[LLVMdev] Break operands into multiple instructions

[John Criswell]

On 8/19/11 12:58 PM, Kodakara, Sreekumar V wrote:
>
> Hi All,
>
> I am creating an instrumentation pass using LLVM libraries. I am using 
> clang++ (version 2.9) to compile cpp code into LLVM bit code. When I 
> generate the llvm bit code using clang++ using --O3, I get many 
> instances of instructions that look like the following.
>
> br i1 icmp ne (i8* bitcast (i32 (i32)* @pthread_cancel to i8*), i8* 
> null), label %bb23, label %bb25
>
> Here three LLVM instructions are used in one instruction, namely br, 
> icmp and bitcast. Is there a way to tell clang++ to break the operand 
> of br into 3 separate instructions or a pass that exists that does it. 
> Ideally, I would like to see the following
>

There is a pass in SAFECode that breaks constant expression GEPs into 
GEP instructions.  Since some constant expression GEPs are casted using 
constant expression casts, it will also convert cast constant 
expressions into cast instructions as needed.  You should be able to 
easily modify the code to do what you need.

You can find the code at 
http://llvm.org/viewvc/llvm-project/safecode/trunk/lib/ArrayBoundChecks/BreakConstantGEPs.cpp?view=log 
(i.e., lib/ArrayBoundsChecks/BreakConstantGEPs.cpp in the SAFECode 
project).  The code written for LLVM 2.7 and may need minor changes to 
compile against mainline LLVM, but that should not be too hard.

-- John T.

> I8* tmp0 = i8* bitcast (i32 (i32)* @pthread_cancel to i8*
>
> I1 tmp = icmp ne i8* tmp0, i8* null
>
> br i1 tmp, label %bb23, label %bb25
>
> Thanks
>
> Sreekumar
>
>
>
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