Builtin memcpy() requirements

[Sebastian Huber]

Hello,

while working on a test case for

https://devel.rtems.org/ticket/3767

I tried to find the right test suite for it. At first I thought that 
this is a unit test 
(testsuites/unit/compiler/tc-misaligned-builtin-memcpy.c). However, I 
now think that we should have a requirement for this function. The test 
case is more or less clear:

T_TEST_CASE(MisalignedBuiltinMemcpy)
{
	double a;
	double b;
	char buf[2 * sizeof(double)];
	void *p;

	p = &buf[0];
	p = (void *)((uintptr_t)p | 1);
	RTEMS_OBFUSCATE_VARIABLE(p);
	a = 123e4;
	RTEMS_OBFUSCATE_VARIABLE(a);
	a *= a;
	memcpy(p, &a, sizeof(a));
	RTEMS_OBFUSCATE_VARIABLE(p);
	memcpy(&b, p, sizeof(b));
	T_eq(a, b, "%f == %f", a, b);
}

It is obvious that such a memcpy() operation should work. In general, we 
assume in RTEMS that the compiler does the right job. However, on some 
architectures we have to select the right compiler options and this is 
clearly under full control of the RTEMS BSP. I think we should add 
something like this to the RTEMS requirements:

A call of memcpy() in C from a pointer to a double value to a misaligned 
destination buffer shall store the double value in the destination buffer.

The formulation is quite specific, but I was not able to formulate it 
more generic and keep it testable. We have to keep in mind here that we 
are not interested in the memcpy() C library function, but rather the 
compiler optimized code for the memcpy() builtin function.

An architecture-specific requirement could be:

The alignment exception shall terminate the system with a fatal error.

If we use the wrong compiler options we may not fulfil both requirements 
on some platforms.

-- 
Sebastian Huber, embedded brains GmbH

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