aarch64: Fix invalid nested subregs [PR115464]

The testcase extracts one arm_neon.h vector from a pair (one subreg)
and then reinterprets the result as an SVE vector (another subreg).
Each subreg makes sense individually, but we can't fold them together
into a single subreg: it's 32 bytes -> 16 bytes -> 16*N bytes,
but the interpretation of 32 bytes -> 16*N bytes depends on
whether N==1 or N>1.

Since the second subreg makes sense individually, simplify_subreg
should bail out rather than ICE on it.  simplify_gen_subreg will
then do the same (because it already checks validate_subreg).
This leaves simplify_gen_subreg returning null, requiring the
caller to take appropriate action.

I think this is relatively likely to occur elsewhere, so the patch
adds a helper for forcing a subreg, allowing a temporary pseudo to
be created where necessary.

I'll follow up by using force_subreg in more places.  This patch
is intended to be a minimal backportable fix for the PR.

Bootstrapped & regression tested on aarch64-linux-gnu.  OK for trunk
and GCC 14 branch?

Richard


gcc/
	PR target/115464
	* simplify-rtx.cc (simplify_context::simplify_subreg): Don't try
	to fold two subregs together if their relationship isn't known
	at compile time.
	* explow.h (force_subreg): Declare.
	* explow.cc (force_subreg): New function.
	* config/aarch64/aarch64-sve-builtins-base.cc
	(svset_neonq_impl::expand): Use it instead of simplify_gen_subreg.

gcc/testsuite/
	PR target/115464
	* gcc.target/aarch64/sve/acle/general/pr115464.c: New test.
---
 gcc/config/aarch64/aarch64-sve-builtins-base.cc   |  2 +-
 gcc/explow.cc                                     | 15 +++++++++++++++
 gcc/explow.h                                      |  2 ++
 gcc/simplify-rtx.cc                               |  5 +++++
 .../aarch64/sve/acle/general/pr115464.c           | 13 +++++++++++++
 5 files changed, 36 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/gcc.target/aarch64/sve/acle/general/pr115464.c

On Thu, 13 Jun 2024, Richard Sandiford wrote:

> The testcase extracts one arm_neon.h vector from a pair (one subreg)
> and then reinterprets the result as an SVE vector (another subreg).
> Each subreg makes sense individually, but we can't fold them together
> into a single subreg: it's 32 bytes -> 16 bytes -> 16*N bytes,
> but the interpretation of 32 bytes -> 16*N bytes depends on
> whether N==1 or N>1.
> 
> Since the second subreg makes sense individually, simplify_subreg
> should bail out rather than ICE on it.  simplify_gen_subreg will
> then do the same (because it already checks validate_subreg).
> This leaves simplify_gen_subreg returning null, requiring the
> caller to take appropriate action.
> 
> I think this is relatively likely to occur elsewhere, so the patch
> adds a helper for forcing a subreg, allowing a temporary pseudo to
> be created where necessary.
> 
> I'll follow up by using force_subreg in more places.  This patch
> is intended to be a minimal backportable fix for the PR.
> 
> Bootstrapped & regression tested on aarch64-linux-gnu.  OK for trunk
> and GCC 14 branch?

OK for trunk and branch after it settles for a while.

Richard.

> Richard
> 
> 
> gcc/
> 	PR target/115464
> 	* simplify-rtx.cc (simplify_context::simplify_subreg): Don't try
> 	to fold two subregs together if their relationship isn't known
> 	at compile time.
> 	* explow.h (force_subreg): Declare.
> 	* explow.cc (force_subreg): New function.
> 	* config/aarch64/aarch64-sve-builtins-base.cc
> 	(svset_neonq_impl::expand): Use it instead of simplify_gen_subreg.
> 
> gcc/testsuite/
> 	PR target/115464
> 	* gcc.target/aarch64/sve/acle/general/pr115464.c: New test.
> ---
>  gcc/config/aarch64/aarch64-sve-builtins-base.cc   |  2 +-
>  gcc/explow.cc                                     | 15 +++++++++++++++
>  gcc/explow.h                                      |  2 ++
>  gcc/simplify-rtx.cc                               |  5 +++++
>  .../aarch64/sve/acle/general/pr115464.c           | 13 +++++++++++++
>  5 files changed, 36 insertions(+), 1 deletion(-)
>  create mode 100644 gcc/testsuite/gcc.target/aarch64/sve/acle/general/pr115464.c
> 
> diff --git a/gcc/config/aarch64/aarch64-sve-builtins-base.cc b/gcc/config/aarch64/aarch64-sve-builtins-base.cc
> index 0d2edf3f19e..c9182594bc1 100644
> --- a/gcc/config/aarch64/aarch64-sve-builtins-base.cc
> +++ b/gcc/config/aarch64/aarch64-sve-builtins-base.cc
> @@ -1174,7 +1174,7 @@ public:
>         Advanced SIMD argument as an SVE vector.  */
>      if (!BYTES_BIG_ENDIAN
>  	&& is_undef (CALL_EXPR_ARG (e.call_expr, 0)))
> -      return simplify_gen_subreg (mode, e.args[1], GET_MODE (e.args[1]), 0);
> +      return force_subreg (mode, e.args[1], GET_MODE (e.args[1]), 0);
>  
>      rtx_vector_builder builder (VNx16BImode, 16, 2);
>      for (unsigned int i = 0; i < 16; i++)
> diff --git a/gcc/explow.cc b/gcc/explow.cc
> index 8e5f6b8e680..f6843398c4b 100644
> --- a/gcc/explow.cc
> +++ b/gcc/explow.cc
> @@ -745,6 +745,21 @@ force_reg (machine_mode mode, rtx x)
>    return temp;
>  }
>  
> +/* Like simplify_gen_subreg, but force OP into a new register if the
> +   subreg cannot be formed directly.  */
> +
> +rtx
> +force_subreg (machine_mode outermode, rtx op,
> +	      machine_mode innermode, poly_uint64 byte)
> +{
> +  rtx x = simplify_gen_subreg (outermode, op, innermode, byte);
> +  if (x)
> +    return x;
> +
> +  op = copy_to_mode_reg (innermode, op);
> +  return simplify_gen_subreg (outermode, op, innermode, byte);
> +}
> +
>  /* If X is a memory ref, copy its contents to a new temp reg and return
>     that reg.  Otherwise, return X.  */
>  
> diff --git a/gcc/explow.h b/gcc/explow.h
> index 16aa02cfb68..cbd1fcb7eb3 100644
> --- a/gcc/explow.h
> +++ b/gcc/explow.h
> @@ -42,6 +42,8 @@ extern rtx copy_to_suggested_reg (rtx, rtx, machine_mode);
>     Args are mode (in case value is a constant) and the value.  */
>  extern rtx force_reg (machine_mode, rtx);
>  
> +extern rtx force_subreg (machine_mode, rtx, machine_mode, poly_uint64);
> +
>  /* Return given rtx, copied into a new temp reg if it was in memory.  */
>  extern rtx force_not_mem (rtx);
>  
> diff --git a/gcc/simplify-rtx.cc b/gcc/simplify-rtx.cc
> index 9bc3ef9ad9f..b6bb7e1f9e9 100644
> --- a/gcc/simplify-rtx.cc
> +++ b/gcc/simplify-rtx.cc
> @@ -7735,6 +7735,11 @@ simplify_context::simplify_subreg (machine_mode outermode, rtx op,
>        poly_uint64 innermostsize = GET_MODE_SIZE (innermostmode);
>        rtx newx;
>  
> +      /* Make sure that the relationship between the two subregs is
> +	 known at compile time.  */
> +      if (!ordered_p (outersize, innermostsize))
> +	return NULL_RTX;
> +
>        if (outermode == innermostmode
>  	  && known_eq (byte, 0U)
>  	  && known_eq (SUBREG_BYTE (op), 0))
> diff --git a/gcc/testsuite/gcc.target/aarch64/sve/acle/general/pr115464.c b/gcc/testsuite/gcc.target/aarch64/sve/acle/general/pr115464.c
> new file mode 100644
> index 00000000000..d728d1325ed
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/aarch64/sve/acle/general/pr115464.c
> @@ -0,0 +1,13 @@
> +/* { dg-options "-O2" } */
> +
> +#include <arm_neon.h>
> +#include <arm_sve.h>
> +#include <arm_neon_sve_bridge.h>
> +
> +svuint16_t
> +convolve4_4_x (uint16x8x2_t permute_tbl)
> +{
> +    return svset_neonq_u16 (svundef_u16 (), permute_tbl.val[1]);
> +}
> +
> +/* { dg-final { scan-assembler {\tmov\tz0\.d, z1\.d\n} } } */
>