[llvm-bugs] [Bug 38751] New: Missed optimization in 16-bit __builtin_add_overflow on aarch64

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38751

            Bug ID: 38751
           Summary: Missed optimization in 16-bit __builtin_add_overflow
                    on aarch64
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: AArch64
          Assignee: unassignedbugs at nondot.org
          Reporter: tblodt at icloud.com
                CC: llvm-bugs at lists.llvm.org

The following C function generates this code on aarch64 with -O3:

void g(int a, int b, int *c, int *o, int *v) {
    *o = __builtin_add_overflow((int16_t) a, (int16_t) b, (int16_t *) c);
}

  sxth w8, w0
  add w8, w8, w1, sxth
  sxth w9, w8
  cmp w9, w8
  cset w9, ne
  strh w8, [x2]
  str w9, [x3]
  ret

The sequence
  sxth w9, w8
  cmp w9, w8
can be rewritten as
  cmp w8, w8, sxth
which is one instruction shorter and uses one less register. So the following
code should be generated instead:

  sxth w8, w0
  add w8, w8, w1, sxth
  cmp w8, w8, sxth
  str w8, [x2]
  cset w8, ne
  str w8, [x3]
  ret

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