[llvm-dev] How to prevent optimizing away a call + its arguments

[David Majnemer via llvm-dev]

noinline should, and does, not mean "do not do IPO"; we will still do IPCP.
The easiest way to defeat IPO is to use __attribute__((weak)) as it makes
isDefinitionExact false: https://godbolt.org/g/VVBcgF

On Thu, Jun 22, 2017 at 10:35 AM, David Blaikie via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> optnone should work, but really noinline should probably (Chandler: Can
> you confirm: is it reasonable to model noinline as "no interprocedural
> analysis across this function boundary" (so FunctionAttrs should do the
> same thing for noinline as it does for optnone, for example? ie: not derive
> any new attributes) - allowing the function to be optimized internally
> (unlike optnone) but not allowing interprocedural analysis inside the
> function to be used in callers (unlike optnone)) work as well?
>
>
> On Thu, Jun 22, 2017 at 7:35 AM Joerg Sonnenberger via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> 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
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>
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