Would you provide a test case, please?  Cf. https://gcc.gnu.org/bugs/#need
For example there is nothing like "panVel" in your code.

all code samples from https://github.com/night-ghost/minimosd-extra/tree/master/MinimOsd_Extra. When I try to isolate a piece of code with a bug in one file, it is compiled differently than in their native place.

Same stack usage in this code:

#include <Arduino.h>

#define SCK_PIN   13
#define MISO_PIN  12
#define MOSI_PIN  11

class SPI
{
  public:
    SPI(void);
     static byte transfer(byte);
};

SPI::SPI()
{
  // initialize the SPI pins
  pinMode(SCK_PIN, OUTPUT);
  pinMode(MOSI_PIN, OUTPUT);
  pinMode(MISO_PIN, INPUT);
}

byte SPI::transfer(byte value){
  SPDR = value;
  while (!(SPSR & (1<<SPIF))) ;
  return SPDR;
}

void MAX_write(byte addr, byte data){
    register byte d=data;
    SPI::transfer(addr);
    SPI::transfer(d);
}

will be 

void MAX_write(byte addr, byte data){
     f06:<----->cf 93       <-->push<-->r28
     f08:<----->df 93       <-->push<-->r29
     f0a:<----->1f 92       <-->push<-->r1
     f0c:<----->cd b7       <-->in<---->r28, 0x3d<----->; 61
     f0e:<----->de b7       <-->in<---->r29, 0x3e<----->; 62
    register byte d=data;
    SPI::transfer(addr);
     f10:<----->69 83       <-->std<--->Y+1, r22<------>; 0x01
     f12:<----->0e 94 e2 2e <-->call<-->0x5dc4<>; 0x5dc4 <_ZN3SPI8transferEh>
    SPI::transfer(d);
     f16:<----->69 81       <-->ldd<--->r22, Y+1<------>; 0x01
     f18:<----->86 2f       <-->mov<--->r24, r22
     f1a:<----->0e 94 e2 2e <-->call<-->0x5dc4<>; 0x5dc4 <_ZN3SPI8transferEh>
}
     f1e:<----->0f 90       <-->pop<--->r0
     f20:<----->df 91       <-->pop<--->r29
     f22:<----->cf 91       <-->pop<--->r28
     f24:<----->08 95       <-->ret

as one can see, "data" variable saved in stack, not in register

You can follow the bug reporting instructions an provide the preprocessed code and the compiler output as described in https://gcc.gnu.org/bugs/#need

Just add -v -save-temps to the compiler's command line options, re-build the project, and supply the output of the compiler and the generated *.i file (in case of C) resp. *.ii file (in case of C++).

This is needed because you have to resolve the non-standard stuff like "INPUT" or "Ardiuno.h".  There's no need that you change the suorces to accomplish this (if you can manage to find a smalle test case, this is nice but not mandatory).

Created attachment 38550 [details]
testcase in attachment

for AVR platform arduino.h is the most that neither is a standard :)

Maybe -fno-caller-saves is what you are looking for?

Here is a C test case guessed from your first code snipped:

typedef struct
{
    unsigned char x, y;
} point;

extern void printSpeed (long, unsigned char);
extern long cnvGroundSpeed (void);

void panVel (point p)
{
    printSpeed (cnvGroundSpeed(), p.y & 0x40);
}

compiled with avr-gcc 5.x

$ avr-gcc -S -Os

we'll get

panVel:
	push   r28
	push   r29
	push   __zero_reg__
	in r28,__SP_L__
	in r29,__SP_H__

	mov   r20,r25
	andi  r20,lo8(64)
	std   Y+1,r20
	rcall cnvGroundSpeed
	ldd   r20,Y+1

	pop   __tmp_reg__
	pop   r29
	pop   r28
	rjmp  printSpeed


Adding -fno-caller-saves:

panVel:
	push  r28

	mov   r28,r25
	andi  r28,lo8(64)
	rcall cnvGroundSpeed
	mov   r20,r28

	pop   r28
	rjmp  printSpeed


I actually don't know whether this is a flaw in the avr backend (like a cost issue) or wrong assumptions in the middle-end.  Saving / Restoring in the frame is actually not more costly than saving in a call-saved register;  what makes it expensive is the frame setup in prologue and epilogue...

Thank you again for such good key - it saves me 150 bytes. Moreover this key is not listed in GCC's possible optimisations for AVR. 


PS. Why it not included by default to -Os? What can be affected by it?

Author: gjl
Date: Wed Aug  3 15:46:11 2016
New Revision: 239080

URL: https://gcc.gnu.org/viewcvs?rev=239080&root=gcc&view=rev
Log:
	PR 70677
	* common/config/avr/avr-common.c (avr_option_optimization_table)
	[OPT_LEVELS_ALL]: Turn off -fcaller-saves.


Modified:
    branches/gcc-6-branch/gcc/ChangeLog
    branches/gcc-6-branch/gcc/common/config/avr/avr-common.c

Author: gjl
Date: Thu Aug  4 07:52:38 2016
New Revision: 239117

URL: https://gcc.gnu.org/viewcvs?rev=239117&root=gcc&view=rev
Log:
	PR 70677
	* common/config/avr/avr-common.c (avr_option_optimization_table)
	[OPT_LEVELS_ALL]: Turn off -fcaller-saves.


Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/common/config/avr/avr-common.c

For 6.2+, use -fno-caller-saves by default.  It still can be re-enabled by explicit -fcaller-saves.

*** Bug 52025 has been marked as a duplicate of this bug. ***