llvm dev - Jun 2017 - How to prevent optimizing away a call + its arguments

Hi llvm-dev,

I have a C function:

__attribute__((__visibility__("default")))
__attribute__((used))
__attribute__((noinline))
void please_do_not_optimize_me_away(int arg1, void *arg2) {
  asm volatile("" :::);
}

(the purpose is that this function will be used dynamically at runtime, perhaps
by interposing the function, or via the debugger)

I really thought this will not get optimized out, but I've realized (the
hard way) that LLVM will happily optimize a call to this function, and replace
all arguments with undef, because it figures out that they're not really
needed.

I'm going to fix this by passing the arguments explicitly as inputs to the
asm, but is that expected?  Is there any more reasonable way (attribute) of
telling that the compiler should really not expect anything from the body of the
function, not assume that it's not doing anything, and not optimizing out
arguments?

Thanks,
Kuba

Hi Kuba,

Try:

__attribute__(optnone)

See
https://clang.llvm.org/docs/AttributeReference.html#optnone-clang-optnone

-- 
Mehdi


2017-06-21 16:16 GMT-07:00 Kuba Mracek via llvm-dev <llvm-dev at
lists.llvm.org
>:
> Hi llvm-dev,
>
> I have a C function:
>
> __attribute__((__visibility__("default")))
> __attribute__((used))
> __attribute__((noinline))
> void please_do_not_optimize_me_away(int arg1, void *arg2) {
>   asm volatile("" :::);
> }
>
> (the purpose is that this function will be used dynamically at runtime,
> perhaps by interposing the function, or via the debugger)
>
> I really thought this will not get optimized out, but I've realized
(the
> hard way) that LLVM will happily optimize a call to this function, and
> replace all arguments with undef, because it figures out that they're
not
> really needed.
>
> I'm going to fix this by passing the arguments explicitly as inputs to
the
> asm, but is that expected?  Is there any more reasonable way (attribute) of
> telling that the compiler should really not expect anything from the body
> of the function, not assume that it's not doing anything, and not
> optimizing out arguments?
>
> Thanks,
> Kuba
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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On Wed, Jun 21, 2017 at 05:25:04PM -0700, Mehdi AMINI via llvm-dev
wrote:
> Hi Kuba,
> 
> Try:
> 
> __attribute__(optnone)
> 
> See
> https://clang.llvm.org/docs/AttributeReference.html#optnone-clang-optnone
Actually, it should be enough to use:

__attribute__((noinline))
void please_do_not_optimize_me_away(int arg1, void *arg2) {
  asm volatile("":::"memory");
}

Creating a real barrier is important.

Joerg