Set TARGET_OMIT_STRUCT_RETURN_REG to true

Set TARGET_OMIT_STRUCT_RETURN_REG to true

The reason this caught my eye on aarch64 is because
the return value register (x0) is not identical to the register in which
the hidden parameter for AArch64 is set (x8). Thus setting this to true
seems to be quite reasonable and shaves off 100 odd mov x0, x8's from
cc1 in a bootstrap build.

I don't expect this to make a huge impact on performance but as they
say every little counts.  The AAPCS64 is quite explicit about not
requiring that the contents of x8 be kept live.

Bootstrapped and regression tested on aarch64.

Ok to apply ?

Ramana

gcc/
* config/aarch64/aarch64.c (TARGET_OMIT_STRUCT_RETURN_REG): Set to
true.

gcc/testsuite

* gcc.target/aarch64/struct_return.c: New test.

From-SVN: r236197