[llvm-dev] Prevent LLVM optimizations from erasing unused basic blocks

[Gleb Popov via llvm-dev]

On Tue, Oct 9, 2018 at 11:42 PM Friedman, Eli <efriedma at codeaurora.org>
wrote:

> On 10/9/2018 1:03 PM, Gleb Popov wrote:
>
>
>
> On Tue, Oct 9, 2018 at 10:39 PM Friedman, Eli <efriedma at codeaurora.org>
> wrote:
>
>> On 10/9/2018 11:58 AM, Gleb Popov wrote:
>>
>>
>>
>> On Tue, Oct 9, 2018 at 9:39 PM Friedman, Eli <efriedma at codeaurora.org>
>> wrote:
>>
>>> On 10/9/2018 11:31 AM, Gleb Popov via llvm-dev wrote:
>>> > Hello LLVM Devs.
>>> >
>>> > In my compiler I attach some arbitrary data to functions by creating
>>> > BBs with inline assembly. However, these blocks are "unused" from LLVM
>>> > point of view and get erased from the function.
>>> >
>>> > To counter that I started adding checks for conditions that are
>>> > guaranteed to be true or false. I ended up with calling
>>> > @llvm.returnaddress(i32 0) intrinsic and comparing the result with 0.
>>> > It worked well until in one function I had two such calls and SROA
>>> > replaced one of checks with constant 1 and erased the BB.
>>> >
>>> > I should probably stop trying to fool LLVM and "do it right", but
>>> > don't have any idea how. Note that I can't use global variables for a
>>> > reason, so the data has to be encoded in a BB using inline assembly.
>>> > All I need is just prevent optimizations from erasing it.
>>>
>>> A reachable inline asm won't be erased if LLVM thinks it has some
>>> side-effect.  The simplest way to do this is the "sideeffect" marking
>>> (in C++, it's a parameter to InlineAsm::get()).  See
>>> http://llvm.org/docs/LangRef.html#inline-assembler-expressions .
>>>
>>
>> The problem is exactly reachability. Here is a simple example:
>>
>> define void @foo() {
>> entry:
>>   ...
>>   ret void
>> data:
>>   call void asm sideeffect inteldialect ".byte 0xB2",
>> "~{dirflag},~{fpsr},~{flags}"()
>>   call void asm sideeffect inteldialect ".byte 0xB9",
>> "~{dirflag},~{fpsr},~{flags}"()
>>   ...
>> }
>>
>> To make "data" reachable I change entry's terminator to br %tobool, label
>> %exit, label %data, where %tobool is a result of icmp eq that is always
>> true. However, I can't come up with such a condition that didn't get erased
>> by SROA.
>>
>>
>> Even if you manage to trick LLVM into emitting the inline asm, it won't
>> be in a predictable location in the emitted assembly; some LLVM transforms
>> will rearrange the code in a function.
>>
>
> Won't @llvm.returnaddress() always get me correct location of my inline
> asm block?
>
>
> I'm very confused... how could you possibly use @llvm.returnaddress to
> return the address of a block of code that's never executed?
>

Sorry, it is a typo. I meant blockaddress constant, not @llvm.returnaddress
intrinsic.


> -Eli
>
> --
> Employee of Qualcomm Innovation Center, Inc.
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
>
>
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