On Wed, Aug 27, 2025 at 6:12 PM Andrew Pinski
<[email protected]> wrote:
>
> Currently the code rejects:
> ```
> tmp = *a;
> *b = tmp;
> ```
> (unless *a == *b). This can be improved such that if a and b are known to
> share the same base, then only reject it if they overlap; that is the
> difference of the offsets (from the base) is maybe less than the size.
>
> This fixes the testcase in comment #0 of PR 107051.
>
> PR tree-optimization/107051
>
> gcc/ChangeLog:
>
> * tree-ssa-forwprop.cc (optimize_agr_copyprop_1): Allow for
> memory sharing the same base if they known not to overlap over
> the size.
>
> gcc/testsuite/ChangeLog:
>
> * gcc.dg/tree-ssa/copy-prop-aggregate-union-1.c: New test.
>
> Signed-off-by: Andrew Pinski <[email protected]>
> ---
> .../tree-ssa/copy-prop-aggregate-union-1.c | 24 +++++++++++++++++
> gcc/tree-ssa-forwprop.cc | 27 ++++++++++++++++++-
> 2 files changed, 50 insertions(+), 1 deletion(-)
> create mode 100644
> gcc/testsuite/gcc.dg/tree-ssa/copy-prop-aggregate-union-1.c
>
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/copy-prop-aggregate-union-1.c
> b/gcc/testsuite/gcc.dg/tree-ssa/copy-prop-aggregate-union-1.c
> new file mode 100644
> index 00000000000..206f6e1be55
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/copy-prop-aggregate-union-1.c
> @@ -0,0 +1,24 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O1 -fdump-tree-forwprop1-details" } */
> +/* PR tree-optimization/107051 */
> +
> +
> +union U2 {
> + unsigned f0;
> + char * f1;
> +};
> +
> +/* Since g_284[0] and g_284[1] are known not overlap,
> + copy prop can happen. */
> +union U2 g_284[2] = {{0UL},{0xC2488F72L}};
> +
> +int e;
> +void func_1() {
> + union U2 c = {7};
> + int *d[2];
> + for (; e;)
> + *d[1] = 0;
> + g_284[0] = c = g_284[1];
> +}
> +
> +/* { dg-final { scan-tree-dump-times "after previous" 1 "forwprop1" } } */
> diff --git a/gcc/tree-ssa-forwprop.cc b/gcc/tree-ssa-forwprop.cc
> index e0f25a12f34..82344f4020d 100644
> --- a/gcc/tree-ssa-forwprop.cc
> +++ b/gcc/tree-ssa-forwprop.cc
> @@ -1455,7 +1455,32 @@ optimize_agr_copyprop_1 (gimple *stmt, gimple
> *use_stmt,
> */
> if (!operand_equal_p (dest2, src, 0)
> && !DECL_P (dest2) && !DECL_P (src))
> - return false;
> + {
> + /* If *a and *b have the same base see if
> + the offset between the two is greater than
> + or equal to the size of the type. */
> + poly_int64 offset1, offset2;
> + tree len = TYPE_SIZE_UNIT (TREE_TYPE (src));
> + if (len == NULL_TREE
> + || !tree_fits_poly_int64_p (len))
> + return false;
> + tree base1 = get_addr_base_and_unit_offset (dest2, &offset1);
> + if (!base1)
> + return false;
> + tree base2 = get_addr_base_and_unit_offset (src, &offset2);
> + if (!base2)
> + return false;
> + if (!operand_equal_p (base1, base2))
I'll note that exact overlap, thus
b = a;
tmp = *a;
*b = tmp;
is fine. VN uses alignment as additional non-partial overlap test,
so when !operand_equal_p here you could see whether
get_object_alignment () for both is > size (mostly interesting for
small sizes, of course). With -fstrict-aliasing if both have the
same type then partial overlaps also cannot happen (but this
condition is a bit difficult to apply I think).
> + return false;
> + poly_int64 size = tree_to_poly_int64 (len);
> + /* Make sure [offset1, offset1 + len - 1] does
> + not overlap with [offset2, offset2 + len - 1]
> + or overlaps fully. */
There's ranges_may_overlap_p you might want to use here.
Richard.
> + if (!known_eq (offset2, offset1)
> + && !known_ge (offset2 - offset1, size)
> + && !known_ge (offset1 - offset2, size))
> + return false;
> + }
>
> if (dump_file && (dump_flags & TDF_DETAILS))
> {
> --
> 2.43.0
>
