RE: [PATCH 06/12] openrisc: Use of_get_cpu_hwid()

From: Segher Boessenkool

Sent: 06 October 2021 22:27

On Thu, Oct 07, 2021 at 05:44:00AM +0900, Stafford Horne wrote:

quoted

You have defined of_get_cpu_hwid to return u64, will this create compiler
warnings when since we are storing a u64 into a u32?

It seems only if we make with W=3.

Yes.  This is done by -Wconversion, "Warn for implicit conversions that
may alter a value."

quoted

I thought we usually warned on this.

The microsoft compiler does - best to turn all those warnings off.

This warning is not in -Wall or -Wextra either, it suffers too much from
false positives.  It is very natural to just ignore the high bits of
modulo types (which is what "unsigned" types *are*).  Or the bits that
"fall off" on a conversion.  The C standard makes this required
behaviour, it is useful, and it is the only convenient way of getting
this!

I've also seen a compiler convert:
	struct->char_member = (char)(int_val & 0xff);
into:
	reg = int_val;
	reg &= 0xff; // for the & 0xff
	reg &= 0xff; // for the cast
	struct->char_member = low_8bits(reg);

You really don't want the extra noise.

I'll bet that (char)int_val is actually an arithmetic expression.
So its type will be 'int'.

	David

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