[llvm-dev] simplify CFG Pass in llvm

[Friedman, Eli via llvm-dev]

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

-- 
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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