Georg-Johann Lay writes:
> On 02.08.2016 06:50, Senthil Kumar Selvaraj wrote:
>>
>> Denis Chertykov writes:
>>
>>> 2016-08-01 15:17 GMT+03:00 Georg-Johann Lay <a...@gjlay.de>:
>>>> Problem with -fcaller-saves is that there are situations where it triggers
>>>> an expensive frame just to store a variable around a function call even
>>>> though there are plenty of call-saved registers.
>>>>
>>>> Example:
>>>>
>>>> typedef __UINT8_TYPE__ uint8_t;
>>>>
>>>> extern uint8_t uart0_getc (void);
>>>>
>>>> void foo (uint8_t *buffer, uint8_t cnt)
>>>> {
>>>> while (--cnt)
>>>> {
>>>> *buffer++ = uart0_getc();
>>>> }
>>>> }
>>>>
>>>> $ avr-gcc -Os -S -dp -mmcu=atmega8 loop-buf.c
>>>>
>>>> $ avr-gcc gcc -B$TV -Os -c -save-temps -dp -mmcu=atmega8 loop-buf.c &&
>>>> avr-size loop-buf.o
>>>> text data bss dec hex filename
>>>> 50 0 0 50 32 loop-buf.o
>>>>
>>>> $ avr-gcc -Os -c -save-temps -dp -mmcu=atmega8 loop-buf.c -fno-caller-saves
>>>> && avr-size loop-buf.o
>>>> text data bss dec hex filename
>>>> 32 0 0 32 20 loop-buf.o
>>>>
>>>> I actually came never across a situation where -fcaller-saves improved the
>>>> code performance, hence this patch proposes to switch off -fcaller-saved
>>>> per
>>>> default.
>>
>> Like you mentioned in the bug report, would fixing the costs be a better
>> way to fix this rather than a blanket disabling of the option?
>
> What costs specifically? Where could the costs of different epilogues /
> prologues be described?
I don't know either - I'd have to dig in to find that out too. Let me
check that out.
Regards
Senthil
