[LLVMdev] a LICM bug (in LLVM-2.7)?

[Chuck Zhao]

  Thank you very much, Eugene,

This explains it nicely.

Chuck

On 8/15/2010 10:36 AM, Eugene Toder wrote:
> I don't think licm looks at loads/stores to allocas -- these are
> usually handled by mem2reg which happens much earlier (if you run your
> example with -mem2reg you'll see it already deleted the store). In
> fact, licm sinks the stores by converting them to stores to allocas
> first and running mem2reg on that.
>
> If you change your example to
>
> void testLICM(int* restrict p)
> {
>    int i,N=100;
>    for(i=0;i<N;i++){
>      *p = 1;
>      printf("i: %d\n",i);
>    }
>    printf("data: %d\n", *p);
> }
>
> and run opt with -mem2reg -loop-rotate -licm you can see the store being sunk.
>
> Eugene
>
> On Sun, Aug 15, 2010 at 3:47 AM, Chuck Zhao<czhao at eecg.toronto.edu>  wrote:
>> I am studying the Transform/Scalar/LICM.cpp pass, and wrote a simple test
>> program to validate.
>>
>> void testLICM(void){
>>    int i,N=100;
>>    int data;
>>    for(i=0;i<N;i++){
>>      data = 1;
>>      printf("i: %d\n",i);
>>    }
>>    printf("data: %d\n", data);
>> }
>>
>>
>> I expect the "data=1" will be moved out of loop (either hoist or sink).
>> However, to my surprise, that statement remains within the loop after -licm.
>>
>> This is the licm produced IR:
>>
>> define void @testLICM() nounwind {
>> entry:
>>    %i = alloca i32                                 ;<i32*>  [#uses=3]
>>    %N = alloca i32                                 ;<i32*>  [#uses=2]
>>    %data = alloca i32                              ;<i32*>  [#uses=2]
>>    %"alloca point" = bitcast i32 0 to i32          ;<i32>  [#uses=0]
>>    call void @llvm.dbg.declare(metadata !{i32* %i}, metadata !0), !dbg !7
>>    call void @llvm.dbg.declare(metadata !{i32* %N}, metadata !8), !dbg !7
>>    call void @llvm.dbg.declare(metadata !{i32* %data}, metadata !9), !dbg !7
>>    store i32 100, i32* %N, align 4, !dbg !10
>>    store i32 0, i32* %i, align 4, !dbg !11
>>    %0 = load i32* %N, align 4, !dbg !11            ;<i32>  [#uses=1]
>>    %i.promoted = load i32* %i                      ;<i32>  [#uses=1]
>>    br label %bb1, !dbg !11
>>
>> bb:                                               ; preds = %bb1
>>    store i32 1, i32* %data, align 4, !dbg !12
>>    %1 = call i32 (i8*, ...)* @printf(i8* noalias getelementptr inbounds ([7 x
>> i8]* @.str, i32 0, i32 0), i32 %i.tmp.0) nounwind, !dbg !13 ;<i32>
>> [#uses=0]
>>    %2 = add nsw i32 %i.tmp.0, 1, !dbg !11          ;<i32>  [#uses=1]
>>    br label %bb1, !dbg !11
>>
>> bb1:                                              ; preds = %bb, %entry
>>    %i.tmp.0 = phi i32 [ %i.promoted, %entry ], [ %2, %bb ] ;<i32>  [#uses=4]
>>    %3 = icmp slt i32 %i.tmp.0, %0, !dbg !11        ;<i1>  [#uses=1]
>>    br i1 %3, label %bb, label %bb2, !dbg !11
>>
>> bb2:                                              ; preds = %bb1
>>    store i32 %i.tmp.0, i32* %i
>>    %4 = load i32* %data, align 4, !dbg !14         ;<i32>  [#uses=1]
>>    %5 = call i32 (i8*, ...)* @printf(i8* noalias getelementptr inbounds ([10
>> x i8]* @.str1, i32 0, i32 0), i32 %4) nounwind, !dbg !14 ;<i32>  [#uses=0]
>>    br label %return, !dbg !15
>>
>> return:                                           ; preds = %bb2
>>    ret void, !dbg !15
>> }
>>
>>
>> I ran the program with licm only option through opt:
>> opt -licm<  test.bc>  test.licm.bc
>>
>>
>> Is this a bug, or am I running it in a wrong way?
>>
>> Thank you
>>
>> Chuck
>>
>>
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