[llvm-dev] questions about AliasAnalysis(Basic Alias Analysis) in llvm3.3

[Vedant Kumar via llvm-dev]

Hi,

ISTM that you want:

  canInstructionRangeModRef(BB.begin(), BB.end(), MemoryLocation(P, Size), MRI_Ref)

You could also walk the uses of the GV in question and check if their parent BB's are the BB of interest.

vedant


> On May 4, 2016, at 5:34 AM, PeiLIU via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hi, My name is Pei Liu, I am using llvm-3.3 do some developing. I want to confirm that basic blocks operate global variables or not.  For example:
> C-Language source code:
> int ga;
> 
> int main()
> {
> 	int a = 10;
> 	int b = 100;
> 
> 	if (a > 8) {
> 		int ma = ga;
> 	}
> 	return 0;
> }
> 
> The llvm IR .ll file looks like this:
> ; Function Attrs: nounwind uwtable
> define i32 @main() #0 {
> entry:
>   %retval = alloca i32, align 4
>   %a = alloca i32, align 4
>   %b = alloca i32, align 4
>   %ma = alloca i32, align 4
>   store i32 0, i32* %retval
>   call void @llvm.dbg.declare(metadata !{i32* %a}, metadata !23), !dbg !24
>   store i32 10, i32* %a, align 4, !dbg !24
>   call void @llvm.dbg.declare(metadata !{i32* %b}, metadata !25), !dbg !26
>   store i32 100, i32* %b, align 4, !dbg !26
>   %tmp = load i32* %a, align 4, !dbg !27
>   %cmp = icmp sgt i32 %tmp, 8, !dbg !27
>   br i1 %cmp, label %if.then, label %if.end, !dbg !27
> 
> if.then:                                          ; preds = %entry
>   call void @llvm.dbg.declare(metadata !{i32* %ma}, metadata !28), !dbg !30
>   %tmp1 = load i32* @ga, align 4, !dbg !30
>   store i32 %tmp1, i32* %ma, align 4, !dbg !30
>   br label %if.end, !dbg !31
> 
> if.end:                                           ; preds = %if.then, %entry
>   ret i32 0, !dbg !32
> }
> 
> I want to use AliasAnalysis to confirm that the if.then block operate the global variable. I use the function canBasicBlockModify(const BasicBlock &BB, const Value *P, uint64_t Size) which belong to the class of AliasAnalysis. However, I use every block to test then it always return true of the function canBasicBlockModify. 
> 
> Does this property which basic block operates global variable can be confirmed? If it can be confirm, then how? Thanks for your help. Best wishes! 
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