Re: [PATCH] avoid-store-forwarding: Reject BLKmode memory operands [PR126686]

On Thu, Aug 20, 2026 at 7:07 AM Konstantinos Eleftheriou
<[email protected]> wrote:
>
> Memory operands were only rejected when their size is unknown or
> non-constant, so a BLKmode store with a known size could become a
> forwarding candidate. Building the bit insert sequence for it asserts
> in store_bit_field, as BLKmode has no corresponding integer mode.
> Reject such memory when collecting candidates.
>
>         PR rtl-optimization/126686
>
> gcc/ChangeLog:
>
>         * avoid-store-forwarding.cc
>         (store_forwarding_analyzer::avoid_store_forwarding): Reject
>         BLKmode stores and loads as forwarding candidates.
>
> gcc/testsuite/ChangeLog:
>
>         * gcc.target/s390/pr126686.c: New test.
> ---
>  gcc/avoid-store-forwarding.cc            | 10 +++++++---
>  gcc/testsuite/gcc.target/s390/pr126686.c | 16 ++++++++++++++++
>  2 files changed, 23 insertions(+), 3 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.target/s390/pr126686.c
>
> diff --git a/gcc/avoid-store-forwarding.cc b/gcc/avoid-store-forwarding.cc
> index 141f96c3d848..99feebafe126 100644
> --- a/gcc/avoid-store-forwarding.cc
> +++ b/gcc/avoid-store-forwarding.cc
> @@ -565,11 +565,15 @@ store_forwarding_analyzer::avoid_store_forwarding (basic_block bb)
>        /* The mem RTX if INSN is a store, NULL_RTX otherwise.  */
>        rtx store_mem = MEM_P (SET_DEST (set)) ? SET_DEST (set) : NULL_RTX;
>
> -      /* We cannot analyze memory RTXs that have unknown size. */
> +      /* We cannot analyze memory RTXs that have unknown size.  BLKmode
> +        memory is rejected as well, as there is no mode for the forwarded
> +        value, even when its size is known.  */
>        if ((store_mem && (!MEM_SIZE_KNOWN_P (store_mem)
> -                        || !MEM_SIZE (store_mem).is_constant ()))
> +                        || !MEM_SIZE (store_mem).is_constant ()
> +                        || GET_MODE (store_mem) == BLKmode))
>           || (load_mem && (!MEM_SIZE_KNOWN_P (load_mem)
> -                          || !MEM_SIZE (load_mem).is_constant ())))
> +                          || !MEM_SIZE (load_mem).is_constant ()
> +                          || GET_MODE (load_mem) == BLKmode)))

Can you place the GET_MODE check first in the list since it should be
the cheapest test?
Otherwise ok.


>         {
>           store_exprs.truncate (0);
>           continue;
> diff --git a/gcc/testsuite/gcc.target/s390/pr126686.c b/gcc/testsuite/gcc.target/s390/pr126686.c
> new file mode 100644
> index 000000000000..ab25d109ec35
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/s390/pr126686.c
> @@ -0,0 +1,16 @@
> +/* PR rtl-optimization/126686 */
> +/* { dg-do compile } */
> +/* { dg-options "-O3 -favoid-store-forwarding" } */
> +
> +int tmp;
> +short d_e;
> +
> +int
> +foo ()
> +{
> +  long f = 0;
> +  __builtin_memset ((char *) &f + sizeof f - 2, d_e, 2);
> +  tmp = f;
> +
> +  return f;
> +}
> --
> 2.55.0
>