Experimental Physics and
Industrial Control System

Dirk Zimoch <[email protected]> · Wed, 3 Oct 2018 16:12:24 +0200

I had removed all the -fno-strict-aliasing flags from my configurations

because I thought it was not necessary any more with EPICS 7, but the

warnings are back:

/usr/local/epics/base-7.0.1/include/pv/anyscalar.h:111: warning:

dereferencing pointer ‘<anonymous>’ does break strict-aliasing rules

As I understand, strict aliasing rules allow the compiler to do some

aggressive optimization by assuming an assignment to a pointer of type

A* can never change a variable of type B, given A and B are sufficiently

different types. Thus a variable of type B can be safely kept in a

register and does not need to be reloaded from memory only because a

pointer of type A* has been written to.

The option -fno-strict-aliasing switches off that optimization and thus

the warning as well.

But I don't know if the warning means that the optimizer will not touch

the expressions that break strict-aliasing rules or if the optimizer

will do its work anyway, potentially leading to very subtle and hard to

debug run-time errors.

I propose to either

a) Fix the code and be careful to never break strict-aliasing rules, i.e

fix it immediately when such warnings pop up.

b) Use -fno-strict-aliasing for all compilers that support it (gcc 3+ ?)

Dirk

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System