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Re: x86: combined usage of "-Os -mregparm=3" leads to broken codes


On Tue, Sep 23, 2014 at 11:39 AM, Bin Meng <bmeng.cn@gmail.com> wrote:
> On Tue, Sep 23, 2014 at 5:11 PM, Bin Meng <bmeng.cn@gmail.com> wrote:
>>
>> Sorry I still don't get it. The inline-asm codes are put in the very end
>> of the test function just before return, so the clobbered registers are
>> not used by gcc. The %edx gets overwritten before the inline-asm block.
>>
>> I just did a modification to the inline-asm codes to indicate to gcc that
>> %edx is clobbered, but it did not help.
>>
>
> PS: if turning on WORKAROUND define in my test code, the generated codes
> are correct.
>
> I believe the issue is not with the inline assembly constraints but with
> '-Os' plus '-regparm=3' combination. With '-Os' gcc optimization goes too
> far to generate nothing for the C code 'params.a = d;' and gcc simply
> takes %edx (supposed to be the 2nd parameter of test function) for a
> scratch register.

Well, you don't use 'params'.  the "m" (params_ptr) constraint only
puts the pointer in memory, it doesn't make the pointed-to value
used.

Richard.

> Regards,
> Bin


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