[Bug c/101995] New: regression built-in memset missed-optimization arm -Os

[Bug c/101995] New: regression built-in memset missed-optimization arm -Os

[dumoulin.thibaut at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101995

            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 memset 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 = 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=cortex-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 produces
slower code.

There is this PR https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61241 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.

[Bug middle-end/101995] regression built-in memset missed-optimization arm -Os

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101995

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c                           |middle-end
           Keywords|                            |needs-bisection, ra

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
The RA used to know how to rematerialize a register from the memset return
value.
Sth made that no longer work it seems.

[Bug rtl-optimization/101995] [9/10/11/12 Regression] regression built-in memset missed-optimization arm -Os

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101995

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|                            |8.2.0
      Known to fail|                            |9.1.0
   Target Milestone|---                         |9.5
            Summary|regression built-in memset  |[9/10/11/12 Regression]
                   |missed-optimization arm -Os |regression built-in memset
                   |                            |missed-optimization arm -Os

[Bug rtl-optimization/101995] [9/10/11/12 Regression] regression built-in memset missed-optimization arm -Os

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101995

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2021-08-21
             Target|arm                         |arm, aarch64
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
On x86_64 for some reason it works in GCC 9+; tested with " -Os
-mstringop-strategy=libcall" and adding noinline to doStuff just to force to
make sure it does not inline.

Anyways confirmed, worked in GCC 8.2.0 for both aarch64 and arm but does not
work in GCC 9.

[Bug rtl-optimization/101995] [9/10/11/12 Regression] regression built-in memset missed-optimization arm -Os

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101995

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
REG_RETURNED is no longer there.

Looks like there is an extra move which caused the code to be different.

For aarch64 we have:
(insn 18 4 2 2 (set (reg:DI 94)
        (reg:DI 0 x0 [ foo ])) "/app/example.cpp":17:42 47 {*movdi_aarch64}
     (nil))
(insn 2 18 3 2 (set (reg/v/f:DI 90 [ foo ])
        (reg:DI 94)) "/app/example.cpp":17:42 47 {*movdi_aarch64}
     (expr_list:REG_DEAD (reg:DI 94)
        (nil)))
Before the call to memset.

[Bug rtl-optimization/101995] [9/10/11/12 Regression] regression built-in memset missed-optimization arm -Os

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101995

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note I suspect it is r9-3594 is what makes the difference here. Also this is
just very fragile really.

[Bug rtl-optimization/101995] [9/10/11/12 Regression] regression built-in memset missed-optimization arm -Os

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101995

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
For reference, it's find_call_crossed_cheap_reg finding (or not) such
candidate.

[Bug rtl-optimization/101995] [9/10/11/12 Regression] regression built-in memset missed-optimization arm -Os since r9-3594

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101995

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|needs-bisection             |
                 CC|                            |marxin at gcc dot gnu.org,
                   |                            |segher at gcc dot gnu.org
            Summary|[9/10/11/12 Regression]     |[9/10/11/12 Regression]
                   |regression built-in memset  |regression built-in memset
                   |missed-optimization arm -Os |missed-optimization arm -Os
                   |                            |since r9-3594

--- Comment #6 from Martin Liška <marxin at gcc dot gnu.org> ---
Really started with r9-3594.

[Bug rtl-optimization/101995] [9/10/11/12 Regression] regression built-in memset missed-optimization arm -Os since r9-3594

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101995

--- Comment #7 from Segher Boessenkool <segher at gcc dot gnu.org> ---
I don't see any problem with aarch64 fwiw.

If RA decides it does not want to tie the new pseudo to the argument
register, it may have a reason for it?  Or suboptimal heuristics.

What is this REG_RETURNED thing?

[Bug rtl-optimization/101995] [9/10/11/12 Regression] regression built-in memset missed-optimization arm -Os since r9-3594

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101995

--- Comment #8 from Segher Boessenkool <segher at gcc dot gnu.org> ---
Also, what is fragile here?  This is *removing* fragility and premature
choices!

[Bug rtl-optimization/101995] [9/10/11/12 Regression] regression built-in memset missed-optimization arm -Os since r9-3594

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101995

--- Comment #9 from Segher Boessenkool <segher at gcc dot gnu.org> ---
(In reply to Segher Boessenkool from comment #7)
> What is this REG_RETURNED thing?

Ah, something added in ira-lives.c, and you call *that* code fragile?
I agree :-)

[Bug rtl-optimization/101995] [9/10/11/12 Regression] regression built-in memset missed-optimization arm -Os since r9-3594

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101995

--- Comment #10 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
>I don't see any problem with aarch64 fwiw.
I have to try it on aarch64 but it failed there at one point.

(In reply to Segher Boessenkool from comment #8)
> Also, what is fragile here?  This is *removing* fragility and premature
> choices!

(In reply to Segher Boessenkool from comment #9)
> (In reply to Segher Boessenkool from comment #7)
> > What is this REG_RETURNED thing?
> 
> Ah, something added in ira-lives.c, and you call *that* code fragile?
> I agree :-)

You got to what I thought was fragile in the end. And yes the whole
REG_RETURNED mechanism seems very fragile (not the change r9-3594 ). I could
not figure out how it was being known to be set on which function call even.

[Bug rtl-optimization/101995] [9/10/11/12 Regression] regression built-in memset missed-optimization arm -Os since r9-3594

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101995

--- Comment #11 from Segher Boessenkool <segher at gcc dot gnu.org> ---
Yeah, that could be much more robust.

OTOH this stuff is completely opportunistic in the first place: it handles
only function return values, not any other hard registers (like local
register vars).

[Bug rtl-optimization/101995] [9/10/11/12 Regression] regression built-in memset missed-optimization arm -Os since r9-3594

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101995

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2

[Bug rtl-optimization/101995] [10/11/12/13 Regression] regression built-in memset missed-optimization arm -Os since r9-3594

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101995

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|9.5                         |10.4

--- Comment #12 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 9 branch is being closed

[Bug rtl-optimization/101995] [10/11/12/13 Regression] regression built-in memset missed-optimization arm -Os since r9-3594

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101995

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|10.4                        |10.5

--- Comment #13 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 10.4 is being released, retargeting bugs to GCC 10.5.

[Bug rtl-optimization/101995] [11/12/13/14 Regression] regression built-in memset missed-optimization arm -Os since r9-3594

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101995

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|10.5                        |11.5

--- Comment #14 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 10 branch is being closed.