On Tue, Aug 12, 2025 at 10:14 AM Richard Sandiford
<richard.sandif...@arm.com> wrote:
>
> For the reasons explained in the comment, fwprop shouldn't even
> try to propagate an asm definition.
>
> Tested on aarch64-linux-gnu. Bordering on obvious, but just in case:
> OK to install?
OK.
Richard.
> Richard
>
>
> gcc/
> PR rtl-optimization/121253
> * fwprop.cc (forward_propagate_into): Don't propagate asm defs.
>
> gcc/testsuite/
> PR rtl-optimization/121253
> * gcc.target/aarch64/pr121253.c: New test.
> ---
> gcc/fwprop.cc | 14 ++++++++++++++
> gcc/testsuite/gcc.target/aarch64/pr121253.c | 16 ++++++++++++++++
> 2 files changed, 30 insertions(+)
> create mode 100644 gcc/testsuite/gcc.target/aarch64/pr121253.c
>
> diff --git a/gcc/fwprop.cc b/gcc/fwprop.cc
> index 4ee44adcde2..59df7cad0de 100644
> --- a/gcc/fwprop.cc
> +++ b/gcc/fwprop.cc
> @@ -836,6 +836,20 @@ forward_propagate_into (use_info *use, bool
> reg_prop_only = false)
> if (def_insn->is_artificial ())
> return false;
>
> + /* Do not propagate asms. The only kind of propagation that would
> + succeed is propagation into a register move. Such a propagation
> + is neutral if the destination of the move is a pseudo and unnecessarily
> + restricts the register allocator if the destination of the move is
> + a hard register.
> +
> + Furthermore, unlike for a normal instruction, we cannot take a SET from
> an
> + asm and try dropping the CLOBBERs. The recog process does not (and
> should
> + not try to) second-guess whether what the user wrote can be changed and
> + so it has to assume that any asm given to it is a fair reflection of
> + what the user wrote. */
> + if (def_insn->is_asm ())
> + return false;
> +
> rtx_insn *def_rtl = def_insn->rtl ();
> if (!NONJUMP_INSN_P (def_rtl))
> return false;
> diff --git a/gcc/testsuite/gcc.target/aarch64/pr121253.c
> b/gcc/testsuite/gcc.target/aarch64/pr121253.c
> new file mode 100644
> index 00000000000..37de605c646
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/aarch64/pr121253.c
> @@ -0,0 +1,16 @@
> +/* { dg-options "-O" } */
> +
> +struct s128 {
> + long a, b;
> +};
> +
> +struct s128 foo(void) {
> + struct s128 ret;
> + asm("mov %0, #0 \n\t"
> + "mov %R0, #0 \n\t"
> + "mov x0, #12345"
> + : "=r" (ret) : : "x0");
> + return ret;
> +}
> +
> +/* { dg-final { scan-assembler-not {mov x0, #0} } } */
> --
> 2.43.0
>
