On Fri, Feb 28, 2025 at 5:25 AM Kyrylo Tkachov <ktkac...@nvidia.com> wrote:
>
> Hi all,
>
> In this PR late-combine was failing to merge:
> dup v31.4s, v31.s[3]
> fmla v30.4s, v31.4s, v29.4s
> into the lane-wise fmla form.
> This is because late-combine checks may_trap_p under the hood on the dup insn.
> This ended up returning true for the insn:
> (set (reg:V4SF 152 [ _32 ])
> (vec_duplicate:V4SF (vec_select:SF (reg:V4SF 111 [ rhs_panel.8_31 ])
> (parallel:V4SF [
> (const_int 3 [0x3])]))))
>
> Although mem_trap_p correctly reasoned that vec_duplicate and vec_select of
> floating-point modes can't trap, it assumed that the V4SF parallel can trap.
> The correct behaviour is to recurse into vector inside the PARALLEL and check
> the sub-expression. This patch adjusts may_trap_p_1 to do just that.
> With this check the above insn is not deemed to be trapping and is propagated
> into the FMLA giving:
>
> fmla vD.4s, vA.4s, vB.s[3]
>
> The testcase is reduced from a larger example so it has some C++ cruft around
> the main part but still demonstrates the desired effect.
>
> Bootstrapped and tested on aarch64-none-linux-gnu.
> Apparently this also fixes a regression in
> gcc.target/aarch64/vmul_element_cost.c that I observed.
>
> Ok for trunk?
This looks ok but here is a better/more reduced testcase without the
extra C++ism:
```
/* { dg-do compile } */
/* { dg-additional-options "-O2" } */
#include <arm_neon.h>
float32x4_t madd_helper_1(float32x4_t a, float32x4_t b, float32x4_t d)
{
float32x4_t t = a;
t = vfmaq_f32 (t, vdupq_n_f32(vgetq_lane_f32 (b, 1)), d);
t = vfmaq_f32 (t, vdupq_n_f32(vgetq_lane_f32 (b, 1)), d);
return t;
}
/* { dg-final { scan-assembler-not {\tdup\tv[0-9]+\.4s, v[0-9]+.s\[3\]\n} } } */
/* { dg-final { scan-assembler-times {\tfmla\tv[0-9]+\.4s,
v[0-9]+\.4s, v[0-9]+\.s\[3\]\n} 1 } } */
```
Thanks,
Andrew Pinski
> Thanks,
> Kyrill
>
> Signed-off-by: Kyrylo Tkachov <ktkac...@nvidia.com>
>
> gcc/
>
> PR rtl-optimization/119046
> * rtlanal.cc (may_trap_p_1): Don't mark FP-mode PARALLELs as trapping.
>
> gcc/testsuite/
>
> PR rtl-optimization/119046
> * g++.target/aarch64/pr119046.C: New test.
>
