https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70677
--- Comment #2 from night_ghost at ykoctpa dot ru --- 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
