On 07.12.2016 13:42, Segher Boessenkool wrote:
On Wed, Dec 07, 2016 at 01:35:59PM +0100, Georg-Johann Lay wrote:
So the constant 317 made its way into the code of mod_mul (insn 14),
and insn 9 also uses this value.
>From my understanding "noclone" should avoid any such propagations?
noclone prevents cloning the function, and it indeed has not been cloned.
Because the function is static, GCC knows all callers, and it determines
all callers have n=317, so it optimises with that knowledge.
I understand this. What I am after is a means to get a generated
code that matches the source, no matter what context.
Don't make the function static, or preferable compile it completely
separately (in its own translation unit), and make sure there is no LTO
done on it either?
Imagine you called that function from assembler, then the code
would crash.
You cannot (validly) call it from assembler code the way it is now, the
function is local to its translation unit.
Ja, I'm aware of this.
I just wrote "imagine". I'd like to have a means to avoid such
propagations, so that the would would work "even if" it was
called from a different context (like global asm in the same
modul). Don't misinterpret this as if I'd intend to write
such code, I just tried to describe what I am trying to achieve
by means of noinline + noclone + some-magic-attribute
As I understand you, there is NO such way -- even though the docs
for noclone might lead to the (wrong) conclusion noclone would
avoid inter-procedural of constants or value-ranges.
And yes, when I am adding some other usage of
mod_mul or let escape its address, then the propagation is not
performed.
For example, this might have impact on writing test for GCC:
When I am writing a test with noinline + noclone then my
expectation is that no such propagation happens, because
otherwise a test might turn trivial...
The usual ways to prevent that are to add some volatile, or an
asm("" : "+g"(some_var)); etc.
Ja, I'm aware of this.
So the bottom line re $subject is:
"No, there is no general way on GCC to avoid inter-procedural constant
propagations for specific functions."
Thanks.
Johann
