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From: "dumoulin.thibaut at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/101981] GCC10 produces bigger asm for simple switch than
 GCC7 - cortexM4 since r8-2701-g9dc3d6a96167b4c8
Date: Thu, 09 Sep 2021 12:51:43 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101981

--- Comment #7 from Thibaut M. <dumoulin.thibaut at gmail dot com> ---
There are other regressions (in term of code size) but not sure if this is =
the
same.
For example, this code:

```C
#include <stdio.h>

void big_switch(int a) {
  switch (a) {
  default:
    printf("default a(%d)\n", a);
  case 0x1:
  case 0x2:
  // case 0x3:
  // case 0x4:
  case 0x5:
  case 0x6:
  // case 0x7:
  // case 0x8:
  // case 0x9:
  case 0xA:
  case 0xD:
  case 0xE:
  case 0xF:
  // case 0x10:
  case 0x11:
  case 0x12:
  // case 0x13:
  // case 0x14:
  case 0x15:
  case 0x17:
  case 0x18:
  case 0x19:
  case 0x1A:
  // case 0x1B:
  case 0x1C:
  case 0x1D: {
    printf("a(%d)\n", a);
  } break;
  }
}

int main(void) {
  big_switch(2);
  return 0;
}
```

with compile parameters `arm-none-eabi-gcc -Os -mcpu=3Dcortex-m4 ./switch.c`
the asm is lighter with GCC7.3.1 than with GCC10.3.1.


While GCC7.3.1 (20 lines)
```asm
00008134 <big_switch>:
    cmp     r0, #29
    push    {r4, lr}
    mov     r4, r0
    bhi.n   8146 <big_switch+0x12>
    movs    r2, #1
    ldr     r3, [pc, #28]   ; (815c <big_switch+0x28>)
    lsls    r2, r0
    ands    r3, r2
    cbnz    r3, 814e <big_switch+0x1a>
    mov     r1, r4
    ldr     r0, [pc, #20]   ; (8160 <big_switch+0x2c>)
    bl      8264 <printf>
    mov     r1, r4
    ldr     r0, [pc, #16]   ; (8164 <big_switch+0x30>)
    ldmia.w sp!, {r4, lr}
    b.w     8264 <printf>
    nop
    .word   0x37a6e466
    .word   0x0000f5f8
    .word   0x0000f600
```

GCC10.3.1 (24 lines)
```asm
0000813c <big_switch>:
    subs    r3, r0, #1
    push    {r4, lr}
    mov     r4, r0
    cmp     r3, #28
    bhi.n   8168 <big_switch+0x2c>
    tbb     [pc, r3]
    .short  0x1313
    .word   0x13130f0f
    .word   0x130f0f0f
    .word   0x13130f0f
    .word   0x13130f13
    .word   0x0f130f0f
    .word   0x13131313
    .short  0x130f
    .byte   0x13
    .byte   0x00
    mov     r1, r0
    ldr     r0, [pc, #16]   ; (817c <big_switch+0x40>)
    bl      828c <printf>
    mov     r1, r4
    ldr     r0, [pc, #8]    ; (817c <big_switch+0x40>)
    ldmia.w sp!, {r4, lr}
    b.w     828c <printf>
    .word   0x0001011c
```

Not a big difference in term of instructions is this case but as much as the
switch increases, the difference becomes huge (in my case it switched from =
75
to 94 lines)
Code size increases of about 10%.


@Martin, do you know if this is linked?=