bug in longlong.h for aarch64 sub_ddmmss

[Vincent Lefevre]

On 2020-06-16 16:11:53 +0200, Vincent Lefevre wrote:
> Note that I'm rather surprised to see that the issue disappears
> if I add unrelated code. For instance, in the following testcase,
> with GCC 9.1.0 -O2, I get "h = 0xffffffffffffffff", which is
> incorrect, but if I define V, I get the correct "h = 0x0".

Defining V apparently disables some optimization, so that only
the "else" case of sub_ddmmss is used.

Note: in the tests, it is important to test the macro on constants
in order to test the "if" case.

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