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From: "olegendo at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/52394] SH Target: SH2A defunct bitops
Date: Sun, 20 Sep 2015 03:37:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52394

--- Comment #2 from Oleg Endo <olegendo at gcc dot gnu.org> ---
The SH2A bitops seem to produce some of the insns, but it seems the generated
code is either really bad (defeating the original purpose) or broken.

For example:


volatile struct
{
  union
  {
    unsigned char BYTE;
    struct
    {
      unsigned char BIT7:1;
      unsigned char BIT6:1;
      unsigned char BIT5:1;
      unsigned char BIT4:1;
      unsigned char BIT3:1;
      unsigned char BIT2:1;
      unsigned char BIT1:1;
      unsigned char BIT0:1;
    }
    BIT;
  }
  ICR0;
}
USRSTR;

int
main (unsigned char a, unsigned char b, unsigned char c)
{
  USRSTR.ICR0.BIT.BIT5 |= b & 1;
  return 0;
}

compiled with -m2a -mb -mbitops -O2:
        mov.l   .L2,r2        // addr of USRSTR

        mov     #1,r1
        and     r1,r5         // b & 1

        mov     #0,r0
        bor.b   #5,@(0,r2)    // T |= (bit in mem)

        movt    r7            // r7 = T
        mov.b   @r2,r3        // load byte
        cmp/pl  r7            // T = r7
        mov     #5,r7
        movt    r1            // r1 = T
        bclr    #5,r3         // clear bit 5 of loaded byte
        shld    r7,r1         // T << 5
        or      r3,r1         // OR bit
        mov.b   r1,@r2        // write back
        rts/n

It seems that this will produce funny results at the first bor.b insn because
at function entry the T bit is undefined.

The code should actually be something like this:
        mov.l   .L2,r2
        bld     #0,r5
        mov     #0,r0
        bor.b   #5,@(0,r2)
        bst.b   #5,@(0,r2)