[Bug c/93800] New: GCC adds unwanted nops to align loops on powerpc 8xx

[christophe.leroy@c-s.fr]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93800

            Bug ID: 93800
           Summary: GCC adds unwanted nops to align loops on powerpc 8xx
           Product: gcc
           Version: 9.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: christophe.leroy@c-s.fr
  Target Milestone: ---

GCC 9.2 add nops in front of loops. GCC 8.1 didn't when compiled for powerpc
8xx. On the 8xx, a nop is 1 cycle and alignment of loops provide no benefit, so
this is a waste of cycles.

Reproducer:

volatile int g;
int f(int a, int b)
{
        int i;

        for (i = 0; i < b; i++)
                a += g;
        return a;
}

Built with -m32 -mcpu=860 -O2

00000000 <f>:
   0:   2c 04 00 00     cmpwi   r4,0
   4:   4c 81 00 20     blelr   
   8:   3d 40 00 00     lis     r10,0
                        a: R_PPC_ADDR16_HA      g
   c:   7c 89 03 a6     mtctr   r4
  10:   39 4a 00 00     addi    r10,r10,0
                        12: R_PPC_ADDR16_LO     g
  14:   60 00 00 00     nop
  18:   60 00 00 00     nop
  1c:   60 00 00 00     nop
  20:   81 2a 00 00     lwz     r9,0(r10)
  24:   7c 63 4a 14     add     r3,r3,r9
  28:   42 00 ff f8     bdnz    20 <f+0x20>
  2c:   4e 80 00 20     blr



The same with GCC 8.1:

00000000 <f>:
   0:   2c 04 00 00     cmpwi   r4,0
   4:   4c 81 00 20     blelr   
   8:   3d 40 00 00     lis     r10,0
                        a: R_PPC_ADDR16_HA      g
   c:   7c 89 03 a6     mtctr   r4
  10:   39 4a 00 00     addi    r10,r10,0
                        12: R_PPC_ADDR16_LO     g
  14:   81 2a 00 00     lwz     r9,0(r10)
  18:   7c 63 4a 14     add     r3,r3,r9
  1c:   42 00 ff f8     bdnz    14 <f+0x14>
  20:   4e 80 00 20     blr