[llvm-dev] DependenceAnalysisWrapperPass is confused with Simple Flow Dependence

[Bekket McClane via llvm-dev]

Hi,

You can dump the debug trace by adding -debug option or -debug-only=“da” to opt
Normally Pass authors would print details in debug trace.

Or, you can just trace it with gdb, it works well for me all the time.

Bests,
Bekket

> On Sep 21, 2018, at 5:56 PM, Sato, Kento via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hi,
> 
> I’m trying to do dependency analysis using LLVM DependenceAnalysisWrapperPass (Clang/LLVM6.0.0).
> But, I’m confused by the results.
> 
> For example, I tried this simple C program (simple.c).
> 
> int main(int argc, char** argv)
> { 
> volatile int x = 1, y = 2;
> y = x;
> return 0;
> }
> 
> If I output the IR, I get this.
> 
> define i32 @main(i32, i8** nocapture readnone) local_unnamed_addr #0 !dbg !7 {
> %3 = alloca i32, align 4
> %4 = alloca i32, align 4
> call void @llvm.dbg.value(metadata i32 %0, metadata !15, metadata !DIExpression()), !dbg !20
> call void @llvm.dbg.value(metadata i8** %1, metadata !16, metadata !DIExpression()), !dbg !21
> %5 = bitcast i32* %3 to i8*, !dbg !22
> call void @llvm.lifetime.start.p0i8(i64 4, i8* nonnull %5), !dbg !22
> call void @llvm.dbg.value(metadata i32 1, metadata !17, metadata !DIExpression()), !dbg !23
> store volatile i32 1, i32* %3, align 4, !dbg !23
> %6 = bitcast i32* %4 to i8*, !dbg !22
> call void @llvm.lifetime.start.p0i8(i64 4, i8* nonnull %6), !dbg !22
> call void @llvm.dbg.value(metadata i32 2, metadata !19, metadata !DIExpression()), !dbg !24
> store volatile i32 2, i32* %4, align 4, !dbg !24
> %7 = load volatile i32, i32* %3, align 4, !dbg !25
> call void @llvm.dbg.value(metadata i32 %7, metadata !17, metadata !DIExpression()), !dbg !23
> call void @llvm.dbg.value(metadata i32 %7, metadata !19, metadata !DIExpression()), !dbg !24
> store volatile i32 %7, i32* %4, align 4, !dbg !26
> call void @llvm.lifetime.end.p0i8(i64 4, i8* nonnull %6), !dbg !27
> call void @llvm.lifetime.end.p0i8(i64 4, i8* nonnull %5), !dbg !27
> ret i32 0, !dbg !28
> }
> 
> Since there is obvious flow dependence in y = x, that is, READ(%7 = load volatile i32, i32* %3, align 4, !dbg !25) and WRITE(store volatile i32 %7, i32* %4, align 4, !dbg !26),
> I expect that the LLVM DependenceAnalysisWrapperPass can easily detect this flow dependence.
> 
> So I wrote simple dependence analysis pass (libmyirpass.so) to confirm it.
> 
> bool runOnFunction(Function &F)
> {
> DependenceInfo *depinfo;
> depinfo = &getAnalysis<DependenceAnalysisWrapperPass>().getDI();
> for (inst_iterator Iit = inst_begin(F), Iit_end = inst_end(F); Iit != Iit_end; Iit++) {
>   Instruction &Dst = *Iit;
>   for (inst_iterator Jit = inst_begin(F), Jit_end = inst_end(F); Jit != Jit_end; Jit++) {
>     unique_ptr<Dependence> infoPtr;
>     Instruction &Src = *Jit;
>     Dependence *dep;
>     infoPtr = depinfo->depends(&Src, &Dst, false); /* depends(&Src, &Dest, true); */
>     dep = infoPtr.get();
>     if (dep != NULL) {
>       if (dep->isConfused()) errs() << "[C] ";
>       dep->getDst()->print(errs(), false);
>       errs() << "   ---> ";
>       dep->getSrc()->print(errs(), false);
>       errs() << "\n";
>     }
>   }
> }
> return false;
> }
> 
> However, DependenceAnalysisWrapperPass is confused with this simple flow dependence, i.e., dep->isConfused() returns true,
> so it outputs this message if I compile simple.c with libmyirpass.so by clang-6.0.0.
> 
> $ clang++ -g -O0 -std=c++11  -Xclang -load -Xclang ./libmyirpass.so  simple.c
> ...
> [C]   store volatile i32 %7, i32* %4, align 4, !dbg !26   --->   %7 = load volatile i32, i32* %3, align 4, !dbg !25
> …
> 
> I see the same issue with use of both "PassManagerBuilder::EP_EarlyAsPossible" and "PassManagerBuilder::EP_OptimizerLast".
> 
> I’m wondering why DependenceAnalysisWrapperPass is confused with this simple flow dependence.
> If I’m misunderstanding about this dependence analysis, please correct me.
> 
> Thanks,
> Kento
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