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Keil -O0 and -O1 optimize

Mr.Anderson

My code

while(usart_transmit_status);
usart_transmit_status = 1;

translate at -O0

0x08001EB4 4807      LDR      r0,[pc,#28]  ; @0x08001ED4
0x08001EB6 7800      LDRB     r0,[r0,#0x00]
0x08001EB8 2800      CMP      r0,#0x00
0x08001EBA D1FB      BNE      0x08001EB4

and translate at -O1

0x08001A6E 4907      LDR      r1,[pc,#28]  ; @0x08001A8C
0x08001A70 7808      LDRB     r0,[r1,#0x00]
0x08001A72 2800      CMP      r0,#0x00
0x08001A74 D1FD      BNE      0x08001A72

It leads to an error because R0 changes in interruption

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  • 0

    Just for the record: I believe you have the two compiler outputs the wrong way round.  The -O1 version looks exactly like -O0 code would: ponderously re-doing every single step of the loop, without the slightest consideration for efficiency.  The code you show as -O0 has received a bit of optimization: steps that (for non-volatile input) would yield the exact same result every time have been short-circuited.

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  • 0

    Just for the record: I believe you have the two compiler outputs the wrong way round.  The -O1 version looks exactly like -O0 code would: ponderously re-doing every single step of the loop, without the slightest consideration for efficiency.  The code you show as -O0 has received a bit of optimization: steps that (for non-volatile input) would yield the exact same result every time have been short-circuited.

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