[llvm-dev] Prevent LLVM optimizations from erasing unused basic blocks (original) (raw)

Friedman, Eli via llvm-dev llvm-dev at lists.llvm.org
Tue Oct 9 13:42:30 PDT 2018

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_ _<mailto: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 <mailto: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?

-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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