Generate
> 	lowparts for MEMs as well as REGs.
> 	(vt_add_function_parameters): When obtaining the information from
> 	the decl_rtl, adjust the offset to match incoming.  Use track_loc_p
> 	and var_lowpart.

OK, thanks.

Btw, could we do something clever for the many similar calculations in 
alter_subreg and simplify_subreg (and maybe elsewhere):

      /* The SUBREG_BYTE represents offset, as if the value were stored
	 in memory.  Irritating exception is paradoxical subreg, where
	 we define SUBREG_BYTE to be 0.  On big endian machines, this
	 value should be negative.  For a moment, undo this exception.  */
      if (byte == 0 && GET_MODE_SIZE (innermode) < GET_MODE_SIZE (outermode))
	  int difference = (GET_MODE_SIZE (innermode) - GET_MODE_SIZE (outermode));
	    final_offset += (difference / UNITS_PER_WORD) * UNITS_PER_WORD;
	    final_offset += difference % UNITS_PER_WORD;

Eric Botcazou

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