From: Eric Botcazou <ebotcazou@adacore.com>
To: Richard Sandiford <rdsandiford@googlemail.com>
Cc: gcc-patches@gcc.gnu.org, ramrad01@arm.com
Subject: Re: [0/8] Add optabs alternatives for insv, extv and extzv
Date: Thu, 29 Nov 2012 15:31:00 -0000 [thread overview]
Message-ID: <4058772.se91EY6rS5@polaris> (raw)
In-Reply-To: <87hao8vgxs.fsf@sandifor-thinkpad.stglab.manchester.uk.ibm.com>
> I agree that this kind of MEM is less than ideal, but I thought:
>
> set_mem_attributes_minus_bitpos (to_rtx, to, 0, bitpos);
>
> said that the attributes of TO_RTX will to be TO _once a pending offset-and-
> narrowing operation has been applied_. So we have:
>
> /* If we modified OFFSET based on T, then subtract the outstanding
> bit position offset. Similarly, increase the size of the accessed
> object to contain the negative offset. */
> if (apply_bitpos)
> {
> gcc_assert (attrs.offset_known_p);
> attrs.offset -= apply_bitpos / BITS_PER_UNIT;
> if (attrs.size_known_p)
> attrs.size += apply_bitpos / BITS_PER_UNIT;
> }
>
> I didn't think we necessarily expected the width of the reference
> (TO_RTX) and the width of the type (TO) to match at this stage.
> That's different from adjust_bitfield_address, where the
> offset-and-narrowing operation itself is applied.
I was essentially thinking of the size adjustment just above that one: if the
mode size is known, setting a smaller size without further ado seems awkward.
So the questionable MEM doesn't survive long? OK, maybe...
> The difference between the width of the reference and the width
> of T is what led to:
>
> http://gcc.gnu.org/ml/gcc-patches/2012-11/msg00262.html
>
> As things stand, APPLY_BITPOS is only nonzero if we set both the
> MEM_EXPR and MEM_SIZE from T. There are also cases (like this one)
> where we don't set the MEM_EXPR from T but do set the MEM_SIZE from T.
> The bitpos will be applied either way, so I thought MEM_SIZE should be
> the same in both cases. That doesn't fix this problem of course, it's
> just an argument that the relationship between the width of the reference
> mode, the MEM_SIZE and the width of T seems pretty complicated with the
> current interface.
MEM_SIZE and MEM_EXPR are used alone by the aliasing machinery to disambiguate
memory references, so they need be conservative wrt the actual memory access.
> Maybe set_mem_attributes_minus_bitpos (but not set_mem_attributes)
> should only set the MEM_EXPR and leave the MEM_SIZE unchanged?
>
> Before submitting the patched linked above, I tried getting rid
> of set_mem_attributes_minus_bitpos and passing the tree down instead.
> Then we could set the attributes at the time of the offset-and-narrowing
> operation, where the size and offset of the final reference are known.
> That didn't seem like an easy change to make though, and became a
> bit of a distraction from the main patches.
>
> Anyway, given the breakage that this series has already caused,
> I'd prefer not to tackle stuff like this as well. I'd only used
> MEM_SIZE in the first attempted patch out of habit. I think the
> revised patch more obviously matches the *_fixed_bit_field functions
> and is more generally in keeping with the existing checks.
> (It's deliberately more conservative though, only using register
> bitfields if both the bit_field_mode_iterator and strict volatile
> bitfield rules are met.)
Well, rewriting the bitfield machinery of the middle-end is a once-in-a-decade
undertaking, so some fallouts are to be expected. :-) That wasn't too bad in
the end. But I agree with the cautious approach from now on.
--
Eric Botcazou
prev parent reply other threads:[~2012-11-29 15:31 UTC|newest]
Thread overview: 42+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-11-03 11:10 Richard Sandiford
2012-11-03 11:13 ` [1/8] Handle TRUNCATE in make_extraction Richard Sandiford
2012-11-10 15:52 ` Eric Botcazou
2012-11-03 11:14 ` [2/8] Add adjust_bitfield_address_size Richard Sandiford
2012-11-10 15:53 ` Eric Botcazou
2012-11-03 11:16 ` [3/8] Add narrow_bit_field_mem Richard Sandiford
2012-11-10 16:02 ` Eric Botcazou
2012-11-03 11:21 ` [4/8] Add bit_field_mode_iterator Richard Sandiford
2012-11-13 12:44 ` Eric Botcazou
2012-11-13 21:46 ` Richard Henderson
2012-11-13 22:05 ` Eric Botcazou
2012-11-15 12:11 ` Richard Sandiford
2012-11-15 20:39 ` Richard Henderson
2012-11-18 17:34 ` Richard Sandiford
2012-11-18 17:36 ` Richard Sandiford
2012-11-03 11:27 ` [5/8] Tweak bitfield alignment handling Richard Sandiford
2012-11-13 13:52 ` Eric Botcazou
2012-11-18 17:36 ` Richard Sandiford
2012-11-20 2:57 ` John David Anglin
2012-11-20 8:21 ` Mikael Pettersson
2012-11-20 10:32 ` Richard Sandiford
2012-11-20 19:56 ` Richard Sandiford
2012-11-20 22:11 ` Eric Botcazou
2012-11-03 11:28 ` [6/8] Add strict volatile handling to bit_field_mode_iterator Richard Sandiford
2012-11-13 13:57 ` Eric Botcazou
2012-11-15 12:25 ` Richard Sandiford
2012-11-15 17:10 ` Eric Botcazou
2012-11-15 17:47 ` Richard Sandiford
2012-11-15 19:32 ` Eric Botcazou
2012-11-18 17:36 ` Richard Sandiford
2012-11-03 11:39 ` [7/8] Replace mode_for_extraction with new interface Richard Sandiford
2012-11-03 11:41 ` [8/8] Add new optabs and use them for MIPS Richard Sandiford
2012-11-27 17:11 ` [0/8] Add optabs alternatives for insv, extv and extzv Ramana Radhakrishnan
2012-11-27 20:22 ` Richard Sandiford
2012-11-27 22:45 ` Ramana Radhakrishnan
2012-11-28 10:25 ` Richard Biener
2012-11-28 12:06 ` Ramana Radhakrishnan
2012-11-28 12:51 ` Richard Biener
2012-11-28 13:58 ` Richard Sandiford
2012-11-28 23:19 ` Eric Botcazou
2012-11-29 10:31 ` Richard Sandiford
2012-11-29 15:31 ` Eric Botcazou [this message]
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