[PATCH] LowerDbgDeclare - get rid of redundant dbg.values when an alloca survives optimization

[David Blaikie]

On Tue, Oct 15, 2013 at 1:06 PM, Adrian Prantl <aprantl at apple.com> wrote:

> You still need to pass -O2 to clang.
>

Right - but what I mean is, in your test case, rather than running -O2,
could you run only the specific passes that cause the transformation to
occur that's concerned? See, for example, the various tests in
test/Transforms/*, each of which uses opt arguments to run just the
transformation it's testing (-instcombine in the InstCombine tests,
-globalopt in the GlobalOpt tests, etc). You could even put this test in
the directory for the tests for the transform in question.

It looks like it's InstCombine, since that's the only caller of this
function - but you might need to run the IR through some other
transformations first (and checkin the result of that) before passing it to
InstCombine in the test.


>
> -- adrian
>
> On Oct 15, 2013, at 13:01, David Blaikie <dblaikie at gmail.com> wrote:
>
> >
> >
> >
> > On Tue, Oct 15, 2013 at 11:46 AM, Adrian Prantl <aprantl at apple.com>
> wrote:
> >
> > On Oct 15, 2013, at 11:42, David Blaikie <dblaikie at gmail.com> wrote:
> >
> > > Could you attach/paste the IR obtained without your change? I'm
> curious to see what dbg.values are produced.
> >
> > ; Function Attrs: nounwind ssp uwtable
> > define void @foo() #0 {
> >   %offset = alloca i64, align 8
> >   call void @llvm.dbg.declare(metadata !{i64* %offset}, metadata !10),
> !dbg !13
> >   call void @llvm.dbg.value(metadata !14, i64 0, metadata !10), !dbg !13
> >   store i64 0, i64* %offset, align 8, !dbg !13, !tbaa !15
> >   %1 = call i32 @bar(i64* %offset, i64 0) #3, !dbg !19
> >   %2 = icmp eq i32 %1, 0, !dbg !19
> >   br i1 %2, label %6, label %3, !dbg !19
> >
> > ; <label>:3                                       ; preds = %0
> > ; -----> This is the one I care about:
> >   call void @llvm.dbg.value(metadata !{i64* %offset}, i64 0, metadata
> !10), !dbg !21
> >   %4 = load i64* %offset, align 8, !dbg !21, !tbaa !15
> >   %5 = call i32 @bar(i64* %offset, i64 %4) #3, !dbg !21
> >   br label %6, !dbg !21
> >
> > ; <label>:6                                       ; preds = %3, %0
> >   ret void, !dbg !22
> > }
> >
> > > & a little bit of 'asking for a pony' - but is there one specific
> optimization you could run to demonstrate this problem rather than all of
> -O2?
> >
> > No ponies, unfortunately. The specific optimization _is_ LowerDbgDeclare
> and it usually gets run by clang even if you go straight to -emit-llvm.
> >
> > Sorry, I'm not sure I follow. When I run clang -g -O0 -S -emit-llvm on
> the source you listed (and without your change applied) I get one
> dbg.declare, no dbg.values). So I don't know what you mean by "usually gets
> run by clang even if you go straight to -emit-llvm" - if that were true,
> wouldn't I expect to see the spurious dbg.values you described in the
> output of -g -O0 -S -emit-llvm?
>
>
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