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From: "dumoulin.thibaut at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c/101995] New: regression built-in memset missed-optimization
 arm -Os
Date: Fri, 20 Aug 2021 09:35:53 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101995

            Bug ID: 101995
           Summary: regression built-in memset missed-optimization arm -Os
           Product: gcc
           Version: 10.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dumoulin.thibaut at gmail dot com
  Target Milestone: ---

For cortex-m4 -Os, GCC10 produces bigger assembly code than GCC7 when memse=
t is
called.

Here is the C code example to trigger the regression:

```C
#include <stdio.h>
#include <string.h>

struct foo_t {
  int a;
  int b;
  int c;
  int d;
};

/* Random function modifying foo with another value than 0 */
void doStuff(struct foo_t *foo) {
  foo->b =3D foo->a + foo->c;
}

void twoLinesFunction(struct foo_t *foo) {
  /* R0 is saved in GCC10 but not in GCC7 */
  memset(foo, 0x00, sizeof(struct foo_t));
  doStuff(foo);
}

int main(void) {
  struct foo_t foo;
  twoLinesFunction(&foo);
  return 0;
}
```

compile command: `gcc -Os -mcpu=3Dcortex-m4`

GCC7.3.1 produces:
```asm
<twoLinesFunction>:
    push    {r3, lr}
    movs    r2, #16
    movs    r1, #0
    bl      8168 <memset>
    ldmia.w sp!, {r3, lr}
    b.w     8104 <doStuff>
```

While GCC10.3.0 produces:
```asm
<twoLinesFunction>:
    push    {r4, lr}
    movs    r2, #16
    mov     r4, r0        --> backup r0
    movs    r1, #0
    bl      8174 <memset>
    mov     r0, r4        --> restore r0
    ldmia.w sp!, {r4, lr}
    b.w     810c <doStuff>
```

Main function remains the same.

The builtin memset function does not change R0 so there is no need to save =
it
and restore it later. GCC7 is more efficient.
GCC10 should not backup R0 for this builtin function in this case, it produ=
ces
slower code.

There is this PR https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D61241 which=
 is
also referring to this behavior with a patch to implement the optimization =
but
I'm not sure when this optimization has been wiped out.=