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[avr-gcc-list] Volatile bad compiled while

From: max2009tiny
Subject: [avr-gcc-list] Volatile bad compiled while
Date: Wed, 25 Nov 2009 22:57:10 -0800 (PST)

Hi everybody. I make project on Tiny2313 and use 

volatile register unsigned char lastrec asm("r8")

and lastrec is modified on rs232 rx isr.

When compile little function
__attribute__ ((noinline)) void waitACK(u08 cmack)
while(cmack!=lastrec) ; //wait for reply request

compiler make bad asm
  a0:   98 2d           mov     r25, r8
  a2:   98 17           cp      r25, r24
  a4:   e9 f7           brne    .-4             ; 0xa2 <waitACK>

To avoid i change while to asm as follow, but i need know why this gcc make.

000000a0 <waitACK>:
__attribute__ ((noinline)) void waitACK(u08 cmack)
// cmd to wait is in txcmd
// use std ack packet 0x4 0x0 0x2 0xresult 0xtxcmd 0xcksum

asm(" mov       r25, r8 ;\n cp r25, r24 ;\n brne .-6 ;");
  a0:   98 2d           mov     r25, r8
  a2:   98 17           cp      r25, r24
  a4:   e9 f7           brne    .-6             ; 0xa0 <waitACK>

//while(cmack!=lastrec) ;       //wait for reply request
  a6:   08 95           ret

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