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Subject: Re: what is the difference with and without crc extension support
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To: Dongjiu Geng <gengdongjiu1@gmail.com>, gcc@gcc.gnu.org
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From: Richard Earnshaw <Richard.Earnshaw@foss.arm.com>
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On 03/03/2022 13:41, Dongjiu Geng via Gcc wrote:
> Hi,
>       My program does not use CRC instructions，but I find the compiled
> binary has much difference between using "-march=armv8-a+crc" and
> using "-march=armv8-a". Even stranger, when I use
> "-march=armv8-a+crc", I find my compiled binary can not run. but when
> I change -O2 to -O0 based on "-march=armv8-a+crc",  it can run. I  do
> not  know what is the reason.. can you answer it? Thanks.

The most common reason for a program failing to run when the optimizer 
is turned on is that it contains undefined behaviour.  Make sure that 
you turn on GCC's warning options and fix anything that these report.

It's very unlikely that the change of -march=armv8-a to 
-march=armv8-a+crc is the reason for the differences in compiled code 
that you are seeing.  Something else to try here is to identify an 
object file that is different between the two options and then rebuild 
it both ways, but add "-g".  You can then use

   readelf -debug-dump=str <object-file>

to print out the real options used by the compiler (there may be some 
additional strings reported in the dump, but the options are the bit 
that is interesting).  The only difference should be the -march option 
you mentioned above.

R.