[General] GCC v10.3 use more stack size than GCC v6.3 for a function

[yacoutm]

I have an application which I compile using GCC v6.3, when I compile it with GCC v10.3 I find some functions use more stack size than what GCC v6.3 uses, for example this function uses zero stack for GCC v6.3 but uses 8 bytes for GCC v10.3

int func(int *x, int y, int z, int a)
{
   switch (y) {
       case 1:
           *x = a;
           y = z;
           return y;
   }
}

GCC v6.3 assembly:

int func(int *x, int y, int z, int a)
{
   switch (y) {
  0:   2901            cmp     r1, #1
  2:   d101            bne.n   8 <func+0x8>
       case 1:
           *x = a;
  4:   6003            str     r3, [r0, #0]
           y = z;
           return y;
  6:   0010            movs    r0, r2
   }
}
8: 4770 bx lr

and GCC v10.3 assembly:

int func(int *x, int y, int z, int a)
{
  0:   b510            push    {r4, lr}
  2:   0004            movs    r4, r0
  4:   0010            movs    r0, r2
   switch (y) {
  6:   2901            cmp     r1, #1
  8:   d100            bne.n   c <func+0xc>
       case 1:
           *x = a;
  a:   6023            str     r3, [r4, #0]
           y = z;
           return y;
   }
}
c: bd10 pop {r4, pc}

In GCCv10.3, it push {r4, lr} which is not the case in GCC v6.3
so why does this happen?, it costs the application more stack area over the old compiler, so how to avoid it to get less stack size?
also, why did it stack the lr although it's a leaf function?
plus, why does it return z in all cases although the c code returns it inside the case condition?

Notes:
1. This function is a dummy one just to reproduce the issue, so don't consider rewriting it.
2. Building flags are:
arm-none-eabi-gcc -O0 -c -std=c99 -fmessage-length=0 -fomit-frame-pointer -Wno-aggressive-loop-optimizations -Werror -Werror=strict-prototypes -pedantic-errors -Wconversion -pedantic -Wall -Wextra -Wno-unused-function -Wextra -Wpointer-arith -Wsign-compare -Wswitch -Wno-maybe-uninitialized -fno-strict-aliasing -fshort-wchar -mfix-cortex-m3-ldrd -gdwarf-3 -gstrict-dwarf -mabi=aapcs -mthumb -mcpu=Cortex-M0 -g3 -Os -mthumb -ffunction-sections -fdata-sections -MMD -MP -MF"xyz.d" -MT"xyz.o"