Problem understand optimization with inline asm

[Kalamatee <kalamatee@gmail.com>]

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Good afternoon,

I have spent a while debugging an issue in an operating system I contribute
to, and it comes down to the behaviour of optimizing code.

We have code which is compiled for a native, or "hosted" (e.g. under linux)
environment, and have code which does the following -:

foo(struct mystruct1 *bar)
{
 int somevar = 0; // variable is initialized.
int anothervar = 0;
#if (some clause to check if it is not hosted)
asm volatile(
<some asm block which sets "somevar" if a feature is enabled, otherwise
leaves it as its initialized state>
: "=m" (somevar)
);
#endif
if (somevar)
{
//use the feature
 bar->param = 20;
anothervar = 1;
}
if (!anothervar)
{
 bar->param = 10;
}
)


Without optimization, the code works as intended for both the native and
hosted cases, but once optimization is enabled the following things appear
to happen.

(1) - gcc concludes the asm always overwrites somevar, and optimizes the
initialization away leaving it uninitialized.
(2) it doesn't now warn that the code is using an uninitialized variable
but silently compiles it as if all is well.
(3) at runtime if (somevar) has random contents, so runs the //use feature
case when it shouldnt.

Now, I can work around this by using "+" (even though I don't read the
variable), or initializing the value a second time in the asm block - but
should I have to? This sounds like the compiler is doing the wrong thing
based on the assertion/assumption "=" means I will 100% change some
variable, and worse after optimizing the code and creating a case where I
now have an uninitialized variable being used - does not warn me this is
happening?

Would it not be better to interpret "=" to mean "may" change the value,
which would prevent it incorrectly optimizing the initialization away, or
if it does warn that the value may be used uninitialized?

Yours,
Nick Andrews.