Hi.
Thanks for doing this! This was on my TODO list for some time. I didn't
realize it was as simple as this.
Just something I've noticed: When both lhs and rhs have alignment/range info,
maybe_duplicate_ssa_info_at_copy doesn't do anything. So we lose any
additional info asociated with lhs. I assume that this also happens in other
copy propagation passes. I wonder if we could perhaps intersect the infos in
that case.
But then I guess a situation like this could happen:
unsigned x, y;
x = getchar();
if (x <= 4)
error();
y = x;
if (y <= 4)
<do something>
<never use y again>
GCC maybe computes range of y to be [0,4]? Then if we copy propagate y = x
and intersected the range information, we would get that range of x is [0,4]
but that is not true and will lead to a miscompilation.
Filip
On Tue 2026-06-02 19:06:34, Andrew Pinski wrote:
> Currently if the ssa info (range or aliasing) is different
> between the src and dest of a copy, sccopy will reject it.
> This fixes the FIXME and handles it in a similar way as copyprop
> handles it. This is needed to able to move sccopy earlier (before phiopt1)
> and to act like a copyprop pass.
>
> Bootstrapped and tested on x86_64-linux-gnu with no regressions.
>
> gcc/ChangeLog:
>
> * gimple-ssa-sccopy.cc (stmt_may_generate_copy): Remove restriction
> on the ssa info being different for the "copy".
> (scc_copy_prop::replace_scc_by_value): Call
> maybe_duplicate_ssa_info_at_copy
> when copyproping a ssa name.
>
> Signed-off-by: Andrew Pinski <[email protected]>
> ---
> gcc/gimple-ssa-sccopy.cc | 18 ++----------------
> 1 file changed, 2 insertions(+), 16 deletions(-)
>
> diff --git a/gcc/gimple-ssa-sccopy.cc b/gcc/gimple-ssa-sccopy.cc
> index 0badfb11b93..4252e5295df 100644
> --- a/gcc/gimple-ssa-sccopy.cc
> +++ b/gcc/gimple-ssa-sccopy.cc
> @@ -405,22 +405,6 @@ stmt_may_generate_copy (gimple *stmt)
> if (SSA_NAME_OCCURS_IN_ABNORMAL_PHI (rhs))
> return false;
>
> - /* It is possible that lhs has more alignment or value range information.
> By
> - propagating we would lose this information. So in the case that
> alignment
> - or value range information differs, we are conservative and do not
> - propagate.
> -
> - FIXME: Propagate alignment and value range info the same way copy-prop
> - does. */
> - if (POINTER_TYPE_P (TREE_TYPE (lhs))
> - && POINTER_TYPE_P (TREE_TYPE (rhs))
> - && SSA_NAME_PTR_INFO (lhs) != SSA_NAME_PTR_INFO (rhs))
> - return false;
> - if (!POINTER_TYPE_P (TREE_TYPE (lhs))
> - && !POINTER_TYPE_P (TREE_TYPE (rhs))
> - && SSA_NAME_RANGE_INFO (lhs) != SSA_NAME_RANGE_INFO (rhs))
> - return false;
> -
> return true; /* A statement of type _2 = _1;. */
> }
>
> @@ -497,6 +481,8 @@ scc_copy_prop::replace_scc_by_value (vec<gimple *> scc,
> tree val)
>
> }
> replace_uses_by (name, val);
> + if (TREE_CODE (val) == SSA_NAME)
> + maybe_duplicate_ssa_info_at_copy (name, val);
> bitmap_set_bit (dead_stmts, SSA_NAME_VERSION (name));
> didsomething = true;
> }
> --
> 2.43.0
>
