[Bug rtl-optimization/96756] New: [AArch64] Missed FPSR description on saturating instruction patterns

[xiezhiheng at huawei dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96756

            Bug ID: 96756
           Summary: [AArch64] Missed FPSR description on saturating
                    instruction patterns
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: xiezhiheng at huawei dot com
  Target Milestone: ---
            Target: aarch64

Test case:

#include <arm_neon.h>
#include <stdlib.h>

typedef union {
  struct {
    int _xxx:24;
    unsigned int FZ:1;
    unsigned int DN:1;
    unsigned int AHP:1;
    unsigned int QC:1;
    int V:1;
    int C:1;
    int Z:1;
    int N:1;
  } b;
  unsigned int word;
} _ARM_FPSCR;

static volatile int __read_neon_cumulative_sat (void) {
    _ARM_FPSCR _afpscr_for_qc;
    asm volatile ("mrs %0,fpsr" : "=r" (_afpscr_for_qc));
    return _afpscr_for_qc.b.QC;
}

int main()
{
  uint32x2_t op0, op1, res;

  op0 = vdup_n_u32 ((uint32_t)0xfffffff0);
  op1 = vdup_n_u32 ((uint32_t)0x20);

  _ARM_FPSCR _afpscr_for_qc;
  asm volatile ("mrs %0,fpsr" : "=r" (_afpscr_for_qc));
  _afpscr_for_qc.b.QC = (0);
  asm volatile ("msr fpsr,%0" :  : "r" (_afpscr_for_qc));

  res = vqadd_u32 (op0, op1);
  if (__read_neon_cumulative_sat () != 1)
    abort ();

  return 0;
}

Command line (GCC version 11.0): aarch64-linux-gnu-gcc test.c -O0 -o test
$ ./test
Run successfully

Command line (GCC version 11.0): aarch64-linux-gnu-gcc test.c -O2 -o test
$ ./test
Aborted (core dumped)

The test case has different results under -O0 and -O2.
In RTL phase insn
  (insn 11 10 12 2 (set (reg:V2SI 97)
        (us_plus:V2SI (reg:V2SI 98)
            (reg:V2SI 99))) {aarch64_uqaddv2si}
     (nil))
would be eliminated because saturating instruction patterns have no side
effect of set the FPSR register and this insn is treated as dead insn.

Reference mails:
https://gcc.gnu.org/pipermail/gcc-patches/2020-August/552367.html