On Sun, Apr 15, 2012 at 8:11 AM, Richard Sandiford
<rdsandif...@googlemail.com> wrote:
> The comment in alias.c says:
>
> The contents of an ADDRESS is not normally used, the mode of the
> ADDRESS determines whether the ADDRESS is a function argument or some
> other special value. Pointer equality, not rtx_equal_p, determines whether
> two ADDRESS expressions refer to the same base address.
>
> The only use of the contents of an ADDRESS is for determining if the
> current function performs nonlocal memory memory references for the
> purposes of marking the function as a constant function. */
>
> The first paragraph is a bit misleading IMO. AFAICT, rtx_equal_p has
> always given ADDRESS the full recursive treatment, rather than saying
> that pointer equality determines ADDRESS equality. (This is in contrast
> to something like VALUE, where pointer equality is used.) And AFAICT
> we've always had:
>
> static int
> base_alias_check (rtx x, rtx y, enum machine_mode x_mode,
> enum machine_mode y_mode)
> {
> ...
> /* If the base addresses are equal nothing is known about aliasing. */
> if (rtx_equal_p (x_base, y_base))
> return 1;
> ...
> }
>
> So I think the contents of an ADDRESS _are_ used to distinguish
> between different bases.
>
> The second paragraph ceased to be true in 2005 when the pure/const
> analysis moved to its own IPA pass. Nothing now looks at the contents
> beyond rtx_equal_p.
>
> Also, base_alias_check effectively treats all arguments as a single base.
> That makes conceptual sense, because this analysis isn't strong enough
> to determine whether arguments are base values at all, never mind whether
> accesses based on different arguments conflict. But the fact that we have
> a single base isn't obvious from the way the code is written, because we
> create several separate, non-rtx_equal_p, ADDRESSes to represent arguments.
> See:
>
> for (i = 0; i < FIRST_PSEUDO_REGISTER; i++)
> /* Check whether this register can hold an incoming pointer
> argument. FUNCTION_ARG_REGNO_P tests outgoing register
> numbers, so translate if necessary due to register windows. */
> if (FUNCTION_ARG_REGNO_P (OUTGOING_REGNO (i))
> && HARD_REGNO_MODE_OK (i, Pmode))
> static_reg_base_value[i]
> = gen_rtx_ADDRESS (VOIDmode, gen_rtx_REG (Pmode, i));
>
> and:
>
> /* Check for an argument passed in memory. Only record in the
> copying-arguments block; it is too hard to track changes
> otherwise. */
> if (copying_arguments
> && (XEXP (src, 0) == arg_pointer_rtx
> || (GET_CODE (XEXP (src, 0)) == PLUS
> && XEXP (XEXP (src, 0), 0) == arg_pointer_rtx)))
> return gen_rtx_ADDRESS (VOIDmode, src);
>
> I think it would be cleaner and less wasteful to use a single rtx for
> the single "base" (really "potential base").
>
> So if we wanted to, we could now remove the operand from ADDRESS and
> simply rely on pointer equality. I'm a bit reluctant to do that though.
> It would make debugging harder, and it would mean either adding knowledge
> of this alias-specific code to other files (specifically rtl.c:rtx_equal_p),
> or adding special ADDRESS shortcuts to alias.c. But I think the code
> would be more obvious if we replaced the rtx operand with a unique id,
> which is what we already use for the REG_NOALIAS case:
>
> new_reg_base_value[regno] = gen_rtx_ADDRESS (Pmode,
> GEN_INT (unique_id++));
>
> And if we do that, we can make the id a direct operand of the ADDRESS,
> rather than a CONST_INT subrtx[*]. That should make rtx_equal_p cheaper too.
>
> [*] I'm trying to get rid of CONST_INTs like these that have
> no obvious mode.
>
> All of which led to the patch below. I checked that it didn't change
> the code generated at -O2 for a recent set of cc1 .ii files. Also
> bootstrapped & regression-tested on x86_64-linux-gnu. OK to install?
>
> To cover my back: I'm just trying to rewrite the current code according
> to its current assumptions. Whether those assumptions are correct or not
> is always open to debate...
>
> Richard
>
>
> gcc/
> * rtl.def (ADDRESS): Turn operand into a HOST_WIDE_INT.
> * alias.c (reg_base_value): Expand and update comment.
> (arg_base_value): New variable.
> (unique_id): Move up file.
> (unique_base_value, unique_base_value_p, known_base_value_p): New.
> (find_base_value): Use arg_base_value and known_base_value_p.
> (record_set): Document REG_NOALIAS handling. Use unique_base_value.
> (find_base_term): Use known_base_value_p.
> (base_alias_check): Use unique_base_value_p.
> (init_alias_target): Initialize arg_base_value. Use unique_base_value.
> (init_alias_analysis): Use 1 as the first id for REG_NOALIAS bases.
>
This breaks bootstrap on Linux/x86:
home/regress/tbox/native/build/./gcc/xgcc
-B/home/regress/tbox/native/build/./gcc/
-B/home/regress/tbox/objs/i686-pc-linux-gnu/bin/
-B/home/regress/tbox/objs/i686-pc-linux-gnu/lib/ -isystem
/home/regress/tbox/objs/i686-pc-linux-gnu/include -isystem
/home/regress/tbox/objs/i686-pc-linux-gnu/sys-include -g -O2 -O2
-g -O2 -DIN_GCC -W -Wall -Wwrite-strings -Wcast-qual
-Wstrict-prototypes -Wmissing-prototypes -Wold-style-definition
-isystem ./include -fpic -g -DIN_LIBGCC2 -fbuilding-libgcc
-fno-stack-protector -fpic -I. -I. -I../.././gcc
-I/home/regress/tbox/svn-gcc/libgcc
-I/home/regress/tbox/svn-gcc/libgcc/.
-I/home/regress/tbox/svn-gcc/libgcc/../gcc
-I/home/regress/tbox/svn-gcc/libgcc/../include
-I/home/regress/tbox/svn-gcc/libgcc/config/libbid
-DENABLE_DECIMAL_BID_FORMAT -DHAVE_CC_TLS -DUSE_TLS -o __main_s.o -MT
__main_s.o -MD -MP -MF __main_s.dep -DSHARED -DL__main -c
/home/regress/tbox/svn-gcc/libgcc/libgcc2.c
In file included from
/home/regress/tbox/svn-gcc/libgcc/unwind-dw2-fde-dip.c:79:0:
/home/regress/tbox/svn-gcc/libgcc/unwind-dw2-fde.c: In function 'search_object':
/home/regress/tbox/svn-gcc/libgcc/unwind-dw2-fde.c:997:1: internal
compiler error: Segmentation fault
}
^
Please submit a full bug report,
with preprocessed source if appropriate.
See <http://gcc.gnu.org/bugs.html> for instructions.
make[3]: *** [unwind-dw2-fde-dip.o] Error 1
make[3]: *** Waiting for unfinished jobs....
make[3]: Leaving directory
`/home/regress/tbox/native/build/i686-pc-linux-gnu/libgcc'
make[2]: *** [all-stage1-target-libgcc] Error 2
make[2]: Leaving directory `/home/regress/tbox/native/build'
make[1]: *** [stage1-bubble] Error 2
make[1]: Leaving directory `/home/regress/tbox/native/build'
make: *** [bootstrap] Error 2
--
H.J.
