[llvm-dev] simplify CFG Pass in llvm

Jajoo, Malhar via llvm-dev
Mon, 19 Jun 2017 13:06:23 -0000

Hi Friedman ,


I did not put the whole IR because it would be context sensitive.

I am trying to use non-concrete types in my language but trying to
first generate the IR using dummy types and then transform the IR

to contain the correct types.


hence,I am forced to use the allocas in the middle ,

again because of the problem above that I am trying to solve.


Thanks,

Malhar

________________________________
From: Friedman, Eli <efriedma at codeaurora.org>
Sent: Friday, June 16, 2017 8:29:25 PM
To: Jajoo, Malhar; llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] simplify CFG Pass in llvm

On 6/16/2017 9:24 AM, Jajoo, Malhar via llvm-dev wrote:
I was trying to run the simplify CFG Pass in LLVM , and delete an unreachable
basic block ("continuation" below ) after running one of my own IR
transforms , but I keep getting the
error -

While deleting: i8* %g

Use still stuck around after Def is destroyed:  store i8 0, i8* %g

I am well aware of what that means, but isn't the whole purpose of the
"simplifyCFGPass" to delete the unreachable basic blocks for us ? Why
must it then throw this error ? I would assume it should simply be able to
manage all the use-def dependencies and delete the instructions in the
unreachable "continuation" basic block below.


Following is the relevant IR

    entry:
      %a3 = alloca i32
      store i32 %a, i32* %a3
      %a4 = load i32, i32* %a3
      %ifcond = icmp ne i32 %a4, 0
      br i1 %ifcond, label %then, label %else


    then:                                             ; preds = %entry
      %gclone1 = alloca i32
      store i32 0, i32* %gclone1
      ret i5 0

    else:                                             ; preds = %entry
      %gclone4 = alloca i64
      store i64 0, i64* %gclone4
      ret i5 0

    continuation:                                     ; No predecessors!
      %iftmp = phi i32 [ 32, %then ], [ 64, %else ], !range !0
      %datasize = alloca i32
      store i32 %iftmp, i32* %datasize

      %g = alloca i8 ---------------------> Issue
      store i8 0, i8* %g ---------------------> Issue
      ret i5 0
    }


Can someone please explain why this error crops up ? Isn't the API supposed
to handle this ?

1. If you're going to send a testcase to llvmdev, please make sure it is
complete: if it isn't possible to reproduce the issue, we're going to be
left guessing what the issue is.
2. Running the verifier can catch a lot of issues (llvm::verifyModule in the C++
API).  If your IR doesn't follow the rules correctly, you can get weird
results.
3. It's generally a bad idea to put "alloca" instructions anywhere
other than the entry block of a function, if you can avoid it; see
http://llvm.org/docs/tutorial/LangImpl07.html .


-Eli

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Employee of Qualcomm Innovation Center, Inc.
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