On Wed, 16 Jun 2021, Jakub Jelinek wrote:
> Hi!
>
> The following testcase is miscompiled on x86_64-linux, the bitfield store
> is implemented as a RMW 64-bit operation at d+24 when the d variable has
> size of only 28 bytes and scheduling moves in between the R and W part
> a store to a different variable that happens to be right after the d
> variable.
>
> The reason for this is that we weren't creating
> DECL_BIT_FIELD_REPRESENTATIVEs for bitfields in unions.
>
> The following patch does create them, but treats all such bitfields as if
> they were in a structure where the particular bitfield is the only field.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
OK.
Thanks,
Richard.
> 2021-06-16 Jakub Jelinek <ja...@redhat.com>
>
> PR middle-end/101062
> * stor-layout.c (finish_bitfield_representative): For fields in unions
> assume nextf is always NULL.
> (finish_bitfield_layout): Compute bit field representatives also in
> unions, but handle it as if each bitfield was the only field in the
> aggregate.
>
> * gcc.dg/pr101062.c: New test.
>
> --- gcc/stor-layout.c.jj 2021-03-30 18:11:52.537092233 +0200
> +++ gcc/stor-layout.c 2021-06-15 10:58:59.244353965 +0200
> @@ -2072,9 +2072,14 @@ finish_bitfield_representative (tree rep
> bitsize = (bitsize + BITS_PER_UNIT - 1) & ~(BITS_PER_UNIT - 1);
>
> /* Now nothing tells us how to pad out bitsize ... */
> - nextf = DECL_CHAIN (field);
> - while (nextf && TREE_CODE (nextf) != FIELD_DECL)
> - nextf = DECL_CHAIN (nextf);
> + if (TREE_CODE (DECL_CONTEXT (field)) == RECORD_TYPE)
> + {
> + nextf = DECL_CHAIN (field);
> + while (nextf && TREE_CODE (nextf) != FIELD_DECL)
> + nextf = DECL_CHAIN (nextf);
> + }
> + else
> + nextf = NULL_TREE;
> if (nextf)
> {
> tree maxsize;
> @@ -2167,13 +2172,6 @@ finish_bitfield_layout (tree t)
> tree field, prev;
> tree repr = NULL_TREE;
>
> - /* Unions would be special, for the ease of type-punning optimizations
> - we could use the underlying type as hint for the representative
> - if the bitfield would fit and the representative would not exceed
> - the union in size. */
> - if (TREE_CODE (t) != RECORD_TYPE)
> - return;
> -
> for (prev = NULL_TREE, field = TYPE_FIELDS (t);
> field; field = DECL_CHAIN (field))
> {
> @@ -2233,7 +2231,13 @@ finish_bitfield_layout (tree t)
> if (repr)
> DECL_BIT_FIELD_REPRESENTATIVE (field) = repr;
>
> - prev = field;
> + if (TREE_CODE (t) == RECORD_TYPE)
> + prev = field;
> + else if (repr)
> + {
> + finish_bitfield_representative (repr, field);
> + repr = NULL_TREE;
> + }
> }
>
> if (repr)
> --- gcc/testsuite/gcc.dg/pr101062.c.jj 2021-06-15 10:42:58.642919880
> +0200
> +++ gcc/testsuite/gcc.dg/pr101062.c 2021-06-15 10:42:40.897171191 +0200
> @@ -0,0 +1,29 @@
> +/* PR middle-end/101062 */
> +/* { dg-do run } */
> +/* { dg-options "-O2 -fno-toplevel-reorder -frename-registers" } */
> +
> +union U { signed b : 5; };
> +int c;
> +volatile union U d[7] = { { 8 } };
> +short e = 1;
> +
> +__attribute__((noipa)) void
> +foo ()
> +{
> + d[6].b = 0;
> + d[6].b = 0;
> + d[6].b = 0;
> + d[6].b = 0;
> + d[6].b = 0;
> + e = 0;
> + c = 0;
> +}
> +
> +int
> +main ()
> +{
> + foo ();
> + if (e != 0)
> + __builtin_abort ();
> + return 0;
> +}
>
> Jakub
>
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)
