[llvm-dev] Avoiding during my pass the optimization (copy propagation) of my LLVM IR code (at generation)

[Michael Kuperstein via llvm-dev]

You can avoid at least some of it by using the Instruction interfaces (e.g.
CallInst::Create()) directly instead of IRBuilder.

But (a) I'm not sure if this will change what happens in this particular
case, and (b) I would strongly suggest you not do this, though, unless you
have a really good reason (you're trying to generate a specific instruction
sequence for, say, fuzzing).

On 30 December 2016 at 19:20, Mehdi Amini via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
> > On Dec 30, 2016, at 3:03 PM, Alex Susu via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> >  Hello.
> >    I'm writing an LLVM pass that is working on LLVM IR.
> >    To my surprise the following LLVM pass code generates optimized code
> - it does copy propagation on it.
>
> It does *constant* propagation to be exact.
>
> >      Value *vecShuffleOnePtr = Builder.CreateGEP(ptr_B, vecShuffleOne,
> "VectorGep");
> >      ...
> >      packed_gather_params.push_back(vecShuffleOnePtr);
> >      CallInst *callGather = Builder.CreateCall(func_llvm_
> masked_gather_v128i16,
> >                                                packed_gather_params);
> >      callGather->addAttribute(AttributeSet::FunctionIndex,
> Attribute::NoUnwind);
> >
> >      DEBUG(dbgs() << "callGather: " << *callGather << "\n");
> >
> >    When running this code we get at stderr something like:
> >        callgather: %50 = call <4 x i16> @llvm.masked.gather.v4i16(<4 x
> i16*> getelementptr (i16, i16* inttoptr (i16 51 to i16*), <4 x i64> <i64 1,
> i64 1, i64 1, i64 1>), i32 0, <4 x i1> <i1 true, i1 true, i1 true, i1
> true>, <4 x i16> undef)
> >    Also, more importantly, the resulting LLVM program will also contain
> this 1 gather LLVM IR instruction.
> >
> >    So, although I instructed LLVM to generate 2 instructions, it creates
> only one instruction, a call to gather containing as parameter not the
> VectorGep variable name, but its value/definition, the getelementptr
> instruction (and this happens even if VectorGep is used more than once in
> my program).
> >
> >    Is there a way to specify programmatically in my LLVM pass to avoid
> doing this copy propagation optimization (and obtain an LLVM program with a
> separate getelementptr instruction, besides the gather instruction)?
>
> No: you can’t really force constant to not be propagated (At least I don’t
> know an easy way for that).
>
> —
> Mehdi
>
>
>
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