[Bug target/61578] [4.9 regression] Code size increase for ARM thumb compared to 4.8.x when compiling with -Os

["fredrik.hederstierna@securitas-direct.com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61578

--- Comment #23 from Fredrik Hederstierna <fredrik.hederstierna@securitas-direct.com> ---
Thanks for your patch, I tried it out, and it solves the small example fine,
the code now is similar to GCC 4.8 for this particular example.

Though when I ran the full CSiBE benchmark, the code size unfortunately grew
approx +150 bytes overall for the full suite. So the patch did not solve the
generic root problem with code size increase unfortunately.

This is strange and I'm thinking of how to continue from here, this issue has
diverged a bit too much (mostly because of my own fault) with several examples
etc. Do you think we should create separate issues for different small examples
that compiles bad perhaps? but on the same time we need to keep track on the
'generic' overall code size issue as eg. CSiBE points out.

Here's is another small example I tested yesterday that also gives unnecessary
moves, both for arm7tdmi, arm966e-s and cortex-m0 tested.

extern void func(int data);
char cdata[4];
void test(void) {
  int *idata = (int*)cdata;
  func(*idata);
}

Compiles with GCC 4.8.5 (cortex-m0):

00000000 <test>:
   0:   b508            push    {r3, lr}
   2:   4b07            ldr     r3, [pc, #28]   ; (20 <test+0x20>)
   4:   7858            ldrb    r0, [r3, #1]
   6:   781a            ldrb    r2, [r3, #0]
   8:   0200            lsls    r0, r0, #8
   a:   4310            orrs    r0, r2
   c:   789a            ldrb    r2, [r3, #2]
   e:   78db            ldrb    r3, [r3, #3]
  10:   0412            lsls    r2, r2, #16
  12:   4310            orrs    r0, r2
  14:   061b            lsls    r3, r3, #24
  16:   4318            orrs    r0, r3
  18:   f7ff fffe       bl      0 <func>
  1c:   bd08            pop     {r3, pc}
  1e:   46c0            nop                     ; (mov r8, r8)
  20:   00000000        .word   0x00000000

With GCC 6 master with latest LRA patch (+4 bytes):

00000000 <test>:
   0:   b510            push    {r4, lr}
   2:   4c08            ldr     r4, [pc, #32]   ; (24 <test+0x24>)
   4:   7863            ldrb    r3, [r4, #1]
   6:   7821            ldrb    r1, [r4, #0]
   8:   78a0            ldrb    r0, [r4, #2]
   a:   021b            lsls    r3, r3, #8
   c:   430b            orrs    r3, r1
   e:   0400            lsls    r0, r0, #16
  10:   001a            movs    r2, r3   ??? MOVE
  12:   0003            movs    r3, r0   ??? MOVE
  14:   78e0            ldrb    r0, [r4, #3]
  16:   4313            orrs    r3, r2
  18:   0600            lsls    r0, r0, #24
  1a:   4318            orrs    r0, r3
  1c:   f7ff fffe       bl      0 <func>
  20:   bd10            pop     {r4, pc}
  22:   46c0            nop                     ; (mov r8, r8)
  24:   00000000        .word   0x00000000

Kind Regards, Fredrik