IBM z13s Technical Manual page 62

To obtain the best performance, you must specify an architecture level of 11 by using the
–qarch=arch11 option for the XL C/C++ for Linux on z Systems compiler or the ARCH(11)
option for the other compilers. This option grants the compiler permission to use machine
instructions that are available only on z13s servers.
Because specifying the architecture level of 11 results in a generated application that uses
instructions that are available only on the z13s or z13 servers, the application will not run on
earlier versions of the hardware. If the application must run on z13s servers and on older
hardware, specify the architecture level corresponding to the oldest hardware on which the
application needs to run. For more information, see the documentation for the ARCH or -qarch
options in the guide for the corresponding compiler product.
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IBM z13s Technical Guide