[Bug rtl-optimization/46888] New: missed optimization of zero_extract with constant inputs

[ams at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46888

           Summary: missed optimization of zero_extract with constant
                    inputs
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: ams@gcc.gnu.org


The compiler fails to do constant folding for bit fields, at least on ARM
targets, and instead builds the constant at run time.

Test case:

  struct bits
  {
     unsigned a:5;
     unsigned b:5;
     unsigned c:5;
     unsigned d:5;
  };

  struct bits
  f (unsigned int a)
  {
     struct bits bits = {0,0,0,0};
     bits.a = 1;
     bits.b = 2;
     bits.c = 3;
     bits.d = a;
     return bits;
  }

Output, compiled for ARM with "-O2 -mcpu=cortex-a8 -mthumb":
        movs    r2, #1
        movs    r3, #0
        bfi     r3, r2, #0, #5
        movs    r2, #2
        bfi     r3, r2, #5, #5
        movs    r2, #3
        bfi     r3, r2, #10, #5
        bfi     r3, r0, #15, #5
        mov     r0, r3
        bx      lr