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

[Gleb Popov via llvm-dev]

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.


> -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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